EE6801 ELECTRIC ENERGY GENERATION, UTILIZATION AND ...vel tech high tech dr. rangarajan...

EE6801 ELECTRIC ENERGY GENERATION, UTILIZATION AND CONSERVATION / EEE DEPT/ R SENTHI KUMAR

VEL TECH HIGH TECH DR. RANGARAJAN DR.SAKUNTHALA ENGINEERING COLLEGE

EE6702 PROTECTION AND SWITCHGEAR / EEE DEPT/ R SENTHIL KUMAR VEL TECH HIGH TECH DR. RANGARAJAN DR.SAKUNTHALA ENGINEERING COLLEGE




EE6801 ELECTRIC ENERGY GENERATION, UTILIZATION AND CONSERVATION L T P C 3 0 0 3 OBJECTIVES: • To analyze the various concepts behind renewable energy resources. • To introduce the energy saving concept by different ways of illumination. • To understand the different methods of electric heating and electric welding.

• To introduce knowledge on Solar Radiation and Solar Energy Collectors • To introduce concepts of Wind Energy and its utilization

UNIT I ELECTRIC DRIVES AND TRACTION 9 Fundamentals of electric drive - choice of an electric motor - application of motors for particular services - traction motors - characteristic features of traction motor - systems of railway electrification - electric braking - train movement and energy consumption - traction motor control - track equipment and collection gear.

UNIT II ILLUMINATION 9 Introduction - definition and meaning of terms used in illumination engineering - classification of light sources - incandescent lamps, sodium vapour lamps, mercury vapour lamps, fluorescent lamps – design of illumination systems - indoor lighting schemes - factory lighting halls - outdoor lighting schemes - flood lighting - street lighting - energy saving lamps, LED.

UNIT III HEATING AND WELDING 9 Introduction - advantages of electric heating – modes of heat transfer - methods of electric heating - resistance heating - arc furnaces - induction heating - dielectric heating - electric welding – types - resistance welding - arc welding - power supply for arc welding - radiation welding.

UNIT IV SOLAR RADIATION AND SOLAR ENERGY COLLECTORS 9 Introduction - solar constant - solar radiation at the Earth’s surface - solar radiation geometry – estimation of average solar radiation - physical principles of the conversion of solar radiation into heat – flat-plate collectors - transmissivity of cover system - energy balance equation and collector efficiency - concentrating collector - advantages and disadvantages of concentrating collectors - performance analysis of a cylindrical - parabolic concentrating collector – Feeding Invertors.

UNIT V WIND ENERGY 9 Introduction - basic principles of wind energy conversion - site selection considerations - basic components of a WECS (Wind Energy Conversion System) - Classification of WECS - types of wind Turbines - analysis of aerodynamic forces acting on the blade - performances of wind.

TOTAL : 45 PERIODS




OUTCOMES:

Ability to understand and analyze power system operation, stability, control and protection.

Ability to handle the engineering aspects of electrical energy generation and utilization.

TEXT BOOKS:

1. N.V. Suryanarayana, “Utilisation of Electric Power”, Wiley Eastern Limited, New Age International Limited,1993. 2. J.B.Gupta, “Utilisation Electric power and Electric Traction”, S.K.Kataria and Sons, 2000. 3. G.D.Rai, “Non-Conventional Energy Sources”, Khanna Publications Ltd., New

Delhi, 1997. AULibrary.com 82

REFERENCES: 1. R.K.Rajput, Utilisation of Electric Power, Laxmi publications Private Limited.,2007. 2. H.Partab, Art and Science of Utilisation of Electrical Energy”, Dhanpat Rai and Co., New Delhi, 2004. 3. C.L.Wadhwa, “Generation, Distribution and Utilisation of Electrical Energy”, New Age International Pvt.Ltd., 2003. 4. S. Sivanagaraju, M. Balasubba Reddy, D. Srilatha,’ Generation and Utilization of

Electrical Energy’, Pearson Education, 2010. 5. Donals L. Steeby,’ Alternative Energy Sources and Systems’, Cengage Learning,

2012




CONTENTS

S.No Particulars Page

1 Unit – I 6

2 Unit – II 40

3 Unit – III 86

4 Unit – IV 129

5 Unit – V 160




Unit – I

Electric Drives and Traction

Part – A

1. What is scheduled speed of train? [CO1 – L1 - APRIL/MAY 2008 ] The ratio of distance covered between two stops and total time of run including

time of stop is known as schedule speed. Schedule speed= Distance between stops in km / Actual time of run in hours + stop time in hours. The schedule speed is always smaller than the average speed. The difference is large in case of urban and suburban services and is negligibly small in case of main line service.

2. What is load equalization? [ CO1 – L1 - APRIL/MAY 2008, MAY/JUNE 2009].

When heavy load is applied, the motor speed decreases and flywheel will supply kinetic energy to the motor. During light load condition, the motor speed increases and the flywheel stores the energy. Thus the load on the motor is equalized. So we define load equalization as “during the operation of drives, load torque fluctuates widely within shot intervals of time this may cause more problems and affect the stability of the drive. These problems of fluctuating loads are overcome by mounting a flywheel on the motor shaft in non-reversible drives”. This process is known as load equalization.

3. Distinguish between individual drive and group drive.[ CO1 – L1 - APR 2008] Group electric drive

The group electric drive was used in the earlier days. It had a single motor of sufficient capacity to drive an entire group of machines used in a shop.

The motor was connected to a line shaft and through the use of belts and pulleys all the machines were driven.

This form of drive was very inefficient, difficult to control and unsafe.

4. Define tractive effort. [ CO1 – L1 - NOV /DEC 2008, APR 2014] The effective force necessary to propel the train at the wheels of the locomotive

to which the motor is geared is called the tractive effort. It is measured in Newtons and is tangential to the driving wheels

5. What is meant by specific energy consumption? [ CO1 – L1 - NOV 2008, APR

2015] The energy input to the motors which gives tractive effort is called the energy

consumption of the train, since it is the energy consumed for propelling the train. The total energy drawn from the distribution system will be greater than this by the quantity required for lighting, heating, and control and braking.




6. What are the advantages of individual drive? [ CO1 – L1 - NOV /DEC 2008] The advantages are

If there is a fault in one motor, the effect on the production or output of the industry will not be appreciable

Machines can be located at convenient places Continuity in the production of the industry is ensured to a higher degree.

7. What type of electrical drive is used in (a) cranes (b) blowers? [ CO1 – L1 - NOV /DEC 2008] Cranes: DC series motor and AC Schrage motor Blowers: DC shunt motor and AC single phase induction motors and synchronous motors.

8. List two merits of series-parallel starting of traction motors. [CO1 – L1 - MAY/JUNE 2009]

Efficiency is more Without wasting energy, more than one economical speed is possible The energy lost in the starting resistance is low

9. What are the factors affecting specific energy consumption [ CO1 – L1 – APR 2015, MAY/JUNE 2009]

Distance between the stops Train resistance Acceleration and retardation Gradient Type of train equipment.

10. What is meant by electrical drives? [ CO1 – L1 ] Systems employed for motion control are called “DRIVES” and drives employ any of the prime movers such as, diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors for supplying mechanical energy for motion control. Drives employing electric motion known as “Electric Drives”.

11. Define an electric drive. [ CO1 – L1 ] The combination of an electric motor, the energy transmitting shaft and the controlling devices for controlling the performance of the motor is called an electric drive

12. What are the essential requirements of braking in an electrical drive? [CO1- L1] 1. Fast reliable and controllable 2. Stored energy should be dissipated efficiently 3. Failure in any part should result in braking only.




13. State the merits and de merits of electrical braking. [CO1 – L1 ] Merits:

Less maintenance No dirt Regenerative braking

Possible Demerits: Motor should have suitable braking characteristics. No holding torque During failure of supply mechanical braking needed

14. Define and mention different types of braking in a dc motor? [ CO1 – L1 ]

In braking, the motor works as a generator developing a negative torque which opposes the motion. Types of regenerative braking are Dynamic (or) Rheostat braking; and plugging (or) reverse voltage braking.

15. What is meant by regenerative braking? [ CO1 – L1 ]

Regenerative braking occurs when the motor speed exceeds the synchronous speed. In this case, the induction motor would runs as the induction machine is converting the mechanical power into electrical power, which is delivered back to the electrical system. This method of braking is known as regenerative braking.

16. What is meant by dynamic braking? [ CO1 – L1 ]

Dynamic braking of electric motor occurs when the energy stored in the rotating mass is dissipated in an electrical resistance. This requires the motor to operate as a generator to convert this stored energy into electrical.

17. What is meant by plugging? [ CO1 – L1 ]

It is one method of braking of induction motor. When phase sequence of supply of the motor running at a speed is reversed, by interchanging connections of any two phases of stator with respect to supply terminals, operation shifts from motoring to plugging region.

18. Give the merits and demerits of group drive. [ CO1 – L1 - MAY/JUNE 2009] Merits:

Initial cost of installing the drive is low In certain industry processes one process is connected to another process and

it will be advantageous if all these interconnected processes are stopped simultaneously.

Demerits: If at certain instance all the machines are not in operation, then the motor will be

working at low capacity It is not possible to install any machine at a distance place. The possibility of installation of additional machines in an existing industry is




limited




In case of fault in the motor all the machines connected to this motor will cease to operate thereby paralyzing either complete or part of the industry until the time the fault is removed.

19. With respect to the traction systems, explain the term ‘free running’.[ CO1 – L1 - NOV/DEC 2009] There are five distinct periods in the running of train

Notching up period Acceleration period Free running period Coasting period Braking period In traction system, free running is the status of train run which indicates the train

runs at constant speed attained at the end of speed curve running. Free running period: During this period on level track the power output from the

driving axle balances the rate at which energy is expended against the resistance to motion. At the end of speed curve running train reaches maximum speed.

20. What is the voltage level used in traction distribution network? [ CO1 – L1 - NOV/DEC 2009] DC system: 1500 V to 3000 V (AC source is 33KV) 25 KV single phase AC systems: voltage level used to 300V to 400 V at 25 or 16.67 Hz Three phase system: voltage level used is 300 V to 3600V at normal frequency or 16.67 Hz Finally the distribution network is fed at voltage varying between 15Kv to 25Kv at normal frequency at 50Hz.The ac supply is stepped down and converted to dc.

21. Write the applications of DC shunt motor. [ CO1 – L1 - NOV/DEC 2009]

The main characteristics of d.c.shunt motor are its nearly constant speed over wide range of loading and its ability of operating at any speed within a wide range. Shunt motor mostly used for constant speed applications

Lathe machines Drilling machines Grinder Line shafting and fans.

22. Name two types of loads, related to electrical drives. [ CO1 – L1 – NOV 2009] Active loads: which are due to the forces of gravity, tension or compression. Active loads are independent of loads, ex: paper mill drive. Passive loads: which are due to the friction, cutting and deformation of inelastic bodies. Example for passive loads is fans, compressors, airplanes, centrifugal pumps, ship propellers, high speed hoists, traction etc.




23. What type of motor is used for electric traction? Why? [ CO1 – L1 - MAY 2010] Series and compound motors are employed in d.c traction systems. D.C series motor

Advantages High starting torque Simple speed control Better commutation up to twice full load Simple and robust in construction Less susceptible to variations in supply voltage Capable of withstanding excessive loads.

These motors are more particularly suitable for suburban and urban services where high rate of acceleration is essential.

Series Motor Many single phase a.c. motors have been developed for traction purposes but only compensated series type commutator motor is best suited for traction. Single phase induction motors are not capable of developing high starting torque hence it is not used.

Advantages Higher efficiency Improved commutation The weight per kw output is greater for the higher frequency because of larger

dimensions of the motor. Efficient speed control of motor by providing taps on a transformer which is not

possible in d.c series motor. These motors are used for main line services. These are not suitable for urban

and sub urban services because of low starting torque and poor power factor at start.

24. What are the requirements of an ideal traction system? [ CO1 – L1 ] The requirements of an ideal traction system are as follows

The starting tractive effort should be high so as to have rapid acceleration. The wear on the track should be minimum. Pollution free Speed control should be easy. The equipment should be capable of withstanding large temporary loads. Low initial and maintenance cost. There should be no interference to the communication lines running along the

lines. Braking should be such that minimum wear is caused on the brake shoes.

25. Name the various systems of traction. [ CO1 – L1 ] Direct steam engine drive Direct Internal Combustion Engine Drive Steam Electric Drive




Internal Combustion Engine Electric Drive




Petrol Electric traction Electric Drive

26. Classify the supply system for electric traction. [ CO1 – L1 ]

D.C system A.C system

Single phase

Three phase Composite system

Single phase AC-DC

single phase-Three phase

27. What are the advantages of electric traction? [ CO1 – L1 – NOV 14] High starting torque Less maintenance cost Cheapest method of traction Rapid acceleration and braking Less vibration Coefficient of adhesion is better It has great passenger carrying capacity at higher speed.

28. What are the disadvantages of electric traction? [ CO1 – L1 – NOV 14 ] High capital cost Problem of supply failure Additional equipment is required for achieving electric braking and control The leakage of current from the distribution mains and drop of volts in the track

are to be kept within the prescribed limits. The electrically operated vehicles have to move on guided track only.

29. Name the different stages of train movement [ CO1 – L1 ] Acceleration Constant speed or free running Coasting, running with power switched off and brake not applied Retardation, with braking

30. What are the essential features (electrical )of an ideal traction motor [ CO1 – L1 ]

High starting torque. Series speed torque characteristics Simple speed control Possibility of regenerative braking




31. What is the need for traction motor control? [ CO1 – L1 ] To limit starting current 2Smooth acceleration without jerk Both manual and automatic control should be possible.

32. Give any two advantages of electric traction. [ CO1 – L1 - APRIL/MAY 2010] Advantages of electric traction High starting torque

Less maintenance cost Cheapest method of traction

Free from smoke and fluke gases hence used for underground and tubular and braking Less vibration Coefficient of adhesion is better It has great passenger carrying capacity at higher speed.

33. Define continuous rating of motor. [ CO1 – L1 - APRIL/MAY 2010]

It is the output which a motor can give continuously for long time without exceeding the given temperature rise and the motor should be able to give 20% overload for two hours

34. Why is induction motor commonly preferred drive industrial application? [ CO1 – L1 - APRIL/MAY 2010]

Though three-phase induction motors have the advantages of simple and robust construction, high voltage operation, less maintenance etc., they are commonly used for industrial application due to their flat speed torque characteristics, constant speed operation, and low starting torque. Advantages of AC 3 phase induction motors.

It has very simple and extremely rugged, almost unbreakable construction especially squirrel cage type.

Its cost Is low and it is very reliable It has sufficiently high efficiency. In normal running condition, no brushes are

needed, hence frictional losses are reduced. It has a reasonably good power factor

It requires minimum maintenance it start up from rest and needs no extra starting motor and has not to be

synchronized. Its starting arrangement is simple especially for squirrel cage type motor.

35. List the factors affecting scheduled speed of a train. [ CO1 – L1 - MAY 2011]

The scheduled speed of a train when running on a given service is affected by the following factors:

i) Acceleration & braking retardation If the acceleration and braking retardation increases with fixed crest speed for a given run, the schedule speed increase which results in decrease of actual time of run.




ii) Maximum or crest speed If the crest speed increases in case of city and




suburban service, the coasting period and time taken for particular value of accelerating period reduces. Hence schedule speed increases.

iii) Stopping time or duration of stop The stopping time affects the schedule speed in the way that increase of stopping time reduces the schedule speed. This is of particular significance in case of short runs where the stopping time is more compared to total of run. In case of main line service this is not much significance.

36. Define continuous rating of a motor. [ CO1 – L1 - APRIL / MAY 2011] It is the output which a motor can give continuously for long time without

exceeding the given temperature rise and the motor should be able to give 20% overload for two hours.

37. What do you mean by average speed in electric traction? [ CO1 – L1 ]

The mean of the speeds from the start to stop i.e the distance between two

stops divided by the actual time of run is known as average speed.

38. What do you mean by schedule speed in electric traction? [ CO1 – L1 ]

The ratio of distance covered between two stops and total time of run including time of stop is known as schedule speed. The schedule speed is always smaller than the average speed. The difference is large in case of urban and suburban services and is negligibly small in case of main line service.

39. Define dead weight, adhesive weight. [ CO1 – L1 ] (i) Dead weight The total weight of locomotive and train to be pulled by the locomotive is known as dead weight. (ii) Adhesive weight The total weight to be carried on the driving wheels is known as the adhesive weight. 40. Name the various methods of traction motor control. [ CO1 – L1 ] There are various methods for controlling the speed of d.c series motors. They are

Rheostatic control Series parallel control Field control Buck and Boost method Metadyne control Thyristor control

41. What are the basic requirements of braking system? [ CO1 – L1 ] The basic requirements of a braking system are given below

It should be simple, robust, quick and reliable in action. Easy to use for driver to operate.




Maintenance should be minimum.




The braking system should be inexhaustible. In case of emergency braking, safety consideration is taken into account. Kinetic energy of the train must be storable during braking which could be used

subsequently during acceleration of the train.

42. What are the various methods of applying electric braking? [ CO1 – L1 ] There are three methods of applying electric braking are

Plugging or Reverse current braking. Rheostatic braking. Regenerative braking.

43. Name the advanced methods of speed control of traction motors. [CO1 – L1 ] The latest methods of speed control of traction motors are

Tap changer control Thyristor control Chopper control Microprocessor control

44. What are the advantages of microprocessor based control of traction motors? [ CO1 – L1 ]

High speed of response High accuracy Over voltage and over speed protection. Electronic interlocking Less sensitive to temperature variations and drift. Numbers of components used are less.




Part-B

1. Explain the working of the Traction system. [ CO1 – L2] The locomotive in which the driving or tractive force is obtained from electric

motors is called Electric traction. Electric traction has many advantages as compared to other non-electrical

systems of traction including steam traction. Electric traction is used in:

i) Electric trains ii) Trolley buses iii) Tram cars iv) Diesel-electric vehicles etc.

Traction systems All traction systems, broadly speaking, can be classified as follows: 1. Non-electric traction systems:

These systems do not use electrical energy at some stage or the other. Examples: (i) Steam engine drive used in railway

(ii) Internal combustion-engine-drive used for road transport 2. Electric traction systems:

These systems involve the use of electric energy at some stage or the other. These are further sub divided into the following two groups: a) Self contained vehicles or locomotives Examples: i) Battery-electric drive

ii) Diesel-electric drive b) Vehicles which receive electric power from a distribution network or suitably placed sub- stations. Examples: i) Railway electric locomotive fed from overhead A.C supply;

ii) Tramways and trolley buses supplied with D.C. supply.

2. List out the Requirements of an ideal traction system.[ CO1 – L2 – APR 2014]

The requirements of an ideal traction systemare: 1. High adhesion coefficient, so that high tractive effort at the start is possible

to have rapid acceleration. 2. The locomotive or train unit should be self contained so that it can run on

any route. 3. Minimum wears on the track. 4. It should be possible to overload the equipment for short periods. 5. The equipment required should be minimum, of high efficiency and low

initial and maintenance cost. 6. It should be pollution free. 7. Speed control should be easy. 8. Braking should be such that minimum wear is caused on the brake shoes,

and if possible the energy should be regenerated and returned to the supply




during braking period.




9. There should be no interference to the communication lines running the track.

3. What are all the Advantages and Disadvantages of Electric Traction? [ CO1 -

L2 ] Advantages and Disadvantages of Electric Traction

Electric traction system has many advantages compared to non-electric traction systems. The following are the advantages of electric traction:

Electric traction system is more clean and easy to handle. No need of storage of coal and water that in turn reduces the

maintenance cost as well as the saving of high-grade coal. Electric energy drawn from the supply distribution system is

sufficient to maintain the common necessities of locomotives such as fans and lights; therefore, there is no need of providing additional generators. The maintenance and running costs are comparatively low. The speed control of the electric motor is easy. Regenerative braking is possible so that the energy can be fed back to

the supply system during the braking period. Electrical features of a traction system

High-starting torque A traction motor must have high-starting torque, which is required to start the

motor on load during the starting conditions in urban and suburban services.

Speed control The speed control of the traction motor must be simple and easy. This is

necessary for the frequent starting and stopping of the motor in traction purpose.

Dynamic and regenerative braking Traction motors should be able to provide easy simple rheostatic and

regenerative braking subjected to higher voltages so that system must have the capability of withstanding voltage fluctuations.

Temperature The traction motor should have the capability of withstanding high

temperatures during transient conditions. Overload capacity The traction motor should have the capability of handling excessive overloads.

No single motor can have all the electrical operating features required for traction

4. Explain the various types of electric traction system. [CO1 – L2 - Nov’13,

NOV 14 ] The various systems of traction commonly used are,

1. Steam engine drive.




2. Internal combustion engine drive.




3. Internal combustion electric drive. 4. Petro-electric traction. 5. Battery electric drive. 6. Electric drive.

Steam engine drive: Steam engine drive, though losing ground gradually due to

various reasons; it is still the amply adopted means of propulsion of railway work in underdeveloped countries. In this type of drive, the reciprocating engine is invariably used for getting the necessary motive power. Advantages

Simplicity in design. Simplified maintenance. Easy speed control. Simplicity of connections between the cylinders and the driving wheels. No interference with communication network. Low capital cost as track electrification is not required. The locomotive and train unit is self contained; therefore, it is not tied to a

route. It is cheap for low density traffic areas and in initial stages of communication by rail Operational dependability.

Disadvantages Low thermal efficiency. Due to the reason of low adhesion coefficient, power- weight ratio of steam locomotive is low. It has strictly limited overload capacity. Steam locomotive cannot be put into service at any moment as time is required for raising of steam. Owing to high centre of gravity of steam locomotive, speed is limited. Steam locomotive requires more repair and maintenance. Extensive and costly auxiliary equipment. Since driving wheels are very close, hence more concentrated adhesive weight is required. Bigger sizes of running sheds and workshop are required.

Internal combustion engine drive:

This drive is widely used for road transport. The motive power is derived from petrol to diesel. It has an efficiency of about 25 percent when operating at normal speed. Various example are buses, cars, trucks etc.,

Advantages Low initial investment. It is self-contained unit and, therefore, it is not tied to any route.




Easy speed control.




Very simple braking system. It is cheap drive for the outer suburbs and country districts.

Disadvantages Limited overload capacity. A gear box is essential for speed control. Higher running and maintenance costs. Operation at any but the normal speed is uneconomical. The life of propulsive equipment is much shorter than that of electrical equipment of a tram car or a trolley bus.

Internal combustion electric drive: In an I.C engine electric drive the reduction gear and gear box

are eliminated as the diesel as the diesel engine is to drive the D.C. generator coupled to it at a constant speed. This type of drive is finding considerable favour for railway work and locomotives of this type are being widely used.

Advantages Low initial investment. No modification of existing tracks is required while converting from steam to

diesel electric traction. As the locomotive and train is a self contained unit, therefore, it is not lied to

any route. Can be put into service at any moment. Loss of power in speed control is very low. It is available for hauling for about 90% of its working days. Overall efficiency is greater than that of steam locomotives.

Disadvantages Limited overload capacity. High running and maintenance cost. Higher dead weight of locomotives; more axles required comparatively. Comparatively costlier than steam or electric locomotives. In such drives, regenerative braking cannot be used. The life of the diesel engine is comparatively shorter.

There is a necessity to provide special cooling system for the diesel engine in addition to motor-generator set.

Petrol-electric traction:

This system, due to electric conversion, provides a very fine and continuous control which makes the vehicle capable of moving slowly at an imperceptible speed and creeping up the steepest slope without throttling the engine.

Petrol-electric traction is employed in heavy Lorries and buses.

Battery electric drive: In this system the locomotive carries the secondary batteries which supply




power to D.C. motors employed for driving the vehicles. This type of drive is well




suited for frequently operated service such as for local delivery of goods in large towns with maximum daily run of 50 to 60 km, shunting and traction in industrial works and mines.

Battery vehicles are started by series-parallel for starting and running at the speed upto half maximum speed and in series for running at full maximum speed. Advantages

Battery driven vehicle is easy to control and very convenient to use. Low maintenance cost. Absence of fumes.

Disadvantages This type of drive is the small capacity of batteries and the necessity for frequent charging.

Limited speed range. Electric drive:

Here the drive is by means of electric motors which are fed from overhead distribution

system. The drive of this type is most widely used. Advantages

As it has no smoke, electric traction is most suited for the underground and tube railways. The motors used in electric traction have a very high starting torque. Hence, it is possible to achieve higher accelerations of 1.5 to 2.5 km/h/s as against 0.6 to 0.8 km/h/s in steam traction. An electric locomotive is ready to start at moment's notice against about two hours required for steam locomotive to heat up. The maintenance cost of an electric locomotive is 50 percent of that of steam locomotive; its maintenance time is also much less comparatively. By the use of electric traction high grade coal can be saved, since electric locomotives can be fed either from hydroelectric stations or thermal power station which use cheap low-grade coal. In electric traction system it is possible to use regenerative braking. Owing to complete absence of smoke and fumes, this system is healthier from the hygienic point of view. The vibrations in electrically operated vehicles are less as the torque exerted by the electric motor is continuous. Electric equipment can withstand large temporary overloads and can draw relatively large power from the distribution system.

Disadvantages High initial cost of laying out overhead electric supply system. Unless the traffic to be handled is heavy, electric traction becomes uneconomical. Power failure for a few minutes can cause traffic dislocation for hours. The electric traction system is tied up to only electrified routes.




Communication lines which usually run parallel to the power supply lines




suffer from electrical interference. Hence, these communication lines have either to be removed away from the rail track or else underground cables have to be used for the purpose which makes the system still more expensive. Additional equipment is required for regeneration. In case of D.C. series motors regeneration is not a simple process. In case of electric traction provision of a negative booster is essential. By avoiding the flow of return currents through earth, it curtails corrosion of underground pipe work and interference with telegraph and telephone circuits. Whereas steam locomotives can use their steam for heating the compartments in cold weather very cheaply, the electric locomotives have to do it at an extra cost. In cold countries a service locomotive is required to run up and down the line in order to prevent the formation of layer of ice on the conductor rails.

5. Draw the Speed time curve of traction and also explain the various periods and

the action [CO1 – L2 - Nov’13]. It is the curve drawn between speed of train in km/hour along y-axis and time

in seconds along x- axis. The speed time curve gives complete information of the motion of the train. This curve gives the speed at various times after the start and run directly.

The distance travelled by the train during a given interval of time can be obtained by determining the area between the curve and the time axis corresponding to this interval. A typical speed time curve for main line service is shown in fig. This curve consists of five sections. 1. Notching up period (0 to t1):

During this period of run (0 to t1), starting resistance is gradually cut so

that the motor current is limited to a certain value and the voltage across the motor is gradually increased and the traction motor accelerates from rest.

To cut the starting resistance, the starter handle has to be moved from one notch to another.

Hence this period is called notching up period.

The acceleration is almost uniform during this period. Therefore speed- time curve is a straight line (OA).




2. Acceleration period (t1 to t2)

When all the starting resistances are cut out, the full voltage is applied to the motor. Now the torque decreases and speed increases according to the speed torque characteristics of the motor.

Now the acceleration gradually decreases with the increase in speed and finally reaches the required torque for the movement of the train (at time t2).

3. Free running period (t2 to t3)

During this period i.e. t2 to t3 the power supplied to the motor is at full voltage

and speed of this period is constant, also during this period power drawn from the supply is constant.

4. Coasting period (t3 to t4)

At the end of free running period supply to the motor is cut off and the train is allowed to run under its own kinetic energy.

Due to train resistance speed of the train gradually decreases.

The rate of decreasing of speed during this period is known as "coasting retardation".

5. Braking or retardation period (t4 to t5)

At the end of coasting period the brakes are applied to bring the train to stop. During this period speed decreases rapidly and finally reduces to zero.

6. Draw the Speed time curve of a various types of tractionservices.[ CO1 - L2 ]

There are three types of electric traction services. 1. Main line service 2. Sub-urban service 3. Urban service

Speed - Time curve for main line service: The distance between two successive stations in main line service is

considerably more (more than 10 km). In this service free run is longer duration. The duration of acceleration and retardation is small.




Speed - Time curve for suburban service

In this type of service the distance between two successive stations is in the range of 1.5 km to 8 km. Fig represents speed-time curve for suburban service. Acceleration and braking retardation required are high. Free running period is not possible and coasting period will be comparatively longer than urban service.

Speed - Time curve for urban or city service

In city service the distance between the two stations is very short i.e., between 0.75 to 1 km. The time required for this run between the adjacent and retardation should be sufficient high. Fig shows the speed-time curve for urban or city service. It will be seen that there will be no free running period.

The coasting period is also small.

7. Explain the types of supply system in electric traction.[ CO1 – L2 – NOV14] DC System

In this system of traction, the electric motors employed for getting necessary propelling torque should be selected in such a way that they should be able to operate on DC supply. Examples for such vehicles operating based on DC system are tramways and trolley buses. Usually, DC series motors are preferred for tramways and trolley buses even though DC compound motors are available where regenerative braking is desired. The operating voltages of vehicles for DC track electrification system are 600, 750, 1,500, and 3,000 V. Direct current at 600–750 V




is universally employed for tramways in the urban areas and for many suburban and




main line railways, 1,500–3,000 V is used. 1-φ AC system

In this system of track electrification, usually AC series motors are used for getting the necessary propelling power. The distribution network employed for such traction systems is normally 15–25 kV at reduced frequency of 163⅔ Hz or 25 Hz. The main reason of operating at reduced frequencies is AC series motors that are more efficient and show better performance at low frequency. These high voltages are stepped down to suitable low voltage of 300–400 V by means of step-down transformer. Low frequency can be obtained from normal supply frequency with the help of frequency converter. Low-frequency operation of overhead transmission line reduces the line reactance

3-φ AC system

In this system of track electrification, 3-φ induction motors are employed for getting the necessary propelling power. The operating voltage of induction motors is normally 3,000–3,600-V AC at either normal supply frequency or 16⅔-Hz frequency.

Usually 3-φ induction motors are preferable because they have simple and robust construction, high operating efficiency, provision of regenerative braking without

placing any additional equipment, and better performance at both normal and seduced frequencies. In addition to the above advantages, the induction motors suffer from some drawbacks; they are low-starting torque, high-starting current, and

the absence of speed control. The main disadvantage of such track electrification system is high cost of overhead distribution structure. This distribution system

consists of two overhead wires and track rail for the third phase and receives power either directly from the generating station or through transformer substation.

Composite system

As the above track electrification system have their own merits and demerits, 1-φ AC system is preferable in the view of distribution cost and distribution voltage can be stepped up to high voltage with the use of transformers, which reduces the transmission losses. Whereas in DC system, DC series motors have most desirable features and for 3-φ system, 3-φ induction motor has the advantage of automatic regenerative braking. So, it is necessary to combine the advantages of the DC/AC and 3-φ/1-φ systems. The above cause leads to the evolution of composite system.

Composite systems are of two types. Single-phase to DC system. Single-phase to three-phase system or kando system.

Single-phase to DC system In this system, the advantages of both 1-φ and DC systems are combined to

get high voltage for distribution in order to reduce the losses that can be achieved with 1-φ distribution networks, and DC series motor is employed for producing the necessary propelling torque. Finally, 1-φ AC distribution network results minimum




cost with high transmission efficiency and DC series motor is ideally suited for




traction purpose. Normal operating voltage employed of distribution is 25 kV at normal frequency of 50 Hz. This track electrification is employed in India.

8. Explain how rheostatic braking is employed in traction motors.[CO1-L2]

In this method, the motor is disconnected from the supply and operated as a generator driver by the momentum, supplying current to the resistance across the motor, thus dissipating away the energy and coming to a quick stop.

In traction work, where two or more motors are employed, they are put in parallel across a resistance for braking, since series givers high voltage, during the braking period the motors are driven as generator owing to the kinetic energy of the train and electrical energy so generated is dissipated in the form of heat in the resistance connected across them. A equalizer connection is used in order to ensure that the two machines share the load equally. If not, the machine that would build up first sends large current through the other in the opposite direction causing reversed voltage.

Hence the second method is advantageous to the first as if the direction of the machine armature reverses, the machines will fail to excited with equalizer connection and no braking effect is produced. 3 – Phase induction motor: This method cannot be employed. AC Series motor:

It is obtained by operating the machines as generators excited from the supply or as self excited DC generators supplying to resistance load. The fields are excited at low voltage from a suitable tap of the main transformer. Fields may be excited from on off motors acting as series generator and de power generated from the other will dissipated through resistive loads.

9. Explain how regenerative braking is employed in traction machines.[CO1-L2]

In regenerative braking, the motor remain connected to the supply and return the braking energy to the supply. It is essential that traction motors must generate power at a voltage higher than the supply voltage and at a reasonable constant voltage. DC series Motor:

Dc series motor cannot be used for regenerative braking in an ordinary way. The reversal of armature current necessary to produce regeneration would cause reversal of field. At the instant of reversal, short circuit will occur.

To overcome these problems, some methods are used. One method of regenerative braking with series motor is the French method. The series motor for trolley bus is equipped with a main series field winding and auxiliary field windings connected in parallel with the main series field windings.

During braking period, the auxiliary field windings are put in series with each other and are switched across the supply. The machine acts as a compound generator with slight differential compounding.




An alternative method of regenerative braking is by using a separate exciter for




controlling the excitation of the field winding during regeneration. The exciter may either be axle driven by a motor operated from the auxiliary supply.

Induction Motor: Regenerative braking with three phase induction motor occurs automatically

when the motor runs at a speed slightly above the synchronous. It then works as an induction generator.

Induction generator is not self – exciting and must be connected to a system supplied from synchronous generator, it can be employ by any one of the following processes.

Switching over from high – low frequency supply in order to reduce the speed of the drive. Downward motion of a loaded hoisting mechanism. Switching over to a larger pole number of operation from a smaller one in multi- speed squirrel cage motors.

Single phase series motors: Braking with AC series motor is much more complicated as compared to DC

series motor as it is necessary to have a high power factor in order to obtain a braking torque. For regenerative braking, the regenerated power should have the same frequency as that of the main supply. Therefore, the fields of motor are energized separately from the AC mains with go out of phase with respect to the supply voltage with phase shifting device.

10. Explain the types of traction motors used in electric traction[CO1-L2-APR14]

In earlier days, DC motor is suited for traction because of the high-starting torque and having the capability of handling overloads. In addition to the above

characteristics, the speed control of the DC motor is very complicated through semiconductor switches. So that, the motor must be designed for high base speed initially by reducing the number of turns in the field winding. But this will decrease the torque developed per ampere at the time of staring. And regenerative braking is also

complicated in DC series motor; so that, the separately excited motors can be preferred over the series motor because their speed control is possible through

semi-controlled converters. DC series motor

From the construction and operating characteristics of the DC series motor, it is widely suitable for traction purpose. Following features of series motor make it suitable for traction.

DC series motor is having high-starting torque and having the capability of handling overloads that is essential for traction drives.

These motors are having simple and robust construction. The speed control of the series motor is easy by series parallel control. Sparkless commutation is possible, because the increase in armature

current increases the load torque and decreases the speed so that the emf induced in the coils undergoing commutation.

Series motor flux is proportional to armature current and torque. But




armature current is independent of voltage fluctuations. Hence, the motor is




unaffected by the variations in supply voltage. DC shunt motor

From the characteristics of DC shunt motor, it is not suitable for traction purpose, due to the following reasons:

DC shunt motor is a constant speed motor but for traction purpose, the speed of the motor should vary with service conditions.

In case of DC shunt motor, the power output is independent of speed and is proportional to torque. In case of DC series motor, the power output is proportional to So that, for a given load torque, the shunt motor has to draw more power from the supply than series motor.

For shunt motor, the torque developed is proportional to armature current (T ∝ Ia). So for a given load torque motor has to draw more current from the supply.

The flux developed by shunt motor is proportional to shunt field current and hence supply voltage. But the torque developed is proportional to φsh and Ia. Hence, the torque

developed by the shunt motor is affected by small variations in supply voltage. AC series motor

Practically, AC series motor is best suited for the traction purpose due to high-

starting torque When DC series motor is fed from AC supply, it works but not satisfactorily due to some of the following reasons:

If DC series motor is fed from AC supply, both the field and the armature currents reverse for every half cycle. Hence, unidirectional torque is developed at double frequency.

Alternating flux developed by the field winding causes excessive eddy current loss, which will cause the heating of the motor. Hence, the operating efficiency of the motor will decrease.

Field winding inductance will result abnormal voltage drop and low power factor that leads to the poor performance of the motor.

Induced emf and currents flowing through the armature coils undergoing commutation will cause sparking at the brushes and commutator segments.

Three-phase induction motor




The three-phase induction motors are generally preferred for traction purpose




due to the following advantages. Simple and robust construction. Trouble-free operation. The absence of commutator. Less maintenance. Simple and automatic regeneration. High efficiency. Three-phase induction motors also suffer from the following drawbacks. Low-starting torque. High-starting current and complicated speed control system. It is difficult to employ three-phase induction motor for a multiple-unit

system used for propelling a heavy train. Three-phase induction motor draws less current when the motor is started at

low frequencies. When a three-phase induction motor is used, the cost of overhead distribution system increases and it consists of two overhead conductors and track rail for the third phase to feed power to locomotive, which is a complicated overhead structure and if any person comes in contact with the third rail, it may cause danger to him or her.

Linear induction motor It is a special type of induction motor that gives linear motion instead of

rotational motion, as in the case of a conventional motor. In case of linear induction motor, both the movement of field and the

movement of the conductors are linear. A linear induction motor consists of 3-φ distributed field winding placed in

slots, and secondary is nothing but a conducting plate made up of either copper or aluminum

The field system may be either single primary or double primary system. In single primary system, a ferro magnetic plate is placed on the other side of the copper plate; it is necessary to provide low reluctance path for the magnetic flux. When primary is excited by 3-φ AC supply, according to mutual induction, the induced currents are flowing through secondary and ferro magnetic plate.

Now, the ferro magnetic plate energized and attracted toward the primary causes to unequal air gap between primary and secondary. This drawback can be overcome by double primary system. In this system, two primaries are placed on both the sides of secondary, which will be shorter in length compared to the other depending upon the use of the motor.




Synchronous Motor The synchronous motor is one type of AC motor working based upon the

principle of magnetic lacking. It is a constant speed motor running from no- load to full load. The construction of the synchronous motor is similar to the AC generator; armature winding is excited by giving three- phase AC supply and field winding is excited by giving DC supply. The synchronous motor can be operated at leading and lagging power factors by varying field excitation.

The synchronous motor can be widely used various applications because of constant speed from no-load to full load.

High efficiency. Low-initial cost. Power factor improvement of three-phase AC industrial circuits.

11. Explain the effective efforts required to run a train on track? [CO1– L2 ] Tractive Effort (Ft)

It is the effective force acting on the wheel of locomotive, necessary to propel the train is known as „tractive effort‟. It is denoted with the symbol Ft. The tractive

effort is a vector quantity always acting tangential to the wheel of a locomotive. It is measured in newton.

The net effective force or the total tractive effort (Ft) on the wheel of a locomotive

or a train to run on the track is equals to the sum of tractive effort:

1. Required for linear and angular acceleration (Fa). 2. To overcome the effect of gravity (Fg). 3. To overcome the frictional resistance to the motion of the train (Fr).

Tractive effort

The effective efforts required to run a train on track is i) Tractive effort needed to provide acceleration (Fa)

ii) Tractive effort needed to overcome the train resistance (Fr)

iii) Tractive effort needed to overcome gradients (Fg)




1. Tractive effort for acceleration (Fa):

Let M is the dead or stationary mass of train in tones. Dead mass of train = M tones

= 1000 M kg Acceleration= km/hr/sec

= x 1000/3600 m/sec2 When a train is accelerated in a linear direction, its rotating parts like the

wheels and armature of motors have to be accelerated in an angular direction. Therefore the accelerating mass of the train is greater than the dead mass of the train.

Generally the effective or accelerating mass is 10% more than the dead mass. i.e. Me = 1.1 M Let the effective

mass of train = Me ton

= 1000 Me kg.

Force required for acceleration = Mass x acceleration. i.e., Fa = Me x a

= 1000 Me x 1000/3600

= 277.8 Me Newtons

2. Tractive effort to overcome the train resistance (Fr):

While moving, the train has to overcome the opposing force due to the surface friction and wind resistance. The train resistance depends upon various factors such as shape, size, condition of track etc. Tractive effort required to overcome the train resistance

Fr = M x r Newtons

Where M = Mass of train in tone r = train resistance in Newtons/tone

3. Tractive effort required to overcome gradients (Fg):

Consider that an electric train is moving upwards on a slope as shown in fig. The dead mass of the train along the slope will tend to bring it downward. To overcome this effect of gravity, tractive efforts are required in opposite direction.




12. Explain the types of Braking with neat diagram [ CO1 – L2 – APR 2015 ] Braking is very frequent in electric drives to stop a motor in a reasonably

short time. For example a plannar must quickly be stopped at the end of its stroke and sometimes must quickly be stopped at the end of its stroke and sometimes it is necessary to stop the motor in order to prevent accident.

The essential of a good braking system should be 1. Reliable and quick in its action.

2. The braking force must be capable of being controlled. 3. Adequate means be provided for dissipating the stored energy that

is kinetic energy of the rotating parts. 4. In case of a fault in any part of the braking system the whole system

must come to instantaneous rest or result in the application of the brakes. There are two types of braking: i) Mechanical braking

The motor in this case is stopped due to friction between the moving part of the motor and the brake shoe that is stored energy is dissipated as heat by a brake shoe or brake lining which rubs against a brake shoe or brake lining which




rubs against a brake drum.




Mechanical brakes are of two types namely i. Compressed air brakes ii. Vacuum brakes

Compressed Air Brakes:

It consists of a reservoir of compressed air, a brake cylinder, a value, pipe, spring piston and piston rod.

The brakes are kept in OFF position by springs in the brake cylinder. When

brakes are to be applied, compressed air is admitted into the cylinder. It presses the piston against the force of the spring. To release the brakes, compressed air is

exhausted for the cylinder of about pressure. Vacuum brakes

It is made up of a vertical cylinder having a piston and a piston rod which operates the braking arrangement through a system of levers. Vacuum is created on the top and the underside of the piston so that in normal condition, the piston rests at the bottom of the cylinder. When brakes are to be applied, the vacuum is broken from the underside by admitting air at atmospheric pressure. The piston moves up and applies the brakes and released either by recreating the vacuum or by making the pressure equal on both sides of the piston.

ii) Electric braking In this method of braking, the kinetic energy of the moving parts that is

motor is converted into electrical energy which is consumed in a resistance as heat or alternatively it is returned to the supply source. Electric braking is superior to the friction braking as it is fast and cheap since there is no cost of maintenance of the brake shoes or lining.

During braking operation a motor has to function as a generator. The motor can be held at stand still. In other words the electric braking cannot hold the motor at rest. Thus it becomes essential to provide mechanical brakes in addition to electric braking.

Various types of electrical braking are: a) Plugging




b) Rheostatic braking c) Regenerative braking




Plugging This is a simple method of electric braking and consists in reversing the

connections of the armature of the motor so as to reverse its direction of rotation which will oppose the original direction of rotation of the motor and will bring it to zero speed when mechanical brakes can be applied.

At the end of the braking period the supply to the motor is automatically cut off. This method of braking can be applied to the following motors.

1) DC motors 2) Induction motors 3) Synchronous motors

DC motors: To reverse a DC motors, it is necessary to reverse the connections of the

armature while the connections of the field are kept the same. The direction of m.m.f remains the same even during braking periods. Series motors:

The arrangements of connection before and after the braking are shown in fig. Shunt motors:

The arrangements of connections before and after braking for shunt motor are shown in fig. Total voltage of V+ Eb is available across the armature terminals which

causes a current I to flow around the circuit. When Eb = V then the voltage across the armature is 2V and at the time of

braking twice the normal voltage is applied to the resistance in series with the armature at this time in order to limit the current.

While the motor is being braked, the current is still being drawn from the supply. This method requires energy from the supply for its action and not only the kinetic energy of the motor is being wasted, but this energy is also being dissipated. Speed and braking torque

Electric braking to torque TB ФI ------------------------------ (1)

TB = K ФI ---------------------------- (2)

Where K is a constant

Current = V+ Eb / R ------------------------ (3)

Eb = K1N Ф --------------------- (4)

Where V is applied voltage Eb is back emf of the motor.

R is the resistance of the motor N is the speed

K1 is a constant

Substitute the value of Eb from

equation (4) in (3)




Current I = (V + K1N Ф)/ R -------------------- (5)

In view of equations (2) and (5) TB = K Ф[(V + K1N Ф)/ R] = K ФV/ R + KK1N Ф2/ R

= K2 Ф + K3 Ф2N ------------- (6)

Where K2 = KV/ R -------------------- (7)

And K3 = KK1/R --------------------- (8)

Apply the results obtained to the series motor, where armature current (Ia) ----------

--------- (9) Then electric braking in series motor, = K4 Ia + K5 Ia2 -------------------- (10)

In the case of shunt motor since flux is constant, so Electric braking torque TB =K6 + K7N ------------------- (11)

Wherever there is a load on the machine the load will also exert braking torque due to it and then the total braking torque (T)

T = Electric braking torque + Load torque ------------- (12) Induction motors

In the case of induction motor its speed can be reversed by inter changing any of the two stator phases which reverses the direction of rotation of motor field.

Actually at the time of braking when the induction motor is running at near synchronous speed.

The point Q represents the torque at the instant of plugging one can notice that the torque increases gradually as one approaches the stand still speed.

Different values of rotor resistance give rise to different shapes of speed torque curve in order to give any desired braking effect.

The rotor current I2 can be calculated during the braking period from the

following relation and is plotted as shown. I2 = SE2 / [Re2 + (SX2)2] ------------------- (13)

Where E2 is the e.m.f. induced in rotor at

standstill R2 is the rotor resistance

X2 is the standstill reactance of the rotor and

S2 is the percentage slip

Synchronous motors Plugging can be applied to the synchronous motors, with the only difference

that the field on the rotor will be rotating in opposite direction to that of the rotating field on the stator with the synchronous speed and the relative velocity between the two will be twice the synchronous speed.

This will meant that there is one synchronous motor torque but the same will be produced by the induction in the starting winding. Since most of the motors are

Ф




equipped with starting winding, a synchronous motor provides satisfactory braking.

Rheostatic braking In this method of braking, the motor is disconnected from the supply and run

as generator driven by the remaining kinetic energy of the equipment that is the energy stored in motor and load which are to be braked.

The following drives can be braked by the rheostatic method: i. DC motor

ii. Induction motor iii. Synchronous motor.

Dc motors: Shunt motor

In this type of motor, the armature is simply disconnected from the supply and is connected to as resistance in series with it, the field; winding remains connect to the supply as shown in fig. The braking can be adjusted suitably by varying the resistance in the armature circuit. In the case of failure of the supply, there is no braking torque because of absence of the field.

Series motor In this case of the connections are made as shown is fig during braking

operation. The motor after disconnection from the supply in made to run as a DC series generator. Resistance inserted in the circuit must be less than the critical resistance otherwise the generator will not be self exciting.

When the series motor is disconnected from the supply the direction of the armature current is reversed. Braking torque and speed

Electric braking torque is given by equation (3) Braking current = Eb / R ---------------------------------- (14)

Hence braking current of equation (14) and (4) = K1ФN/R ------------------------------------ (15)

Substitute the value of braking current is equation (1) Electric braking torque = KK1Ф2N/R = K2 Ф2N ------------------------- (16)

Where K2 = KK1/R -------------------------------- (17)

In the case of a series motor the flux dependent upon the armature current

Series motor

= K3Ia2N ------------------------------- (18)

While in the case of shunt motor since flux is constant Electric braking torque = K4N ------------------------------- (19)

Induction motor In this case the stator is disconnected from the supply and is connected to DC




supply which excites the windings thereby producing a DC field.




The rotor is short-circuited across through resistance in each phase. When the short circuited rotor moves it outs the steady flux produced in the air gap due to DC current flowing in the stator produced in the air gap due to DC current flowing in the stator and an e.m.f is induced in the rotor conductors.

The satisfactory application of this method is applicable only to the phase wound inductor motor where external resistance can be inserted in each phase. Synchronous motors

Rheostatic braking in the synchronous motors is similar to the rheostatic braking in induction motors. In this case the stator is shorted across resistance in star or delta and the machine works like an alternator supplying the current to the resistance, there by dissipating in kinetic energy in the form of losses in the resistances.

Regenerative braking

In this type of braking the motor is not disconnected from the supply but remains connected to it and its feeds back the braking energy or its kinetic energy to the supply system. This method is better than the first and second methods of braking since no energy is wasted and rather it is supplied back to the system. This method is applicable to following motors:

D.C motors Induction motors

D.C motors: Shunt motor In a DC machine where energy will be taken from the supply or delivered to it

depends upon the induced emf, if it in less than the line voltage the machine will operate as motor and if it is more than the line voltage, the machine will operate as generator.

The e.m.f induced in turn depends upon the speed and excitation that is when the field current or the speed is increased the induced e.m.f exceeds the line voltage and the energy will be field into the system. This will quickly decrease the speed of the motor and will bring it to rest. Series motor

In this case, complications arise due to fact that the reversal of the current in the armature would cause a reversal of polarity of the series field and hence back emf would be reversed. Induction motor

In the case of induction motors, the regenerative braking is inherent, since an induction motor act as a generator when running at speeds above synchronous speeds and it feeds power back to the supply system.

No extra auxiliaries are needed for this purpose. This method is however very seldom used for braking but its application is very useful to lifts and hoists for holding a descending load at a speed only slightly above the synchronous speed.




13. Write in detail the recent trends available in electric traction system. [CO1 – L2 - APRIL/MAY 2008]

Three phase AC traction drives The advantages of AC propulsion drive are good reliability due to static

power conversion equipment. The important component of the AC traction is the three phase squirrel cage induction motor. A popular car pair used in modern electric traction uses the pulse width modulation inverter principle. The PWM inverter produces a symmetrical three-phase output voltage, whose amplitude and frequency can be controlled continuously.

Hence the speed and torque of the squirrel cage induction motor used for traction can be adjusted in monitoring and braking, as well as in both directions, of rotation with a fully static device, that is no operational contacts are required.

The PWMAC drive covers subway railcars, LRVS, trolley buses, diesel electric and electric locomotives. Class EA locomotive

The class EA locomotive is a multipurpose locomotive used for fast inter- city trains. Fig shows the main circuit of a class EA locomotive. The four quadrant controllers rectify the AC voltage from the transformer to 2800V DC. The pulsating inverters invert the three phase voltage.

The three phase voltage now fed into traction motors has a variable voltage and frequency. The system can be used for regenerative braking. Three auxiliary converter feed the three-phase fan motors for oil cooling and traction motor cooling, for rectifiers as well as the lubricating oil pump for cooling compressor and pneumatic compressor. Fig shows the block diagram of a locomotive. The locomotive has a transformer suspended under the locomotive body, giving ample space for switch cabinets and equipment in the locomotive. There is a powerful electric brake on the class EA locomotive, which is also of infinitely variable regulation.

Locomotive electrical equipment

A typical scheme of locomotive used consists of eight diodes and twenty thyristors. The current flows from one to another without any stoppage. Here a natural physical phenomenon is used, the motor itself piloting the converter that supplies it. Hence there is no need to fear unbalance in the system.

In earlier days, DC motor is suited for traction because of the high-starting torque and having the capability of handling overloads. In addition to the above characteristics, the speed control of the DC motor is very complicated through semiconductor switches. So that, the motor must be designed for high base speed initially by reducing the number of turns in the field winding. But this will decrease the torque developed per ampere at the time of staring. And




regenerative braking is also complicated in DC series motor; so that, the




separately excited motors can be preferred over the series motor because their speed control is possible through semi-controlled converters.

14. Briefly explain how the current collection system works in the traction motor. [ CO1 – L2 ]

There are mainly two systems for locomotives, tramways or trolley buses. a. Conductor rail system. b. Overhead system.

Conductor Rail System: It is employed at 600V for suburban services since it is cheaper, inspection and

maintenance easier. The current is supplied to the electrically operated vehicle. The insulated return rail is elimination to electrolytic action due to currents on other public services buried in the cicinity of railway tunnels. A special steel alloy (iron 99.63%, carbon 0.05%, manganese 0.2%, phosphorus 0.05%, silicon 0.02% and sulphur

%) is used. It has a resistance of about . The conductor rail is not fixed rigidly to the insulators in order to take care of the contraction and expansion of rails. The current can be collected at about 300 to 500A. At least two shoes must be provided on each side to avoid discontinuity in the current flow and for voltage 1200V. Overhead System:

This system is adopted when the trains are to be supplied at high voltage of 1500V or above. This system is used for ac railways and also used with dc tramways, trolley buses and locomotives operating at voltages 1500V and above with return conductor. Three types of current collectors are commonly used.

a) Trolley Collector: It is employed with tramways and trolley buses. It consists of a grooved gun

metal wheel or grooved slider shoe with carbon insert carried at the end of a long pole. The other end of this pole is hinged to a swiveling base fixed to the roof of the vehicle. The necessary upward pressure for the pole and current collector is achieved by springs. As two trolley wires are required for a trolley bus, a separate trolley collector is provided for each wire.

b) Bow Collector: The low collector consists of light metal strip or low 0.6 or 0.9 m wide pressing against the trolley wire and attached to a framework mounted on the roof of vehicle. Collection strip is made of soft material such as copper, aluminum or carbon that it should wear instead of trolley wire as it is easy to replace worn out collection strip than trolley wire.




c) Pantograph Collector:

The pantograph is employed in railways for collection of current where the operating speed is as high as 100 or 130 kmph and current to be collected are as large as 2000 or 3000A. pantograph are mounted on the roof of the vehicles and usually carry a sliding shoe for contact with the overhead trolley wire. The contact shoes are usually about 1.2m long. There may be a single shoe or two shoes on each pantograph. Materials used for pantograph is oftern steel with wearing plates of copper or bronze inserted. The pressure varies from 5 to 15kg.

15. A train runs with an average speed of 50kmph. Distance between stations

is 2.5 km. Values of acceleration and retardations are 1.8 and 2.4kmphps respectively. Calculate the maximum speed of the train assuming trapezoidal speed - time curve. [CO1- H2- Nov2013,NOV 14]




16. A surban electric train has a maximum speed of 65 km/hr. The scheduled including a station stop of 30 sec is 43.5 km/hr of the acceleration is 1.3 kmphps, find the value of retardation. When the averages distance between stops is 3km. [CO1-H2- May / June 2013 , APR15]




Unit – II

Illlumination

Part – A

1.) Define Luminous Flux [ CO1 – L1 ] It is defined as the total quantity of light energy emitted per second from a

luminous body. It is represented by symbol F and is measured in lumens. The conception of luminous flux helps us to specify the output and efficiency of a given light source.

2) What is meant by candle power? [ CO1 – L1 ]

It is defined as the number of lumens given out by the source in a unit solid angle in a given direction n. It is denoted by CP.

3) Define MHCP. [CO1 – L1- May/June 2012] The mean of candle power in all directions in the horizontal plane containing

the source of light is termed as Mean Horizontal Candle Power.

4) Define utilization factor. [ CO1 – L1 ] It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp.

5) What is meant by luminance? [ CO1 – L1 ] It is defined as the luminous intensity per unit projected area of either a surface source of light or a reflecting surface and is denoted by L.

6) What are the laws of illumination? [ CO1 – L1 - April/May ‘08’10] Law of Inverse Squares:

Illumination at appoint is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction.

If a source of light emits light equally in all directions be placed at the centre of a hollow sphere, the light will fall uniformly on the inner surface of the sphere. If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius. Lambert’s cosine law:




The illumination at a point on a surface is proportional to cosine of the angle




which ray makes with the normal to the surface at that point.

7) Define space-height ratio. [ CO1 – L1 ] It is defined as the ratio of horizontal distance between adjacent lamps and

height of their mountings.

8) What is polar curve? [ CO1 – L1 ] In most lamps or sources of light the luminous intensity is not the same in all

directions. If the luminous intensity, i.e. the candle power is measured in a horizontal plane about a vertical axis and a curve is plotted between candle power and the angular position, a curve is obtained is called as horizontal polar curve.

The luminous intensity in all the directions can be represented by polar curves. If the luminous intensity in a vertical plane is plotted against the angular position, a curve known as vertical polar Curve is obtained.

9) Name the various photometer heads. [ CO1 – L1 ] 1. Bunsen Head (or) Grease spot photometer 2. Lummer-Brodhun photometer

head There are two types of Lummer Brodhun heads

a) Equality of Brightness type photometer head b) Contrast type photometer head

10) What are all the sources of light? [ CO1 – L1 – NOV 2014, APR 2014 ] According to principle of operation the light sources may be grouped as follows.

1. Arc lamps 2. High temperature lamps 3. Gaseous discharge lamps 4. Fluorescent type lamps

11) What is stroboscopic effect of fluorescent tubes? [ CO1 – L1 ]

With A.C. supply frequency of 50 cycles per second, discharge through the lamp becomes zero, 100 times in a second. Due to the persistence of vision, our eyes do not notice this. If this light falls on moving parts, they may appear to be either running slow or in the reverse direction or even may appear stationary. This effect is called stroboscopic effect.

12) Define beam factor. [ CO1 – L1 ] The ratio of lumens in the beam of a projector to the lumens given out by lamps is




called the beam factor. This factor takes into account the absorption of light by reflector and front glass of the projector lamp. Its values vary from 0.3 to 0.6.




13) Mention the types of lighting schemes.[ CO1 – L1 - May/June 2007,13] The distribution of the light emitted by lamps is usually controlled to some extent by means of reflectors and translucent diffusing screens or even lenses. The interior lighting schemes may be classified as

1. Direct lighting 2. Semi-direct lighting 3. Indirect lighting 4. Semi-indirect lighting 5. General lighting

14) What are the drawbacks of discharge lamps? [ CO1 – L1 ] Drawbacks of discharge lamps:

1. Take time to attain full brightness. 2. High initial cost and poor power factor. 3. Starting requires trigger-starter. 4. Light output fluctuates at twice the supply frequency. 5. The flicker causes stroboscopic effect. 6. These lamps can be used only in particular position.

15) What are the requirements of good lighting system? [CO1-L1-May 2012] The following factors are required to be considered while designing the lighting scheme.

1. Illumination level 7. Spacing of luminaries 2. Uniformity of illumination 8. Colour of surrounding walls. 3. Colour of light 4. Shadows 5. Glare 6. Mounting height

16) Specify any four energy efficient lamps. [CO1 – L1 - Nov/Dec 13]

1. Light emitting diode lamp 2. Light emitting electrochemical cell 3. Electromagnetic induction bulbs 4. Fluorescent lamps

17) Why tungsten is selected as the filament material? [CO1 – L1- Nov/Dec 13]

The efficiency, when worked at in an evacuated bulb is 18 lumens per watt. Tungsten metal is most widely used for the purpose.

18) Define luminous efficacy. [CO1 – L1 - Nov/Dec 12] The amount of lumens output radiated from a lamp for per watts of power consumption also taking care of losses in ballast/control gears is termed as




luminous efficacy




19) What is the importance of street lighting system? [CO1 – L1 - Nov/Dec 12] a. To make the traffic and obstruction on the road clearly visible in order to

promote safety and convenience. b. To enhance the community value of the street. c. To make the street more attractive.

20) Define the term MSCP and lamp efficiency [C03 – L1- May/June 12] MSCP The mean of candle power in all direction and in all planes from the

source of light is termed as mean spherical candle power Lamp efficiency: It is defined as the ratio of the luminous flux to the power input.

21) What are the requirements of good lightning? [CO1 – L1 - May/June 12]

The requirements of lightning are 1. Sufficiency 2. Distribution 3. Absence of glare 4. Absence of sharp shadows 5. Steadiness 6. Colour of light 7. Surrounding, 8. Angle of light

22. Define lumen. [ CO1 – L1 – NOV 14, APR 14 ] One lumen is defined as the luminous flux emitted by a source of one

candle power in a solid angle.

Lumen = candle power of source * solid angle




100

300

200

PART B

1. Explain the various factors to be considered, while designing a lighting system [CO1 – L2 - Nov’12, Nov’13, May’13] Factors Affecting the Design of Lighting System: The following factors are required to be considered while designing the lighting scheme.

1. Illumination level 2. Uniformly of illumination 3. Colour of light 4. Shadows 5. Glare 6. Mounting height 7. Spacing of luminaries 8. Colour of surrounding walls

1. Illumination level For each type of work there is a range of brightness most favourable to output i.e., which causes minimum fatigue and gives maximum output in terms of quality and quantity. Degree of illumination, to give necessary brightness to the objects depend upon

i. The size of the object to be seen and its distance from the observer. ii. Contrast between the object and background.

The moving objects require more illumination than those for stationary objects. The illumination level required in various parts of a building is given in the table as per ISI. Table of Building Illumination Level:

100

Stairs

300 Study room

100 Bathroom

200 Laundry

Kitchen

100 Recreation room

200 Dressing table

Bed room

150 Dining room

300 Living room

Entrance

Illumination Level Location




The illumination level required as per ISI, for several of traffic routes is given in the table. Traffic Route Illumination Level

Classification Type of Road Average illumination level (Lux)

Group Important traffic routes carrying fast traffic

30

Group Other main roads arriving mixed traffic like main city street, arterial roads, through ways, etc.

15

Group Secondary roads with considerable traffic like principal local traffic routes, shopping streets, etc.

8

Group Secondary roads with light traffic 4

2. Uniformity of illumination The human eye adjusts itself automatically to the brightness within the field of vision. If there is a lack of uniformity, pupil or iris of the eye has to adjust more frequently and thus fatigue is caused to the eye. Therefore uniformity of illumination is necessary.

3. Colour of light The appearance of the body colour entirely depends upon the colour of the incident light. The composition of the light should be such that the colour appears natural i.e., the appearance by artificial light is not appreciably different form that by day light. Now –a – days day – light fluorescent tubes make it possible to illuminate economically even large spaces with artificial day light giving good colour rendering and at sufficiently high level.

4. Shadows In lighting installations, formation of long and hard shadows causes fatigue of eyes. Hard and long shadows can be avoided by using large number of small luminaries mounted at height not less than 2.5m and by using wide surface sources of light using globes over filament lamps or by using indirect lighting system.

5. Glare: i. Direct glare may come directly from the light source. ii. Reflected glare is glare which comes to the eyes as glint or reflection of the

light source in some polished surface. Toleration of bright light sources in the intermediate vicinity is made possible by locating them at such a height as to place them above the ordinary range of vision. Metal reflectors for industrial lighting are ordinarily provided with a skirt around the rim of the reflector.




6. Mounting height:




The mounting height will largely be governed by the type of the building and type of lighting scheme employed. In case of direct lighting, in rooms of large floor area, the luminaries should be mounted as close to the ceiling as possible. In the case of indirect and semi-indirect lighting, it would of course be desirable to suspend the luminaries for enough down from the ceiling in order to give reasonably uniform illumination on the ceiling.

7. Spacing of luminaries: Correct spacing is of great importance to provide uniform illumination over the whole area and thus do away with comparatively dark areas which are so often found when the luminaries are badly spaced.

8. Colour of surrounding walls: The illumination in the room depends upon the light reflected from the walls and ceilings, While walls and ceiling reflect more light as compared to colored ones.

2. A drawing hall 30 X X is to be provided with a general illumination

of 120 flux. Taking co-efficient of utilization as 0.5, depreciation factor as 1.4, determine the number of fluorescent tubes required, their spacing height, mounting height and total wattage. Take luminous efficiency of fluorescent tubes as lumen/watt for 80 watts tube. [CO1 – H2 - May’ 12] Given data:

Number of fluorescent tubes required =

Number of fluorescent tubes required Considering the use of 50 tubes in 5 rows, each row having 10 tubes giving spacing




of 3m along the length as well as width of the hall.





3. With neat diagram explain the construction and working of sodium lamp. [CO1 – L2 - May’ 12]

Sodium vapour discharge lamp consists of a bulb containing a small amount of metallic sodium, neon gas and two sets of electrodes connected to a pin type base. The presence of neon gas is to start the discharge and to develop enough heat to vaporize the sodium. The ‘u’ shape arrangement is used for discharge.

The sodium – vapor lamp is suitable only for AC supply and therefore requires choke control. This requirement is met by operating the lamp from a stray field, step – up, tapped auto transformer with an open – circuit secondary voltage of 470 to 480 volts. The uncorrected power factor is 0.3, capacitor improve the power factor to about 0.8.

When the lamp is not in operation, the sodium is in the form of solid deposited on

the side walls of the tube; hence at first when it is connected across the supply mains the discharge takes place in the neon gas and gives red – orange glow.

The metallic sodium gradually vaporizes and then ionizes producing yellow light to make the objects appears as grey for rated light within 15 minutes with efficiency of 40 – 50 lumens/watt.

4. Discuss the energy saving opportunities in lighting systems. [CO1 – L2 - Nov’13)

Types of lighting The type of lighting system is as follows:

Direct lighting Semi direct Indirect Semi indirect

Direct lighting In the direct lighting system the luminaries direct the 90 to 100% of the light

output of the lamp towards downward. The distribution may differ from low to high concentration, depending on the reflector material, finish and contour. The lamp surface is visible.





Reflected glare and shadow may occur with direct lighting. Also the illumination is not uniform. Semi direct lighting

In semi direct lighting the luminaries direct the light output of the lamp predominantly (60 to 100%) downward, but with a small upward component illuminate the ceiling and upper walls. The l amp will be used along with louvers. Louvers distribute light. The characteristics are essentially similar to the direct lighting except that the upward component will tend to soften shadows and room brightness. Utilization of lamp output can be equal to that of well shielded direct lighting system. Indirect lighting

In indirect lighting system, all the light output from the lamp is directed upward to the ceiling and upper side of the walls and reflected back to the working plane (Area). The entire ceiling becomes the primary source of illumination. The indirect lighting is uniform and glare free. This type of lighting is highly expensive and has zero eyestrain. Tis is the ideal lighting system for computer centre and software industry, where eyestrain is more. Semi indirect lighting system

In semi indirect lighting the luminaries direct the light output of the l amp partially (10 to 30%) downward, but with a major portion of light output (70 to 90%) upward component illuminate the ceiling and upper walls.

The characteristics are essentially similar to the indirect lighting except that the downward component will tend to brighten the spot and room brightness. Utilization of lamp output can be better than the indirect lighting system.

Design of Lighting System: The lighting system should be such that it may.

i. Provide adequate illumination ii. Provide light of suitable colour iii. Provide light distribution all over the working plane as uniform as possible. iv. Avoid glare and hard shadows as far as possible.

The following steps are involved in design of lighting system:





Calculate area to be illuminated Decide the level of illumination Total illumination = Area x Illumination Level Select utilization factor and depreciation factor Divide total illumination by utilization factor and depreciation factor If depreciation factor is greater that 1, then to find gross Lumen, it is multiplied with total lumen instead of division. Select lamp and luminaries Calculate number of lamps Decide arrangements of lamps for uniform distribution considering space – to – height ratio.

5. Two lamp posts are 14m apart and are fitted with 200 C.P lamps each at a height of 5m above the ground. Calculate,

i. Illumination mid – way between them ii. Illumination under each lamp [CO1 – H2 - May’12, NOV 14]

Given data: Candle power of each lamp = 200 CP Height of each lamp from the ground = 5m Distance between the two lamps = 14m Illumination midway between the lamps EC

= Illumination due to lamp + illumination due to lamp

6. A hall 30m long and 12m wide is to illuminated and the illumination required is 5o lumens/metre2Calculate the number of fitting required, taking depreciation factors of 1.3 and utilization factor of 0.5. Given that the outputs of different types of lamp are given below. [CO1 – H2 - May’13, NOV 2014]

Watts 100 200 300 500 1000

Lumens 1615 3650 4700 9950 21500

Given data: Area Illumination = Depreciation factors = 1.3 Utilization factor = 0.5 Number of fitting required =





------------------------------------------

Depreciation factor x Illumination factor

-------------

1.5 x 0.5

7. Explain the operation of fluorescent lamp in details. [CO1 – L2 - May’13] The fluorescent lamp consists of a glass tube of length varying 2 to 4 feet and is filled with a low pressure argon gas and a drop of mercury. The lamp is connected with a choke and starter. Choke:

A choke is connected in series with fluorescent tube and used to provide a voltage impulse so as to initiate the electron movement. It is an iron cored coil having high inductance. Starters:

The lamp circuit is made such that the starter should conduct when the circuit is ON. Commonly used starter circuit for fluorescent tubes are

Thermal type starter Glow type starter Instant starters

Working: Initially the starter is in closed position. When supply is switched ON the current

heats the filaments and initiates emission of electrons.





After one or two seconds the starter switch gets opened, making the choke to induce a momentary high voltage surge across the two filaments. Due to this, ionization takes place through argon gas.

The mercury vapour are provides a conducting path between the electrodes. The starter used may be of thermal or glow type whose function is to complete the circuit initially for preheating the filaments and then to open the circuit for inducing voltage across choke for initiating ionization.

8. A lamp of uniform intensity of 200 C.P. is enclosed in dins glass globe. 25% of the light emitted by lamp is absorbed by the globe. Determine

1. Brighten of globe

2. CP of globe if diameter of globe is 30 cm. [CO1 – H2 - May’13]

Given data: Candle power = 200 Light absorbed =25% =0.25 Diameter = 30cm

Flux emitted by the globe = candle power

Flux absorbed by the globe = Brighten – flux emitted by the globe

0.25 = Brightness – 565.48 → Brightness = 565.73 lumens Flux absorbed by the globe = Brighten – flux emitted by the globe

0.25 = Brightness – 565.48 → Brightness = 565.73 lumens





9. A lamp of 500 CP is placed 2m below a plane mirror which reflects 80% of light falling on it. Determine illumination at a point 5m away from the foot of the lamp which is hung 5m above the ground. [CO1 – H2 - Nov’12] The lamp L produces an image as far behind the mirror as it is in front.

Height of the image from the ground = 5+2+2 = 9m

Illumination at point B (5m away from the point vertically below the lamp)

10. Describe the construction and principle of operation of mercury vapour lamp. [CO1 – L2 - Nov’12, APR 2014]

Mercury vapour lamp is an electric discharge lamp, in which light is produced by gaseous conduction. The two main electrodes are placed inside a glass tube filled with Argon gas and small quantity of mercury.

Construction:

It consists of two bulbs an arc tube containing the electric discharge and outer bulb which protects the arc – tube from changes in temperature. The inner tube or arc – tube is made of quartz (hard glass) and the outer bulb of hard glass.

The arc – tube contains a small amount of mercury and argon gas. In addition to two main electrodes, an auxiliary starting electrode connected through a high resistance (50kΩ) is also provided. The main electrodes consist of tungsten coils with electron – emitting coating or elements of thorium metal.

Working:

When the supply is switched ON, initial discharge for the few seconds is established in the argon gas between the auxiliary starting electrode and the neighbouring main





electrodes and then in argon between the two main electrodes.

Mercury vapour lamp

The heat produced due to this discharge through the gas is sufficient to vaporise mercury. Consequently, pressure inside the arc tube increases to about one to two atmospheres and potential difference across the main electrodes grows from about 20V to 150V, the operation taking about 5 to 7 minutes. During this time, discharge is established through the mercury vapours which emit greenish – blue light.

The efficiency of this type of lamp is 30 – 40 lumens/ watt. These lamps are manufactured in 250W and 400W ratings for use on 200 – 250V AC supply mains.

Application:

Used for general industrial lighting, railway yards, ports, work areas, shopping

centres etc. where greenish – blue colour light is not objectionable

11. State and explain the Laws of Illumination. [ CO1 – L1 – APR 2014, APR 2015] Mainly there are two laws of illumination.

1. Inverse square law. 2. Lambert's cosine law.

Inverse square law:

This law states that ‘the illumination of a surface is inversely proportional to the square of distance between the surface and a point source’.

Proof:





Let, ‘S’ be a point source of luminous intensity ‘I’ candela, the luminous flux emitting





from source crossing the three parallel plates having areas A1 A2, and A3 square

meters, which are separated by a distances of d, 2d, and 3d from the point source respectively as shown.

Inverse square law

Luminous flux reaching the area A1 = luminous intensity × solid angle

∴ Illumination 'E1' on the surface area 'A1' is:

Similarly, illumination 'E2' on the surface area A2 is:

Lambert's cosine law

This law states that ‘illumination, E at any point on a surface is directly proportional to the cosine of the angle between the normal at that point and the line of flux’.

Proof: While discussing, the Lambert's cosine law, let us assume that the surface is inclined at an angle ‘θ’ to the lines of flux as shown





nd Elecronics Engineering Department 54

Electrical a EEGUC





Substituting d from the above equation

Where d is the distance between the source and the surface in m, h is the height of the surface in m, and I is the luminous intensity in candela.

12. Explain the working of Photometry with its different types of head.[CO1 – L2] Photometry deals with the measurement of candle power or measurement of luminous intensity, for that a simple apparatus called the ‘photometer bench’ is

used.

Figure shows the arrangement. Two lamps are mounted on the bench with a screen in between them. One lamp S may be of known candle power and the





other lamp L whose candle power is to be determined.

The screen is moved in between the two lamps and its position adjusted. The illumination is the same on both sides of the screen. Applying inverse square law, we get,

The screen is called the photometer head. The photometer bench is graduated so that distances may be measured.

The Photometer Bench

It consists of two steel rods, between three to four meters long which carry the stands or saddles for holding the two sources or lamps. One of the steel rods carries a brass strip with a graduated scale in millimeters. The bench must be rigid enough to be free from vibrations and the graduations and distance measurements must be accurate since in calculations squares of distances are used. The photometry room is made dark and its walls and ceiling are painted black.

Photometer Heads

Most commonly used photometer heads are the Bunsen and the Lummer Brodhun type. When lamps of the same or very similar colour are to be compared they give quite accurate results but when two lamps with different colours are to be compared, Flicker photometer gives better result.

Bunsen Head (or) Grease Spot Photometer

It consists of a piece of tissue paper with grease spot in the centre, held vertically in a carrier between the two light sources to be compared. The position of the carrier is then adjusted until the semi-transparent spot and the opaque parts of the paper are equally birth, i.e. the grease spot is invisible. The distances of the tissue paper from both light sources are measured. The candle power of the source under test is then determined from the relation.





Another method is by illuminating one side of the paper from a fixed lamp. On the other side of the photometer, first the test lamp is used and then the standard lamp is used to make the spot to vanish. If are these diameters then we get,

Candle power of test lamp / candle power of standard lamp = d12 / d2

2

Perhaps the most accurate method is when two mirrors are used with the photometer head. The two sides of the spot can be viewed simultaneously and potion of the head for equal ‘contrast’ in illumination between the opaque part and the transparent spot of the paper on the two sides is located. Lummer – Brodhun Photometer Head There are two types of Lummer – Brodhun heads

a. Equality of Brightness type photometer head b. Contrast type photometer head

a. Equality of Brightness type photometer head

The photometer had consists of a plaster of Paris screen, two mirrors M1 and M2, a compound prism P and a telescope. The compound prism is made up of two right angled glass prisms, one of which has principle surface as spherical one with a small flat portion at the centre. This small flat portion at the centre makes optical contact with the flat surface of the other prism a shown in figure.





The light falling on the screen from the test lamp side is reflected by the screen surface on to mirror M2 from where it is reflected to the compound prism. The light

from the standard lamp in a similar manner is reflected by the screen surface and mirror M1 and then it reaches the compound prism. Light from M2 passes through the compound prism direct and into the telescope. That

portion of light from M1 which falls on the surface of contact of the two prisms passes

though the compound prism; the remaining light is reflected and passes through the telescope. When we see through the telescope, we see a central circular area illuminated by the test lamp and a surrounding circular area illuminated by the standard lamp as in figure. b. Contrast type photometer head Arrangement:

Two right angled prisms are joined together. One of the prism has its hypotenuse surface etched away at a, b and c which forms a pattern as shown in figure.

The light falling on the prism from the two sources as reflected from the two sides of the screen passes through the un-etched portion and is reflected at the etched surfaces. The etched portion will be less illuminated than the un-etched portion as shown by the shaded and un-shaded portion in figure.

are glass sheets which give a little reflected light for maintaining some ‘difference’ between the illumination of the etched and un-etched areas





in all positions of the photometer head. the balance position is at which equal contrast and not equal brightness is obtained.

c. Flicker Photometer

Principle:

If the human eye sees two illuminated surfaces alternately and the alternations are quite rapid, the flicker produced disappears when the surfaces are of equal brightness. Colour differences between the two illuminated surfaces do not affect such photometers. The speed of alternation should be kept as low as possible at which the disappearance of the flicker can be obtained for the small variation in brightness.

13. A 2000 square meter shop floor area of an engineering industry is to be illuminated with a light level of 200 lux with the 250 watts metal halide lamp fittings. The coefficient of light utilization is 0.6 and depreciation factor of lamp is 1.2. Calculate the number of lamp fittings required and total lighting power required for the above lighting purpose. The luminous efficiency of the metal halide lamp is 90 lumens per watt. [CO1 – H2 - Nov’ 13] Given data:

Area = 2000m2

Illumination E = 200 Lux Lamp wattage = 250 W Utilization factor = 0.6 Depreciation factor = 1.2 Luminous efficiency = 90 lumens / watt

Gross lumens required =

Manufacturing factor MF = = 0.833

Gross lumens =

Total wattage required =

Number of metal halides lamps required =





14. Discuss the various methods of lightening calculations. [ CO1 – L2 - Nov’ 2013, APR 2015]

Lighting Calculations: Inverse-square law The skilled application of computerized point lighting calculations can optimize lighting levels in both the task and ambient domains in order to minimize energy consumption. The lighting professional should consider the use of point lighting calculations, both to design more energy-efficient spaces, and to create spaces with more drama and visual interest.

Point calculations are an exceptionally accurate way to compare general lighting systems. While the easier lumen method allows the comparison of average luminance, point calculations permit the comparison of uniformity of light on the work plane, the patterns of light produced on ceilings and walls, and task contrast rendering. More specifically, point calculations allow consideration of the effects listed below.

Effect on Room Surfaces. By evaluating the patterns of light on a wall caused by a row of compact fluorescent down lights, an aesthetic evaluation can be made. Artwork locations may be selected or lighting may be designed to highlight artwork. It may also be possible to determine whether the pattern created on a wall will produce luminance extremes that will cause glare or reflections in VDT screens. Indirect Lighting Effects on Ceiling. When they are too close to the ceiling, indirect lighting systems may create definite stripes or pools of light on the ceiling that are distracting and that may image in VDT screens. Careful ceiling luminance calculations can help identify the problem, and allow comparison of lighting products with various optical distributions and suspension lengths to reduce the effect. Gray- scale printouts or shaded VDT screen output of luminance make visual assessments possible. Interior Task-Ambient Lighting. Point calculations should be used for any type of lighting design where the task locations and types are well known and are unlikely to move without a lighting redesign. They may also be used for lighting designs where tasks that move end up in predefined locations.

Cautions for Point Calculations. In the case where a task light is used, or where an indirect fixture is mounted within 12 inches of the ceiling, point calculations are not always appropriate. In general, if the luminaire is close to the surface where lighting patterns are to be evaluated, a near field situation exists. A shortcoming of the mathematics used in point calculations is that these near field calculations are comparatively inaccurate unless near field photometric data is available from the luminaire manufacturer, or the computer program is capable of adjusting the characteristics of the luminaries to improve the accuracy of the results. Otherwise, it may be more accurate to evaluate the light patterns from the task light or indirect fixture empirically.





The most common methods used for lighting calculations are:

(1) Watts per Square Meter Method. This is principally a „rule of thumb‟ method very

handy for rough 2

calculations or checking. It consists of making an allowance of watts/m of area to be illuminated according to the illumination desired on the assumption of an average figure of overall efficiency of the system.

(2) Lumen or Light Flux Method. This method is applicable to those cases where the sources of light are such as to produce an approximate uniform illumination over the working plane or where an average value is required. Lumens received on the working plane may be determined from the relation. Lumens received on the working plane = Number of lamps X wattage of each lamp X lamp efficiency (lumens/watt) X coefficient of utilization/depreciation factor. (3) Point-To-Point or Inverse Square Law Method. This method is applicable where the illumination at a point due to one or more sources of light is required, the candle power of sources in the particular direction under consideration being known. This method is not much used because of its complicated and cumbersome applications.

15. Explain lightening and its type in brief. [ CO1 – L2 – APR 2014 ] Design of lighting system: Direct lighting: Lighting provided from a source without reflection from other surfaces. In day lighting, this means that the light has travelled on a straight path from the sky (or the sun) to the point of interest. In electrical lighting it usually describes an installation of ceiling mounted or suspended luminaires with mostly downward light distribution characteristics.

Indirect lighting: Lighting provided by reflection usually from wall or ceiling surfaces. In day lighting, this means that the light coming from the sky or the sun is reflected on a surface of high reflectivity like a wall, a window sill or a special redirecting device. In electrical lighting the luminaries are suspended from the ceiling or wall mounted and distributes light mainly upwards so it gets reflected off the ceiling or the walls.

Types of Lighting: One of the primary functions of a luminaries is to direct the light to where it is needed. The light distribution produced by luminaries is characterized by the Illuminating Engineering Society as follows:

Indirect Lighting= 90 to 100 percent of the light is directed to the ceilings and upper walls and is reflected to all parts of a room.

Semi-Direct Lighting=60 to 90 percent of the light is directed downward with the remainder directed upward.





Semi-indirect Lighting=60 to 90 percent of the light is directed upward with the remainder directed downward. Highlighting Lighting= the beam projection distance and focusing ability characterize this luminaire Industrial Luminaries: Coming to industrial areas if in the Interior-up to 6m Fluorescent Lamp with matt white reflector are employed. In High bays beyond 6m Discharge Lamps with Mirror Reflectors are employed. Luminaries in Hazardous Areas are specially designed. They are encapsulated in boxes made of steel or cast iron exterior housing to avoid any explosion, sturdy resisting pressure.

Categories of Explosive Areas:

In this respect explosives are as are categorized as Zone 0 – Explosive all the time, Zone 1 – Normally Explosive and Zone 2 – Explosive Abnormally.

Here moisture & dust are taken care by Gasketted Luminnaires – Completely sealed eg: in a Shower or a Laundry. Emergency Lighting is required when normal lighting fails. Escape Lighting sufficient for evacuation typically 1 – 10 lx. Safety Lighting – 5% normal Lighting is provided in Potentially Hazardous areas. Standby power supply required for activation of vital implements. A permanent, separate, self supporting Power system which is reliable and mains rechargeable batteries in each Luminaries are provided Non Permanent - Auto Switching - Emergency Generator - Battery Supply is also used.

Road Lighting: Conventionally by they are arranged in a column, mounted on a wall or suspended by a span wire. Plane of Symmetry being in vertical plane perpendicular to the axis of the road along the road. Catenary – suspended from a catenary cable parallel to the axis of road. Plane of symmetry parallel to the axis of road. They employ Corrosion Resistant sturdy materials and are usually closed.

Flood Lights: Rain Proof Lamp holder with wide / narrow beam Reflectors are used for flood light. They are usually High wattage Incandescent Lamps, Halogen Lamps, High Pressure Mercury Vapor Lamp or Low / high Pressure Sodium Lamp. Spot lights / down lights are usually used with Screens, Reflectors, Filters,





Colored envelope and Closed Lamps. Down lights are Spot lights when suspended.

As already brought out the components of an Illumination system are Lamp, the Radiation Source, Luminaire that directs and controls the light flux. Control Gear is the accessory that helps in controlling the requisite amount of flux on the work plane. Now we take a look at the accessories involved. First of these is Ballast. In a discharge lamp a series impedance to limit the current is required. If the current is allowed to increase there can be explosion of the lamp. This takes the form in a.c. as Inductance-w/o undue loss of power. This is called Ballast. It should have high power factor for economic use of the supply and should generate minimum harmonics. It should offer high impedance to audio frequencies. It should suppress- Electromagnetic interference (Radio interference-TV interference). It is essentially, a reactor of a wound coil on a magnetic core often called Choke and is in series with the lamp. Typical power factor is 0.5 Lag. Power factor is improved by having a capacitor connected across input lines.

Fig 1 shows the connection for a discharge lamp employing ballast formed by a reactor commonly known as choke. Fig 2 shows how the capacitor may be connected to improve the power factor. As may be seen the capacitor is placed in shunt. At times a lead circuit may result by placing a capacitor in series as shown in Fig1 However, when a illumination system employing two lamps is used power factor may be improved by having one with a lead circuit and other with a lag circuit as shown in Fig. 4. Next important accessory is a starter that initiates the discharge in a discharge lamp. Starter is marked as ‘S’ in the Figs.1 to 4. Starter less circuit are shown in Fig 5. They employ pre-heated filament electrodes. The preheating obtained through a small portion of voltage tapped from the input source.





When discharge lamps are used on dc the ballast takes the form of a resistor together with associated power loss. These days they take the form of electronic ballast which converts dc to high frequency ac of around 20 kHZ. Except high pressure mercury lamp where V > VS (starting) all lamps need a

starting device. At times, it is integral part of a lamp. Switch start employs bimetallic strip that opens upon heating. Starterless, rapid start or instant starts are useful for outdoor applications. Other forms of starters employed are three electrode devices called igniters.

Glare Evaluation: Visual comfort system is most common evaluation in the USA/Canada. This is expressed as percentage of people considering an installation comfortable as viewed from one end. Glare tables list various proportions and layout of room for glare free lighting. Figure of merit is based on a source of 1000 lm.from a luminaire. If VCP ≈ 70% then the system is said to be glare free. British method employs Zone of luminaire with a classification for quality of light expressed as Glare index. Luminance limit system is adopted in Australia. Standard code for Luminaire base lamp. dep. on room dimensions, mounting height and a Empirical shielding angle Luminance curve system is employed in Europe. Luminance limits for luminaires critical angles, γ are 45º < γ < 85º. Quality class is expressed from A to E type is based on Luminaire orientation. Type 1. Luminous sides when Luminous side plane> 30 mm

EE6801 ELECTRIC ENERGY GENERATION, UTILIZATION AND CONSERVATION / EEE DEPT/ R SENTHI KUMAR VEL TECH HIGH TECH DR. RANGARAJAN DR.SAKUNTHALA ENGINEERING COLLEGE



General light is predominantly light coming downwards. Typically reflectance of 0.5 for walls /ceiling and 0.25 for furniture. How is Glare evaluated? 1. Determine luminance of the source between 45º - 85º 2. Determine the quality class and illuminance required. 3. Select the curve – class / level.




4. Determine. Max. Angle to be considered from length & height and plane of eye level & plane of luminaires. (Refer to Fig 1) 5. Horizontal limit based on” a / h”, part of the line (or curve) to be ignored. 6. Compare luminance of one luminaire with selected part of the limiting curve.

Interior Lighting: Interior Lighting is a complex problem depending on various factors such as • Purpose intended service,

• Class of Interiors. • Luminaire best suited, • Color effect and • Reflection from ceiling, walls, floors. Good Lighting means intensity should be ample to see clearly and distinctly. The light distribution should be nearly uniform over a part of the room at least. It should be diffused that is soft and well diffused. Color depends on purpose and taste source but should approach daylight / yellow. Source location should be well above range of vision. To avoid glare intrinsic brightness is reduced by diffused glass ware and by remaining objects of secular reflection from range of vision. Shadows are a must for accentuating depth but should not too apparent abruptly or dense, they are not to be harsh and should toned down.

Standard practice is to have general lighting in all areas at a level comfortable to eye. It should eliminate dark shadows and avoid sharp contrast. In order to emphasize on parts that should be shown. Light sources located such that visual importance of object is kept in mind. Lamp may be concealed or counter lighted with a very low attention value to itself. Glare minimized by diffusing.

American Institute of Architects Recommends for Good Illumination.

1. General. Lighting – effectively illuminate all objects/areas with due regard to relative importance in the interior composition. Adequate for eye comfort throughout the room elimination of dark shadows and sharp contrasts – preserve soft shadows for roundness/relief – lighting emphasis on those parts that need first attention.

2. Light sources be subordinated in visual importance to the things intended to illuminate, except rarely when itself is a dominant decorative element. Unless – concealed/counter lighted, that they are not apparent they have extremely high attention value – dominate the scheme. If visible – so disposed – to attract eye to major feature of room than themselves.




3. Glare must be eliminated. Result of intense brightness in concentrated areas within the line of vision. Produced by excess brightness of visible light. reflection of bright lights from – Polished – low diffused surfaces - extreme contrast of light/shade Employ – means of diffusing – at source or finish the room - with Diffusing/Absorbing materials rather than reflecting material.

4. Level of illumination to be adequate for the type of eye work. Local lighting to supplement general lighting adequate illumination – working at m/cs – desks – reading tables High level local lighting is always to be accompanied by general lighting to avoid eye strain and minimize controls. If glare is avoided there is no over illumination. Natural light limits are for outdoor 107600 lux and 1076 lux for indoor. Level should be adequate for eye task expected.

5. General lighting is to be related and controlled to suit the mood. While worship, meditation, introspection need low levels. Gaiety, mental activity, physical activity or intense activity needs high levels. Theaters, homes and restaurants may need levels varied according to mood. Shops level should be appropriate to woo customers through psychological reaction. Offices, factories and schools adequate illumination to work w/o eye strain.

6. Light source must suit interior in style, shape and finish in all architectural aspects.

Sports Lighting:

Lighting for sports facility looks for comfort of four user groups namely

Players, Officials, Spectators and Media. Players and officials should see clearly in the play area to produce best possible results the object used in the game. Spectators should follow the performance of the players. In addition to play area surroundings also need to be illuminated. Lighting should be such that it enables safe entry and exit. With increasing crowd level safety becomes more and more important. Media include TV and film, for which lighting should provide lighting such that conditions are suitable for color picture quality as per CIE 83. This should be suitable for both general pictures as well as close up of players and spectators. Additionally, it should have provisions for emergency power supply to provide continuous transmission.

Criteria relevant for sports lighting are Horizontal Illuminance, Vertical

Illuminance, Illuminance Uniformity, Glare restrictions, Modeling & shadows and Color appearance & rendering




16. Discuss the energy saving opportunities in lighting systems. [ C01 – L2 - Nov’ 2013)

Energy efficiency lamps: Horizontal Illuminance:

This becomes important as major part of view is illuminated playing area. Illuminance on the horizontal plane serves adaptation of the eye. It acts as a background, so adequate illuminance is important. For safe entry and exit adequate illumination is required in the circulation area also.

Vertical Illuminance: Sufficient contrast across players’ body is essential for the identification of

the player. This is possible only if sufficient vertical illumination is there. This is characterized by both magnitude and direction. Players need adequate vertical illumination, from all directions. Spectators and Media need illumination only in defined directions. Generally, if horizontal illuminance is taken care, vertical illuminance levels become adequate. Usually vertical illuminance is specified or measured at a vertical height 1.5 m above the play area. Apart from player recognition and picture quality vertical illuminance should enable observation of movement of ball (or object moving in the sport concerned) above the playing field by both players and spectators. Spectator’s stands are also part of the environment and must also have adequate vertical illuminance, more from the safety point of view.

Illuminance Uniformity: Good illuminance is important in both the horizontal and vertical planes. If

it be good it does away need for continuous adjustment of cameras. This is achieved by having Illuminance Uniformity.

Road Lighting: Road lighting provides visual conditions for safe, quick and comfortable movement of Road users. The factors responsible for the lighting scheme for roads are:

i. Luminance Level. ii. Luminance Uniformity. iii. Degree of Glare limitation. iv. Lamp Spectra and v. Effectiveness of visual guidance.




Luminance Level: As the Luminance of a road influences contrast sensitivity of drivers’ eyes

and contrast of obstacles, relative to back ground. Hence affects performance of Road users. Surrounding brightness affects the adaptation of human eye. Bright surroundings lower contrast sensitivity there by requiring higher luminance for the road surface. Darker surroundings make driver adapted to road (assuming road is brighter). Roads with dark surrounds are to be lit by including surroundings. Otherwise drivers cannot perceive objects in the surroundings. CIE 12 recommends that 5m away from the road on either side should be lit by illuminance level at least 50% of that on the road.

Luminance Uniformity:

Adequate uniformity is necessary for visual performance and visual comfort of the user. From visual performance view point, uniformity ratio is defined by U0 = Lmin / Lavg .U0 should not be below 0.4.From visual comfort view point

uniformity ratio is defined as U1 = Lmin / Lmax measured along the line passing

through the observer positioned in the middle of the traffic facing the traffic flow. Termed longitudinal uniformity ratio.

Glare Limitation: Physiological or disability glare affect visual performance. Psychological

or discomfort glare affect visual comfort. Glare is to be avoided at all costs. Lamp Spectra

Spectral composition determines color appearance of the lamp. The way lamp is going to render color to objects Low pressure sodium vapour lamps give greater visual acuity. Spectrum should be such; there is Great speed of perception, less discomfort glare and shorter recovery time after glare. Visual Guidance:

Visual guidance guides the road user and hence must for user to get a recognizable picture of the course immediately. This is improved by lamp arrangement that follows the run of the road. More so if turns and intersections are there. Lighting scheme must provide visual guidance. On roads having separate lanes with a separator the lighting columns are located on the separator. As is the custom in large avenues in Metros. On a curve the lighting column is located along the outer column. This gives a clear indication of the run of the road on the curvature. Visual guidance pilots traffic through lights of different colors on different routes.




17. Explain in detail about the working of Incandescent lamps with clear sketch. [ CO1 – L2 ]

The incandescent or filament type lamp consists of a glass globe completely evacuated and a fine wire known as filament within it. The glass globe is evacuated to prevent the oxidation and convection currents of the filament and also to prevent the temperature being lowered by radiation.

Materials commonly used for incandescent lamps:

The materials used for making filaments of incandescent lamps are carbon, tantalum and tungsten. Carbon:

Resistivity ρ = 1000 to 7000µ Ω cm

Temperature coefficient α = - 0.0002 to - 0.0008

Melting point = 3500ºC

Density = 1.7 to 3.5

To prevent the blackening of the bulb, the working temperature is 1800ºC. The commercial efficiency of filament lamp is about 4.5 Lumens per watt.

Tantalum:

Resistivity ρ = 12.4µ Ω cm

Temperature coefficient α = - 0.0036


Density = 16.6

The efficiency is low such as 2 lumens per watt. So it is not used much now – a – days.

Tungsten:

Resistivity ρ = 5.6µ Ω cm

Temperature coefficient α = 0.0045


Density = 19.3 The efficiency when worked at 2000 ºC in an evacuated bulb is 18 lumens per watt. This metal is most widely used for the purpose.

To prepare filament, pure tungsten power is pressed in steel mould for small bars. Mechanical strength of bars is improved by heating electrically nearly to the melting point. These bars are the n hammered at red heat and drawn into filaments.




To improve efficiency, the bulb is filled with inert gas argon with a small percentage of nitrogen. To reduce the convection currents, produced by the gas molecules in the bulb, the filament is wound into a close spiral and suspended horizontally in the form of a circular arc.

The efficiency of the gas filled “coiled coil” tungsten filament is about 30 lumens per watt. This is due to high working temperature of 2500 ºC.

The ideal material for the filament of the incandescent lamps is

High melting point

Low vapour pressure

High resistivity

Low temperature coefficient

Ductility

Sufficient mechanical strength to withstand vibrations during use.

18. With neat diagram, explain the working of arc lamps in detail. [ CO1 – L2 ]

These lamps are used in searchlights, projection lamps and other special purpose

lamps like those in flash cameras.

In an arc lamp electric current is made to flow through two electrodes in contact with each




other which are drawn apart. The result is an arc being struck. The arc maintains the current, and is very efficient source of light.

The various forms of arc lamps are: 1. Carbon arc lamp

2. Flame arc lamp

3. Magnetic arc lamp

The carbon rods used with A.C supply are of the same size as that used with

D.C. supply.The positive rod is of larger size than the negative rod.

The craters in the arc at the positive and negative rods are of the same size (With

A.C. supply) while with D.C. supply, the positive crater is bigger and gives 85 percent

light at a temperature of 3500ºC,while the negative crater is of smaller size. The

efficiency of the lamp is 9lumens/watt.

The positive electrode gets consumed earlier than the negative electrode, if the size

of the former is same as the latter. Hence the positive electrode is of twice the

diameter than that of the negative.

A resistance is used to stabilise the arc.

The voltage drop across the arc is about 60 v figure and supply voltage is up to 100 V.

19. Write short notes on Discharge lamps. [ CO1 – L2 ]

In all discharge lamps, an electric current is passed through a gas or vapour which renders it luminous. In this process of producing light by gaseous conduction, the most commonly used elements are neon, mercury and sodium vapours.

The colours (wavelength) of light produced depends on the nature of gas or vapour,

Neon – Orange red light

Mercury vapour – bluish

Sodium vapour – orange yellow




Types:

Those lamps in which colour of light is same as produced by the discharge through the gas or vapour. Eg. Sodium vapour, mercury and neon gas lamps.

Those lamps which use the phenomenon of fluorescence

Eg. Fluorescent mercury vapour tube

Merits:

The discharge lamps are superior to metal filament lamps.

Demerits:

High initial cost

Poor power factor

Starting, being somewhat difficult requires starters/ transformers in different

cases.

Time is needed to attain for full brilliancy.

Since these lamps have negative resistance characteristic ballasts are necessary

to stabilise the arc.

The flicker (caused due to the fluctuation of light output at twice the supply)

causes stroboscopic effect.

They are suitable only for a particular position.

20. Explain the operation of fluorescent lamp in details. [ CO1 – L2 - May’13]

The fluorescent lamp consists of a glass tube of length varying 2 to 4 feet and is filled with a low pressure argon gas and a drop of mercury. The lamp is connected with a choke and starter. Choke:

A choke is connected in series with fluorescent tube and used to provide a voltage impulse so as to initiate the electron movement. It is an iron cored coil having high inductance. Starters:

The lamp circuit is made such that the starter should conduct when the circuit is ON. Commonly used starter circuit for fluorescent tubes are

Thermal type starter

Glow type starter

Instant starters

The mercury vapour are provides a conducting path between the electrodes. The




starter used may be of thermal or glow type whose function is to complete the circuit initially for preheating the filaments and then to open the circuit for inducing voltage across choke for initiating ionization.

Fig 3.10 Fluorescent lamp Working:

When the starter is cold, the electrodes are open. When supply is given, full voltage acts on the starter. A glow discharge is set up in the starter which warms the electrodes and causes the bimetal strip to bend and touch the electrodes.

The circuit becomes a complete series. Current flows and causes emission of free electrons from filaments. At the same time voltage at the starter falls to zero and the bimetal strip cools down. The electrodes of the starter switch the open and interrupt the current in the circuit.

Its effort is to induce high voltage surge of about 1000v in the choke. This voltage produces the flow of electrons between the lamp electrodes and the lamp lights up immediately. Then starting contacts are left open.

In order to improve the power factor, usually a condenser of 4 µF capacity is connected across the supply.




21. Explain the operation of Neon lamp in details. [ CO1 – L2]

Neon lamps belong to cold – cathode category. The electrodes are in the form of

iron shells and are coated on the inside. The colour of light emitted is red. If the

helium gas is used in place of neon, pinkish white light is obtained. Helium and neon

through coloured glass tubing produce a variety of effects.

The transformer has a high leakage reactance which stabilizes the arc in the lamp. A capacitor is used for power factor improvement. High voltages are used for starting.

The efficiency of neon lamp lies between 15 – 40 lumens/watt. These lamps are

used as indicator lamps, night lamps for determination of polarity of DC mains and in larger sizes on neon tubes for purpose of advertising. Neon Tube:

The neon tube which is used in varying lengths up to about 8m may be bent into almost any desired shape during manufacture. It consists of a length of glass tubing containing two electrodes normally cylindrical in shape of iron, steel or copper.

The tubes are mounted either on a wooden frame or a metal base. These are matched with step – up transformers by connecting suitable tapings for the rated current. Connections between letters are made by nickel wires, the glass tubes being slipped over them.

The power factor of neon tube is quite low and is improved by using capacitors. The capacitors can however be placed on the low voltage side of the transformer.

22. With neat diagram explain the construction and working of CFL lamp. [ CO1

– L2 - Apr’15 ] CFLs are a miniature version of the common fluorescent light, glowing phosphor

gas by using an electric current. Earlier the CFLs used magnetic ballasts which usually results into delay or flicker when turned on, whereas, the most new CFLs use electronic ballasts. On comparing with incandescent bulbs, CFLs are approximately four times as efficient (25 Watt CFL will have the same light output as a 100 Watt incandescent bulb).




They also possess high long life (10 times longer than 10 incandescent bulbs). But, a CFL gives off light that looks just like a standard incandescent. Construction

Compact fluorescent lights (CFLs) are created by taking a traditional fluorescent tube and bending it into a compact design that fits easily into ordinary incandescent fixtures.

Electronic ballasts in place instead of electromagnetic ballast, helped in removing most of the flickering and slow starting [considered being the major flaws among the fluorescent lighting].

The compact fluorescent lamp consists of a soda-lime glass tube filled with a few torr of gas basically argon and a drop of mercury. Metal electrodes are sealed at the tube ends and conduct electric current from the external circuit to the gas present internally. There are two main parts in a CFL:

The gas-filled tube (bulb or burner)

The electronic ballast

Electronic ballasts usually contain a small circuit board along with rectifiers, a filter capacitor and two switching transistors. These transistors are usually connected as a high-frequency of 40 kHz resonant series DC to AC inverter.

Operating Principle

CFLs operate by producing light while driving electric current through a tube containing argon and a small proportion of mercury vapour. This will generate an invisible ultraviolet light that excites a fluorescent coating say phosphor, on the inner side of the tube, which finally emits visible light.




When CFLs are first turned on, they require a bit more energy to glow. Ballast helps in starting the bulb and regulates the current once the electricity starts flowing and completed in 30 seconds to three minutes.

The lamp also requires a current to preheat the filaments, a high-voltage for

ignition, and finally a high-frequency AC current during running. In order to fulfill these requirements, the electronic ballast circuit first performs AC-to-DC conversion on low- frequency at the input, followed by DC-to-AC conversion on a high-frequency at the output. There are two types of CFLs:

Integrated CFLs:- Integrated lamp consists of a tube, electronic ballast and either an Edison screw

or bayonet fitted in a single CFL unit. These lamps have allowed consumers to replace incandescent lamps with CFLs. Integrated CFLs work well in many standard incandescent light fixtures, lowering the cost of CFL conversion.

Non-Integrated CFLs:- The ballasts in Non-integrated CFLs are placed in a light fixture that are large

and last longer as compared to the integrated ones. Also, they don't need to be replaced when the bulb reaches its end-of-life.

Non-integrated CFLs can be both expensive and sophisticated than the integrated ones. There are two types of bulbs namely, bi-pin tubes designed for conventional ballasts and quad-pin tubes designed for electronic ballasts and conventional ballasts accompanying external starter.

Advantages

CFLs are cost efficient.

CFLs are energy efficient, since it requires less energy to provide same amount of light.

CFL light bulbs are long lasting and can last upto 10,000 hours.




CFLs are very versatile and can fit into a standard light socket and do not call for any special lighting fixture.

Dimmers are also available for some CFLs so as to control their brightness.

CFLs comes in different colours and various shapes.

Each CFL over its lifetime saves 450 pounds of carbon from being produced, considered to be a powerful saving.

Disadvantages

Frequent use of CFLs, also known as flipping can shorten their life span.

Low temperatures can cause the CFLs to perform with lower light levels, hence reducing their utility.

CFL contains mercury, a toxic substance. But, it does not pose a health threat unless the bulb is broken.

Problems with humming noises and flickering light have largely been addressed by improvements in CFL technology (CFLs using magnetic ballasts).

Applications

Replacing one regular light bulb with a compact fluorescent light bulb helps in saving consumers $30 in energy costs over the life of the bulb.

Compact fluorescent light bulbs has an upper hand over other bulbs as it also generate 70 percent less heat, so they are much safer to operate and can helps in reducing energy costs that are associated with cooling residences and offices.

23. With neat diagram explain the construction and working of LED lamp.[ CO1

– L2 ] Light-emitting diodes (LEDs) has seen from its use in numeric displays and

indicator lights to a range of new and potential new applications, including exit signs, accent lights, task lights, traffic lights, signage, cove lighting, wall sconces, outdoor lighting and down lighting.

LEDs have several advantages such as low heat output, small size, long lamp life, energy savings and durability. LEDs also occupy extraordinary design flexibility in colour changing and dimming. They also possess property of distribution by combining these small units into desired shapes, colours, sizes and lumen packages. LEDs have been in use for decades in house-hold appliances, computers and clocks. But, it has recently gained popularity as an energy-efficient alternative to incandescent bulbs. Construction

LEDs are known as tiny lights which are produced by moving electrons in a semi-conductor. Due to the absence of burning gas or filaments, LEDs are considered more durable and produce very little amount of heat. They are resistant to weather and




can also be water-proofed for extreme weather conditions. LED units are usually small, typically 5mm (T 1-3/4).

The construction of a light emitting diode is quite different as compared to a

normal signal diode. The PN junction of a LED is surrounded by a transparent, hard plastic epoxy mold which is hemispherical shaped shell and protects the LED from both shock and vibrations.

The epoxy resin body is constructed in a manner such that the photons of the

light emitted by the junction are reflected away from the surrounding substrate base because the LED junction does not actually emit a good quantity of light. This light is focused upwards through the top of the LED, which itself acts as a lens and concentrates this amount of light.

This is the reason for the emitted light to appear brightest at the top of the LED.

However, Some LEDs have a rectangular or cylindrical shaped construction having a flat surface on top.

Operation and Biasing

A light emitting diode (LED) is a semiconductor diode that emits light when a current flows from anode to cathode across the P-N junction of the device. To operate an LED, direct current (DC) is supplied to provide the necessary positive bias (forward voltage) across this junction. When an LED is activated, a power supply is required to convert AC voltage into sufficient DC voltage, which is then applied across the diode semiconductor crystal.




This results in electrons (negative charge carriers) in the diode’s electron transport layer and holes (positive charge carriers) in the diode’s hole transport layer which later combines at the P-N junction. Hence, it converts this excess energy into light.

Where, If = forward current

VDC = supply voltage Vf = forward voltage R = ballast resistor

It is feasible to use a rectified and smoothed mains input to power the LED bias

circuit (shown in above figure), but the resulting supply voltage (V) will be higher than the forward voltage (Vf) across a single LED. This implies that considerable power would be wasted in the ballast resistor as compared to the power consumed by the LED.

To resolve this issue, a number of LEDs can be connected in series which only partially addresses this issue, since the cumulative forward voltage will still be less than the voltage drop across the resistor.

In case of LEDs, the External Current Limiting Resistor, R, is given by;

Where VIN = input voltage applied to the circuit

VF = forward voltage of LED emitter at forward current IF

VD = voltage drop across optional reverse transient EMC protection diode y = number of series connected LED emitters x = number of paralleled strings

Advantages

LEDs are available in a variety of shades of white light, ranging from warm hues comparable to traditional incandescent bulbs to cool colors that mimic natural daylight.




A LED style bulb will generally last approximately 100 times as long as incandescent bulbs.

Most Efficient way of illumination and lighting, with an estimated energy efficiency of 80%-90% as compared to traditional lighting and conventional light bulbs.

LED lights are free of toxic chemicals.

LEDs are highly rugged and can withstand even the roughest conditions.

Instant Lighting & Frequent Switching-LED lights brighten up immediately and when switched on.

Disadvantages

LEDs are currently more expensive, considering price per lumen, on an initial capital cost basis.

LED performance depends largely on correctly engineering the fixture which is to manage the heat generated by the LED, thus causing deterioration of the LED chip itself.

LEDs must be supplied with the correct voltage and current at a constant flow. This requires some electronics expertise to design the electronic drivers.

LED’s can shift color due to age and temperature. Also two different white LED will have two different color characteristics, which affect how the light is perceived.

The retina cells are destroyed by prolonged and continuous exposure to LED rays; they cannot be replaced and will not regrow.

24. Explain the various steps followed in calculation of illumination for designing the residential lighting. [ CO1 – L2 - Apr’15]

The street lighting entails the following main objectives:

To make the traffic and obstructions on the road clearly visible in order to

promote safety and convenience.

To enhance the community value of the street.

To make the street more attractive.

Design of street lighting: The following two general principles are employed in the design of street

lighting installations: 1. Diffusion principle 2. Specular reflection principle

Diffusion Principle:

Here the lamps fitted with suitable reflectors are employed.




The design of reflectors is such that they may direct the light downwards and spread as uniform as possible over the surface of the road.

In order to avoid glare the reflectors are made to have a cut-off between 30º to 45º so that the filament is not visible except underneath it.

The diffusing nature of the road surface causes the reflection of a certain proportion of the incident light in the direction of the observer and therefore the road surface appears bright to the observer.

For calculating the illumination at any point on the road surface, point – to – point or inverse – square law method is employed. Over certain proportions of the road the surface is illuminated from two lamps and the resultant illumination is the sum of the illumination due to each lamp.

Specular reflection principle:

Here, the reflectors are curved upwards so that the light is thrown on the road at a very large of incidence. In this method the requirement of a pedestrian who requires seeing objects in his immediate neighbourhood is also fulfilled.

This method is more economical in comparison to diffusion method of lighting. However it has the demerit that it produces glare for the motorists.

Illumination level:

30 lm/m2 - Important shopping centres and road junctions

4 lm/m2 - poorly lighted suburban streets

8 to 15 lm/m2 - average well lighted street Mounting height

Normal spacing for standard lamps is 50 meters with a mounting height of 8 meters.

Types of lamps

Mercury vapour

Sodium discharge lamps Advantages:

There is low power consumption for a given amount of light.

In spite of the higher cost of the lamps makes the overall cost of an installation with discharge lamps less than employing filament lamps.

The colour and monochromatic nature of light produced by discharge lamps does not matter much in street light installations.

Lamps posts should be fixed at the junction of roads.




25. Explain the illumination for designing of factory lighting. [ CO1 – L2 ]

In an industrial establishment an adequate amount of light produces the following good effects:

The productivity of a labour is increased

The quantity of work as improved

Number of work stoppages are reduced

Accidents are reduced A factory – lighting installation with indoor equipments should provide the following:

Adequate illumination on the working plane

Good distribution of light

Simple and easily cleaned fittings

Avoid glare(from the lamp itself as well as from any polished surface, which may be within the line of vision)

General, local and emergency lighting:

In factories and workshops the usual scheme is to mount a number of lamps at a sufficient height so that uniform distribution of light over the working the plane is obtained.

In large machine shops the height is governed by the necessity of keeping the lamps above the travelling crane.

For intense illumination, local lighting can be provided by means of adjustable fittings attached to the machine or bench mounted on portable floor standards. Such lamps should be mounted in deep reflectors to avoid the glare.

Auxiliary lighting from the sources other than the main electric supply preferable from batteries, small petrol driven generator set.

Emergency light circuit are operated from main electric supply should be completely separated from main lighting circuit.

Reflectors must be simple in design and easily cleaned.

Types of fittings:

Industrial lighting fitting

Standard reflectors

Diffusing fittings

Concentrating reflectors

Enclosed diffusing fittings

Angle reflectors.




26. Explain the various steps followed in calculation of illumination for designing the flood lighting in sports ground. [ CO1 – L2 - Nov’15 ]

The flooding of large surface with light from powerful projectors is called flood lighting. As far as possible the projectors should not be visible to the passersby. In some cases the projectors may be housed in ornamental stands.

Employed at:

To enhance the beauty of ancient monuments at night

To illuminate advertisement boards and show cases

To illuminate railway yards, sports stadiums, car parks, construction sites, quarries.

For small buildings:

Uniform flood lighting is used.

Flood lights can be placed on other buildings nearby or on suitable posts at distances of not more than about 60meters.

Light should fall nearly perpendicular to the building. For Large or Tall building:

Non - uniform flood lighting is used.

Flood lights should be so located that contours and features of the building are well defined.

If any shadows are cast, they should enhance the beauty of the building. Types of projectors: (Based on beam spread)

Narrow beam projectors Beam spread between 12 - 25º. These are used for distance beyond 70m. Low wattage standard gas filled tungsten filament lamp (250W, 500W or 1000W) is encouraged

Medium angle projectors

Beam spread between 25 - 40º. These are used for distance beyond 30 - 70m. Narrow beam projector with low wattage standard gas filled tungsten filament lamp (250W, 500W or 1000W) is encouraged.

Wide angle projectors

Beam spread between 40 - 90º. These are used for distance below 30m. High wattage lamp is encouraged.

Flood Lighting Calculations:

1. Waste light factor:

Whatever surface is illuminated by a number of projectors there is certain waste of light. This effect is taken into account by multiplying the theoretical value of light (in lumens) by waste light factor which is 1.2 for rectangular area and 1.5 for irregular objects like statues




2. Depreciation factor:

It is defined as the ratio of illumination under ideal condition to the illumination under normal conditions. The actual amount of light to be provided by the source is greater by 50 – 100% on account of dirt and dust depending on the reflector surface.

3. Coefficient of utilisation: (beam factor)

It is defined as the ratio of beam lumens to the lamp lumens. Its value lies between 0.3 and 0.5.

For any desired intensity over a definite surface the number of projectors required is obtained from the following relations

Where, N – Number of projectors

A – Area of surface to be illuminated, m2

E – Illumination level required lm/m2




Unit – III

Heating and Welding

Part – A

1. What are the advantages of electric heating? [ CO1 – L1] The main advantages of electric heating over other systems of heating such

as coal, oil or gas heating are given below. o Economical and Cleanliness o Absence of flue gases o Ease of control or adaptation o Automatic protection o Upper limit of temperature o Special heating features o High efficiency of utilization o Better working conditions o Safety o Heating of non-conducting materials

2. What are the requirements of a good heating material? [ CO1 – L1 – NOV 2014, APR 2014]

1. High resistivity 2. Low temperature coefficient of resistance 3. High melting point 4. Free from oxidation

3. Classify the methods of electric heating. [ CO1 – L1 ]

Kinds of electric heating A. Power frequency heating

a. Resistance heating i) Direct resistance heating ii) Indirect resistance heating iii) Infrared or Radiant heating

b. Arc heating i) Direct arc heating ii) Indirect arc heating

B. High frequency heating a. Induction heating

i) Direct induction heating




ii) Indirect induction heating b. Dielectric heating

4. What is meant by indirect resistance heating? [ CO1 – L1 ]

In this method, the current is passed through a high resistance wire known

as heating element. The heat produced due to I2 R loss in the element is transmitted by radiation or convection to the body to be heated.

Applications are room heaters, in bimetallic strip used in starters, immersion water heaters and in domestic and commercial cooking and salt bath furnace.

5. What is meant by (1) infra red /radiant heating? (2) Dielectric heating? [ CO1 –

L1 ] (1) When current possess through a resistive element heat energy is produced

and the same is dissipated in the form of infrared radiation this is focused upon a body to be heated. E.g. to dry the wet paint on an object.

(2) When a non metallic material is placed between two electrodes at high voltage the dielectric loss is dissipated in the form of heat which is used for heating purposes.

6. What are the properties of heating element material? [ CO1 – L1 ] The material of the heating elements should posses the following desirable

properties for efficient operation and long life. High resistivity: It should have high specific resistance so that the overall

length to produce a certain amount of heat may be smaller. High melting point: It should have high melting point so that high

temperatures can be produced without jeopardizing the life of the element. Free from oxidation: It should be able to resist oxidation at high

temperatures; otherwise its life will be shortened. Low temperature coefficient: It should have a low temperature coefficient

so that resistance remains appreciably constant even with increases of temperature. This helps in accurate control of temperature.

7. What are the causes of failure of heating elements? [ CO1 – L1 ]

Principle causes are Formation of hot spots General oxidation of the element and intermittency of

operation Embrittlement caused by grain growth Contamination of element or corrosion




8. Write short note on infrared heating. [ CO1 – L1 ] In radiant heating, the elements are of tungsten operating about 2300°C as at

this temperature a greater proportion of infra-red radiation is given off. Heating effect on the charge is greater since the temperature of the heating

element is greater than in the case of resistance heating. Heat emission intensities up to 7500 watts/sq.m can be obtained leading to heat absorption up to 4300 watts/sq.m. This reduces the time taken by various drying process.

9. What is the basic principle of induction heating? [ CO1 – L1 ]

High frequency eddy current heating produced by eddy currents induced by electromagnetic action in the metal to be heated.

It works on the principle of electromagnetic induction as same as a transformer. It has a metal disc surrounded by a copper coil in which a.c supply is flowing. The disc has a finite value of diameter and thickness and is spaced a given distance from the coil and concentric to it. We find that a secondary current is caused to circulate around the outer surface of the disc.

10. What are the different types of resistance welding? [ CO1 – L1 ] The different types are as follows

Butt welding

Spot welding

Projection welding

Seam welding

Percussion welding

11. List the advantages of electric heating. [ CO1 – L1 - May /June 2013]

It is free from dirt. It is a clean system requiring minimum cost of cleaning. The system does not produce any flux gas.

Simple and accurate temperature control can be made either by manual or fully automatic switches.

It is economical as electric furnaces are cheaper in initial and maintenance cost. Overall efficiency of electric heating is much higher.

12. What is meant by resistance welding? [ CO1 – L1 - May /June 2013]

Resistance welding is the process in which a strong electric current is sent through the two metals is constant to be welded which melts the metals by the resistance they offer to the flow of electric current.




13. What is LASER welding? [ CO1 – L1 ] LASER (Light Amplification by Stimulated Emission of Radiation) welding is

a welding process that uses the heat from a laser beam impinging on the joint. The process is without a shielding gas and pressure.

14. Compare DC welding and AC welding. [CO1 – L2 - May/June 2013]

Sl.n Factors D.C welding A.C welding

1. Equipment Motor-generator set or rectifier is required in case of availability of a supply; otherwise oil generator set is required.

Only a transformer is required.

2. Prime Cost Two or three transformer.

times of Comparatively low

3. Operatin g

Low High 85%

4. No-Load voltage Low Too high

5. Power factor High Low

6. Heating Uniform heating Non-uniform heating

7. Arc stability Higher -

8. Arc blow Pronounced Not so pronounced with a.c

15. Draw the voltage versus current characteristics of welding transformer. [CO1 – L1 - Nov/Dec 2013]




16. Give the methods of control of temperature in arc furnaces.[ CO1 – L1 - Nov –

Dec 2012] a) ON – OFF control of temperature b) High – Low control of temperature.

17. List some steps taken to minimize skin effect in induction heating. [CO1 – L1

- Nov/Dec 2012] Armond cables can be used. Bundled conduction (6 conductors) are used. Laminated cores made of thin, naturally insulated sheets are used.

18. What is meant by resistance arc welding? [ CO1 – L1 - May /June 2012]

Resistance arc welding is the process in which a strong electric current is sent through the two metals in contact to be welded which melts the metals by the resistance they offer to the flow of electric current.

19. List a few applications of dielectric heating. [CO1 – L1 - May/June 2012] Domestic applications are room heating, water heating, immersion heater,

electric iron, electric ovens, and hot air driers. Industrial applications are annealing, melting of metals, electric welding,

making plywood, baking of insulators.

20. Explain the method of controlling temperature in resistance heating.[CO1– L2]

Temperature control is necessary in resistance furnaces- temperature may have the be kept constant or varied according to the requirements. Control may be manual or automatic.

The heat developed depends upon so there are three ways by which temperature can be controlled.

1. Voltage or current: By varying voltage across

element Using auto – transformer or induction regulator By series impedance By variable voltage supply.

2. Time : Periodically switching ON and OFF the electric supply.




3. Resistance:

By varying the resistance of elements. By variable number of heating element. By series parallel or star delta arrangement of element .

21. What is the choice of frequency in dielectric heating? [CO1 – L1 – APR15] The process of heating poor conductors of electricity (dielectrics) by-

means of high-frequency electrical currents. The thermoplastic composite

to be heated forms the dielectric of a condenser to which is applied a

high-frequency (20-to-80 mc) voltage. The heat is developed within the

material rather than being brought to it from the outside, and hence the

material is heated more uniformly throughout.

22. What is meant by ARC Welding and list its types. [ CO1 – L1 – APR 2014, APR 2015, NOV 2014]

Flow of electric current through gases accompanied by heat and bright glow

due to ionization and dissipation of energy surrounding the medium.

Types :

Metal arc welding Carbon arc welding Atomic hydrogen welding Helium or Argon arc welding




PART B (16 MARKS)

1. Explain the various method of heating a material. [ CO1 – L2 ] When current is passed through a conductor, the conductor becomes hot.

When a magnetic material is brought in the vicinity of an alternating magnetic field, heat is produced in the magnetic material.

Similarly it was found that when an electrically insulating material was subjected to electrical stresses, it too underwent a temperature rise (Dielectric heating). There are various method of heating a material but electric heating is considered to be far superior for the following reasons:

(i) Cleanliness:

Due to complete elimination of dust and ash, the charges to maintain cleanliness are minimum and the material to be heated does not get contaminated.

(ii) Ease of control: With the help of manual or automatic devices, it is possible to control and

regulate the temperature of a furnace with great ease.

(iii) Uniform heating: Whereas in other forms of heating a temperature gradient is set up from the

outer surface. The core being relatively cooler, in case of electric heating, the heat is uniformly distributed and hence the charge is uniformly heated.

(iv) Low attention and maintenance cost: Electric heating equipments normally do not require much attention and

maintenance is also negligible. Hence labor charges on these items are negligibly small as compared to alternative methods of heating.

2. Explain in detail about the requirement of Heating Material. [ CO1 – L2 ] i) Low Temperature Coefficients of Resistance

Resistance of conducting element varies with the temperature, this variation should be small in case of an element.

Otherwise when switched ON from room temperature to go upto say 1200˚C, the low resistance at initial stage will draw excessively high currents at the same operating voltage.

ii) Resistance coefficient Positive

If temperature is negative the element will draw more current when hot. A higher current means more voltage, a higher temperature or a still lower




resistance, which can instability of operation.

iii) High Melting Point

Its melting point should be sufficiently higher than its operating temperature.

Otherwise a small rise in the operating voltage will destroy the element.

iv) High Specific Resistance

The resistivity of the material used for making element should be high. This will require small lengths and shall give convenient size.

v) High Oxidizing Temperature

Its oxidizing temperature should higher than its operating temperature. Otherwise oxidized layers from the surface will flake off changing the resistance of the filament and giving it a smaller life.

vi) Ductile

To have convenient shapes and sizes, the material used should have high ductility and flexibility. It should not be brittle and fragile.

vii) Should with stand Vibration

In most industrial process quite strong vibrations are produced. Some furnaces have to open or rock while hot. The element material should withstand the vibrations while hot and should not break open.

viii) Mechanical Strength

The material used should have sufficient mechanical strength of its own.




3. Classification of methods of electric heating [ CO1 – L2 ]

(i) Power Frequency Method: Direct resistance heating, indirect resistance heating, direct arc

heating, and indirect arc heating.

(ii) High Frequency Heating: Induction heating and dielectric heating.

4. Explain the various types of resistance heating. [ CO1 – L2 – MAY 2012]

Resistance Heating: This method is based upon the I 2R loss. Whenever current is passed through

a resistive material heat is produced because of I2 R loss.

There are two methods of resistance heating.They are

i) Direct Resistance Heating ii) Indirect Resistance Heating

Direct Resistance Heating: In this method of heating the material or change to be heated is taken as a

resistance and current is passed through it. The charge may be in the form of powder pieces or liquid. The two electrodes are immersed in the charge and

Coreless Type Direct Core

type Indirect Arc Heating

Direct Arc Heating

Indirect Heating Direct Heating

Induction Heating

Dielectric Heating Arc Heating

Resistance Heating

High Frequency Heating

Power Frequency Heating

Electric Heating




connected to the supply. In case of D.C or single phase A.C two electrodes are required but there will be

three electrodes in case of three phase supply. When metal pieces are to be heated a powder of high resistivity material is sprinkled over the surface of the charge to avoid direct short circuit.

The current flows through the charge and heat is produced. This method has high efficiency since heat is produced.

This method has high efficiency since heat is produced is charge itself. Though automatic temperature control is not possible in this method. But it gives uniform heat and high temperature. One of the major application of the process is salt bath furnaces having an operating temperature between 500˚C to 1400˚C.

An immersed electrode type medium temperature salt bath furnace is shown in figure. The bath makes use of supply voltage across two electrodes varying between 5 to 20 volts. In this method the current is passed through a highly resistance element which is either placed above or below the over depending upon the nature of the job

to be performed. The heat proportional to I2R losses produced in heating element delivered to the charge either by radiation or by convection.

For this purpose a special double wound transformer is required which makes use of 3Фprimary and single phase secondary. This speaks of an unbalanced load.

The variation in the secondary voltage is done with the help of an off load tapping switch of the primary side. This is necessary for starting and regulating the bath load. Advantages:

• High efficiency. • It gives uniform heat and high

temperature. Application: • It is mainly used in salt bath furnace and water heaters.




Indirect resistance heating:

Sometimes in case of industrial heating the resistance is placed in a cylinder which is surrounded by the charge placed in the jackets as shown in figure. The arrangement provides as uniform temperature.

Automatic temperature control can be provided in this case. Both A.C and D.C supplies can be used for this purpose at full mains voltage depending upon the design of heating element. Application:

This method is used in room heater, in bimetallic strip used in starters, immersion water heaters and in various types of resistance ovens used in domestic and commercial cooking.

5. Compare the salient features of Arc furnace and induction furnace. [ CO1 –

L2 - NOV/DEC 2005] Arc Furnaces:

There are two common types of arc furnaces: (1) Three-phase furnace and (2) Single phase furnace.

Three phase furnaces are used in the production of alloy steels. Single phase furnaces are used for the manufacture of gray iron casting also. Three phase furnaces are used for power ratings from 250KVA, 10,000KVA and capacities upto 25 tonne.

Generally graphite electrodes are used. As they are subjected to volatilization, they are to be replaced. The arc temperature is between 3000 and 3500˚C, so that the process is carried out between 1500˚C and 2500˚C.

The main components of a three phase furnace are: 1) Variable ratio power transformer 2) Reactors




3) Automatic current regulator 4) Control panel 5) Electric motor and tilting motor 6) Circuit breaker and connecting switches.

The chamber in which arc is struck is placed on a metal frame work. The chamber is lined inside with a refractory lining, which is acidic or basic in nature. The electrodes arc inserted from the top or sides of the chamber, and are placed in such a way as to be replaced easily or adjusted easily. To have a through mixing, the furnace is made amenable for tilting. Direct arc furnace:

The arc is struck directly with the charge, when a current flows through it and

produces intense heat, which results, in high temperature. Although some furnaces up to 100 tonne are made, generally furnaces up to 25 tonne are in general use.

Stirring action is automatic and gives a uniform product. It is used for alloy steel manufacture and gives a purer product. Merits:

When compared with cupola method, • It produces purer products • It is very simple and easy to control the composition of the final product

during refining process. Demerits:

• It is very costlier. • Even though it is used for both melting and refining but wherever electric

energy inexpensive it is economical to use cupola for melting and arc furnace for refining. Application:

The most common application of this type of furnace is to produce steel.




Indirect arc furnace

Electrodes are inserted from the sides and the heat produced is transmitted by radiation to the charge. As there is no inherent stirring action, the furnace should be rocked.

This furnace is used for only single phase supplies. Also the capacity of the furnace is limited up to 100 tonne. The furnace is rocked thoroughly to ensure, that the metal will cover the refactory lining and prevent it from reaching high temperatures. Melting of non-ferrous metals is mostly carried out in this type of furnace. In both the type of furnaces, large quantities of electrodes are used.

The energy used is about 500-800kw/tone corresponding to maximum power input, the power factor is 0.87 and efficiency 70%. Application:

• The main application of this type furnace is melting of non-ferrous metals.

6. What is meant by induction heating? With necessary diagram and derivation, explain the process of induction heating. [ CO1 – L2 - MAY 2007, APR 2014]

Induction heating: Induction heating processes make use of currents induced by electromagnetic

action in the material to be heated. Induction heating is based on the principle of transformers. There is a primary winding through which an a.c current is passed. The coil is magnetically coupled with the metal to be heated which acts as secondary. An electric current is induced in this metal when the a.c current is passed through the primary coil.

The following are different types of induction furnaces 1. Core type and 2. Coreless type

Core type is classified into three types. They are a) Direct core type b) Vertical core type and c) Indirect core type




Direct core type: The direct core type induction furnace is shown in fig.

It consists of an iron core, crucible and primary winding connected to an a.c supply. The charge is kept in the crucible, which forms a single turn short circuited secondary circuit. The current in the charge is very high in the order of several thousand amperes.

The charge is magnetically coupled to the primary winding. The change is

melted because of high current induced in it. When there is no molten metal, no current will flow in the secondary. To start the furnace molten metal is poured in the oven from the previous charge.

This type of furnace has the following drawbacks.

The magnetic coupling between the primary and secondary is very weak; therefore the leakage reactance is very high. This causes low power factor.

Low frequency supply is necessary because normal frequency causes turbulence of the charge.

If current density exceeds about 5 amps/mm2 the electromagnetic force produced by this current density causes interruption of secondary current. Hence the heating of the metal is interrupted. It is called pinch effect. The crucible for the charge id of odd shape and inconvenient from the metallurgical point of view.

The furnace cannot function if the secondary circuit is open. It must be closed. For starting the furnace either molten metal is poured

into the crucible or sufficient molten metal is allowed to remain in the crucible from the previous operation. Such furnace is not suitable for intermittent services.




7. Draw a neat sketch of AJAX-WYATT induction furnace and explain its working. [CO1 – L2 - NOV 2009, APR 2014, APR 2015]

AJAX-WYATT induction furnace It is modified type of core type induction furnace. It has a vertical channel for

the charge, thus the crucible used is also vertical. The construction of AJAXWYATT vertical furnace is shown in fig. The principle of operation is that of a transformer in which the secondary turns are replaced by a closed loop of molten metal. The primary winding is placed on the central limb of the core.

Hence leakage reactance is comparatively low and power factor is high. Inside

of the furnace is lined with refactory depending upon the charge. The top of the furnace is covered with an insulated cover which can be removed for charging. Necessary arrangements are usually made for titling the furnace to take out the molten metal. The molten metal in the 'V' portion acts as a short circuited secondary. When primary is connected to the a.c supply, high current will be accumulated at the bottom and even a small amount of charge will keep the secondary completed.

Hence chances of discontinuity of the circuit are less.

Advantage: High efficiency and low operating cost.




Since both primary and secondary are on the same central core, its power factor is better.

The furnace is operated from the normal supply frequency. Chances of discontinuity of the secondary circuit is less, hence it is useful for intermittent operations.

Applications:

This furnace is used for melting non ferrous metals like brass, zinc, tin, bronze, copper etc.

Indirect core type induction furnace

Indirect core type induction furnace is shown in fig. In this type of furnace induction principle has been used for heating metals. In such furnace an inductively heated element is made to transfer its heat to the change by radiation.

It consists of an iron core linking with the primary winding and secondary. In

this case secondary consists of a metal container forming the walls of the furnace. When the primary winding is connected to the supply, current is induced in the

secondary of the metal container.

So heat is produced due to induced current. This heat is transmitted to the charge by radiation.

The portion AB of the magnetic circuit is made up of a special alloy and is kept inside the chamber of the furnace.

The special alloy will loose its magnetic properties at a particular

temperature and the magnetic properties are regained when the alloy will cooled.

As soon as the furnace attains the critical temperature the reluctance of the magnetic circuit increases many times and the inductive effect correspondingly decreases thereby cutting off the heat supply.




The bar AB is removable type and can be replaced by other, having different critical temperature. Thus the temperature of the furnace can be controlled very effectively.

8. Explain the working of high frequency core-less induction furnace with neat

sketch. [ CO1 – L2 - NOV/DEC 2006, 2012]

Coreless induction furnace: Coreless induction furnace also operates on the principle of transformer. In

this furnace there is no core and thus the flux density will be low. Hence for compensating the low flux density, the current supplied to the primary should have sufficiently high frequency.

The flux set up by the primary winding produces eddy currents in the charge. The heating effect of the eddy currents melts the charge. Stirring of the metals takes place by the action of the electromagnetic forces. Coreless furnace may be having conducting or non conducting containers.

The container acts as secondary winding and the charge can have either conducting or non conducting properties. Thus the container forms a short circuited single turn secondary. Hence heavy current induced in it and produce heat. This heat produced is transferred to the charge by convection.

To prevent the primary winding from high temperature, refractory linings are provided between primary and secondary windings. Fig shows a coreless induction furnace in which the container is made of ceramic material and the charge must necessarily have conducting properties.

The flux produced by the primary winding produces eddy currents in the charge. The heating effects of the eddy currents melt the charge. Stirring action in




the metals takes place by the action of the electromagnetic forces. Advantages:

• Time taken to reach the melting temperature is less. • Accurate power control is possible. • Any shape of crucible can be used. • The eddy currents in the charge results in automatic stirring. • Absence of dirt, smoke, noise, etc. • Erection cost is less.

9. Explain in detail about the working of Dielectric heating: [ CO1 – L2 ] Dielectric heating is also sometimes called as high frequency capacitance

heating. If non metallic materials ie, insulators such as wood, plastics, china clay, glass, ceramics etc are subjected to high voltage A.C current, their temperature will increase in temperature is due to the conversion of dielectric loss into heat.

Equivalent Circuit Phasor Diagram

The dielectric loss is dependent upon the frequency and high voltage.

Therefore for obtaining high heating effect high voltage at high frequency is usually employed. The metal to be heated is placed between two sheet type electrodes which form a Capacitor as shown in fig. The equivalent circuit and vector diagram is also




shown in figure When A.C supply is connected across the two electrodes, the current drawn by

it is leading the voltage exactly 90˚. The angle between voltage and current is slightly less than 90˚, with the result that there is a in phase component of the current (IR).

This current produces power loss in the dielectric of the capacitor. At normal supply

frequency the power loss may be small. But at high frequencies, the loss becomes large, which is sufficient to heat the dielectric. Advantages:

• Uniform heating is obtained. • Running cost is low. • Non conducting materials are heated within a short period. • Easy heat

control. Applications: • For food processing. • For wood processing. • For drying purpose in textile industry. • For electronic sewing.

10. Explain in detail about the working of Welding. [ CO1 – L2 – APR 14 ] Welding is the process of joining two similar metals by heating. The metal

parts are heated to melting point. In some cases the pieces of metal to be joined are heated to plastic stage and are fused together. Electric welding:

In electric welding process, electric current is used to produce large heat, required for joining two metal pieces. There are two methods by which electric welding can be carried out. These are 1. Resistance welding and 2. Arc welding. Types of electric welding 1. Resistance welding

a) Butt welding b) Spot welding c) Seam welding d) Projection welding e) Flash welding

2. Arc welding

a) Carbon arc welding b) Metal arc welding




c) Atomic hydrogen arc welding d) Inert gas metal arc welding e) Submerged arc welding.

Resistance welding: In resistance welding heavy current is passed through the metal pieces to be welded.

Heat will be developed by the resistance of the work piece to the flow of current. The heat produced for welding is given by

H = I2 Rt Where,

H = Heat developed at the contact area. I = Current in amperes.

R = Resistance in ohms. t= time of flow of current.

The A.C supply is given to the primary winding of the transformer through a controlled contactor. The welding transformer is a step down transformer. The secondary voltage is in the order of 1 to 10 volts. But the current may range from 50 to 1000 amperes.

i) Butt welding: In this process heat is generated by the contact resistance between two components.




In this type of welding the metal parts to be joined end to end as shown in fig.

Sufficient pressure is applied along the axial direction. A heavy current is passed from the welding transformer which creates the

necessary heat at the joint due to high resistance of the contact area. Due to the pressure applied, the molten metal forced to produce a bulged joint. This method is suitable for welding pipes, wires and rods.

ii) Spot welding: Spot welding is usually employed for joining or fabricating sheet metal structure.

This type of joint only provides mechanical strength and is not air or water tight.

Spot welding arrangement is shown in fig. The plates to be welded are placed overlapping each other between two electrodes, sufficient mechanical pressure is applied through the electrodes. The welding current flows through electrodes tips producing a spot weld. The welding current and period of current flow depend on the thickness of the plates. iii) Seam Welding:

Rotating wheels are used to rotate the electrods. The sheets travel between these rollers. Heavy current is passd across joint. The weld is obtained which is the series of overlapping spotweld.




Projection Welding: It is in effect, a form of multi-spot welding in which a number of welds are

made simultaneously. The pieces to be welded are arranged between two flat electrodes which exert pressure as the current flows. The projections and the area with which they make contact are raised to welding heat and are joined by the pressure exerted by the electrodes. The projections are flattened during the welding.

Projection Welding

Flash Welding:

In this process, the parts to be welded are clamped to the electrode fixtures as in butt welding but the voltage is applied before the parts are butted together. As the parts touch each other, an arc is established which continues as long as the parts advance at the correct speed. This arc bursts away a portion of the material from each piece. When the welding temperature is reached, the speed of travel is increased, the power switched off and weld is upset.

Flash Welding




11. Discuss with neat diagram different types of Arc welding methods [ CO1 – L2 - NOV 2012, MAY 2012, 13, APR 15, APR 14]

Arc welding: An electric arc is the flow of electric current through gases. An electric arc

is struck by short circuiting two electrodes and then with drawing them apart by small distance. The current continue to flow across the small gap and give intense heat. The heat developed by the arc is also used for cutting of metal.

Carbon arc welding:

In this process D.C is usually employed. The electrode is made of carbon or graphite and is to be kept negative with respect of the work.




The work piece is connected to positive wire as shown in fig. Flux and filler are also used. Filler is made up of similar metal as that of metal to be welded. If the electrode is made positive then the carbon contents may flow into the weld and cause brittleness. The heat from the arc forms a molten pool and the extra metal required to make the weld is supplied by the filler rod. This type of welding is used for welding copper and its alloy. Metal arc welding:

In metal arc welding a metal rod of same material as being welded is used as an electrode.

The electrode also serves the purpose of filler. For metal arc welding A.C or D.C can be used. Electric supply is connected between electrode and work piece. The work piece is then suddenly touched by the electrode and then separated from it a little. This results in an arc between the job and the electrode. A little portion of the work and the tip of the electrode melt due to the heat generated by the arc. When the electrode is removed the metal cools and solidifies giving a strong welded joint.

Atomic hydrogen welding: In this system that is obtained from an alternating current are drawn between

two tungsten electrodes in an atmosphere of hydrogen. As the hydrogen gas passes through the arc, the hydrogen molecules are broken up into atoms and the y recombine on contact with the cooler base metal generating intense heat sufficient to melt the surface to be welded, together with the feller rod (if used). The envelope of hydrogen gas also shields the molten metal from oxygen and nitrogen and thus prevents weld metal from deterioration.




Fig 4.21 Atomic hydrogen welding

Applications: Atomic hydrogen welding, being expensive is used mainly for high grade work

on stainless steel and most non-ferrous metals.

Shield arc welding: In this system molten weld metal is protected from the action of atmosphere

by an envelope of chemically reducing or inert gas. As molten steel has an affinity for oxygen and nitrogen, it will if exposed to the atmosphere, enter into combination with these gases forming oxides and nitrides. Due to this injurious chemical combination metal becomes weak, brittle and corrosion resistance. Thus several methods of shielding have been developed.

The figure the use of a flux coating von the electrode which is addition to

producing a slag which floats on the top of the molten metal and protects if from atmosphere, has organic constituents which turn away and produce an envelope of inert gas around the arc and the weld.

4.22 Shield arc welding

Submerged Arc welding:

The submerged arc process creates an arc column between a base metallic electrode and the work piece. The arc, the end of the electrode and the molten weld




ctronics Engineering Department 111

pool are submerged in a finely divided granulated power that contains appropriate deoxidizers, cleansers and any other fluxing elements. The fluxing power is fed from a hopper that is carried on the welding head. The tube from the hopper spreads the powder in continuous mount in front of the electrode along the line of the weld.

Fig 4.23 Submerged Arc welding This flux mound is of sufficient depth to submerge completely the arc column

so that there is no splatter or smoke and the weld is shielded from all effects at atmospheric gases. As a result, the weld beads are exceptically smooth. The flux adjacent to the arc column melts and floats to the surface of molten pool

Applications:

Fabrication of pipes, boiler, pressure vessels, railroad tank cars, structural shapes.

12. Explain the working of Welding Transformers.[ CO1 – L2 - APR 15 ]

Figure shows a schematic diagram of a welding transformer having thin primary windings with a large number of turns. On the other hand, the secondary has more area of cross-section and less number of turns ensuring less voltage and very high current in the secondary. One end of the secondary is connected to the welding electrode, whereas the other end is connected to the pieces to be welded. If any high current flows, heat is produced due to the contact resistance between the electrode and the pieces to be welded. The generated heat melts a tip of the electrode and the gap between the two pieces is filled.

Electrical and Ele EEGUC





Volt-ampere Characteristic of a Welding Transformer The winding used for the welding transformer is highly reactive. Otherwise, a

separate reactor may be added in series with the secondary winding. Reactors Used with Welding Transformers

To control the arc, various reactors are used with welding transformers. Some methods to control the arc are given below:

Tapped reactor: With the help of taps on the reactor, the output current is regulated. This

has limited number of current settings. Moving coil reactor:

A moving coil reactor in which the reactive distance between primary and secondary is adjusted. The current becomes less if the distance between the coils is large.

Tapped Reactor Moving shunt reactor:

A moving shunt reactor in which the position of the central magnetic shunt can be adjusted. Change of the output current is obtained due to the adjustment of the shunted flux. Continuously variable reactor: A continuously variable reactor in which the height of the reactor is continuously varied. Greater reactance is obtained due to greater core insertion and hence the output current is less.





Saturable reactor: To adjust the reactance of the reactor, the required DC excitation is obtained from a DC controlled transducer. Reactor approaches saturation if the DC excitation current is more. Therefore, changes of current are obtained due to the change of reactance





/ phase by the

13. Calculate the time taken to melt 2 tonnes of steel in a three phase electric are furnace having the following data: Current Resistance of transformer Reactance of transforme, Latent heat of steel = 8.89 K.cals/kg

Specific heat of steel Melting point of steel =

Assume overall efficiency is 85%. Also calculate the energy consumed to melt 2 tonnes of steel. [ CO1 – H2 - NOV/DEC 2013]

Solution: Voltage drop due to transformer resistance

Voltage drop due to transformer reactance Open circuit transformer

Secondary voltage /Phase

Power drawn

secondary

Energy required to melt 3 tonnes of steel

Power actually utilized

Time required for melting steel





14. In a resistance over, 4 Nos. of 120 ohms resistance are used as heating element. Calculate the power drawn by the 4 Nos. of resistance when all are connected in series and all are connected in parallel across a 230 Volts, 50 Hz power supply. [ CO1 – H2 - NOV/DEC 2013] i) When the elements are connected in parallel

Equivalent resistance

Power drawn

ii) When the elements are connected in series Equivalent resistanc

iii) Power drawn

15. An insulating material 2cm thick and 200 cm2 in area is to be heated by dielectric heating. The material has relative permittivity of 5 and power factor of 0.05. Power required is 400 W and frequency of 40MHz is to be used. Determine the necessary voltage and the current that will flow through the material. If the voltage were to be limited to 700 V, what will be the frequency to get the same loss?

[ CO1 – H2 - May/June 2012, APR 2015]

Solution:





16. Calculate the time taken to melt the 3 tons of steel in a three phase are furnace having the following data: [ CO1 – H2 – NOV/DEC 2012 ]





Solution:

17. A furnace consuming 5KW takes 15 mins to just melt 2.5kg of aluminium and its initial temperature being 15degree celcius. Find th efficiency of the furnace when the specific heat of aluminium is 0.212 cal / gm / degree celcius, melting point is 658degree celcius and latent heat of fusion is 320 J/gm. [ CO1 – H2 -





May / June 2013, NOV 2014, APR 2014]

18. Explain the method of controlling temperature in resistance heating. [ CO1 – L2 - Nov’13 ]

Temperature control is necessary in resistance ovens/furnace – temperature may have to be kept constant or varied according to the requirements. The heat developed depends upon I2Rt. There are three ways by which temperature can be controlled.

1. Voltage or current: By varying voltage across element

Using auto – transformer or induction regulator

By series impedance

By variable voltage supply Voltage can be varied by using tapped transformer for supply to the

oven or by using a series resistance so that some voltage is dropped across this series resistor. 2. Time: Periodically switching ON and OFF the electric supply

An ON – OFF switch can be used to control the temperature. The time





for which the oven is connected to the supply and the time for which it remains





isolated from supply will determine the temperature.

The temperature rise is give n by

The higher the ratio the larger will be the temperature of the oven. 3. Resistance: By varying the resistance of the elements

By varying number of heating elements

By series parallel or star – delta arrangement of elements. In single phase supply, various series parallel combination along with some

resistance being in the circuit, other out of the circuit will give various temperatures.

In three phase ovens, different connection with star – delta arrangements will give different temperatures.

19. Write short notes on infrared or radiant heating? [ CO1 – L2 ]

In this method of heating, the heat transfer takes place from the source to the body to be heated through radiation, for low and medium temperature applications, Whereas in resistance ovens, the heat transfer to the charge partly by convection and partly by radiation.

In the radiant heating, the heating element consists of tungsten filament lamps together with reflector and to direct all the heat on the charge. Tungsten filament lamps are operating at 2300ºC instead of 3000ºC to give greater portion of infrared radiation and a longer life.

The radiant energy is mainly used for drying enamel or painted surfaces. The high concentration of the radiant energy enables the heat to penetrate the coating of paint or enamel to a depth sufficient to dry it out without wasting energy in the body of the workpiece. Here the heat absorption remains approximately constant whatever the charge temperature.

The lamp ratings used are usually between 250 W and 1000W and are operating at voltage of 115V, lower voltage results in robust filaments. With is arrangement, charge temperature obtained is between 200ºC to 300ºC and the heat emission intensity obtained is about 7000 W/m2. Applications:

Drying of paints

Drying of radio-cabinets and wood furniture

Drying of pottery, paper, textiles ( where moisture content is not large)

Low temperature heating of plastics.

Dehydration processes Advantages:





Compactness of heating units





rapid heating

Flexibility

Safety

20. What are the requirements of good welding? [ CO1 – L2 - Nov’13 ] Following factors have to be regarded for a good welding result:

Type of tungsten electrode

Diameter of electrode

Distance of electrode to workpiece

Clean and correct grinding angle of electrode tip

Types of Tungsten Electrodes: For the selection of the correct electrode the most common types of electrodes

are listed below with their short description and colouring. Tungsten is used as electrode material due to its high melting point of ~ 3.400° C.

By mixing oxides with pure tungsten (doping) the characteristics and the life time of electrodes can be influenced.

Short symbol colour Oxide addings in weight % Tungsten content

W (WP-00) green

tungsten99,8%

WT-10 * yellow 1,0% Thorium (ThO2) Balance tungsten

WT-20 * red 2,0% Thorium (ThO2) Balance tungsten

WT-30 * purple 3,0% Thorium (ThO2) Balance tungsten

WT-40 * orange 4,0% Thorium (ThO2) Balance tungsten

WZ-08 white 0,8% zirconium (ZrO2) Balance tungsten

WC-20 grey 2,0% Cerium (CeO2) Balance tungsten

WL-10 black 1,0% Lanthanum (LaO2) Balance tungsten

WL-15 gold 1,5% Lanthanum (LaO2) Balance tungsten





WL-20 blue 2,0% Lanthanum (LaO2) Balance tungsten

WM-20 turquoise 2,0% Lanthanide Balance tungsten

The cerium electrode WC 20 is a universal electrode for almost all applications:

DC current and AC current

non-alloyed steel

high-alloyed steel

Aluminum alloys

Titanium alloys

Nickel alloys

Copper alloys

Magnesium alloys

Diameter and length of electrodes The ampacity of electrode depends on its diameter, type of current and

polarity, the alloy additions of electrode and the grinding angle. When overloading the tip of electrode, a distinct molt drop develops at the electrode end which may go over into the molten pool. A load at the electrode tip being too low causes an unsteady burning arc.Standard diameter for tungsten electrodes are: 1,0 - 1,6 - 2,0 - 2,4 - 3,0 - 3,2 - 4,0 - 4,8 - 6,0 - 6,4 mm with standard lengths of 50 - 75 - 150 - 175 mm

Distance from electrode to workpiece

With a different distance of electrode to workpiece the electrode voltage changes, which will lead to different welding results, too. It has to be taken care of, that a constant distance is being kept towards the workpiece. It has proven practical to have a distance between electrode and workpiece which is identical to the electrode diameter used: With an electrode of 2,4 mm diameter => 2,4 mm distance to workpiece.

21. Explain the characteristics of a welding generator [ CO1 – L2 - Nov’15 ] Output Characteristics:

Generator type power sources generally provide welding current adjustment in broad steps called ranges. A rheostat or other control is usually placed in the field circuit to adjust the internal magnetic field strength for fine adjustment of power output, because it regulates the strength of the magnetic field, will also change the open circuit voltage.

When adjusted near the bottom of the range, the open circuit voltage will





normally be substantially lower than at the high end of the range.

The basic machine does not often have the dynamic response required for shielded metal arc welding. Thus, a suitable inductor is generally inserted in series connection in one leg of the output from the rectifier. Welding generators do not normally require an inductor. There is a limited range of overlap normally associated with rotating equipment where the desired welding current can be obtained over a range of open circuit voltages.

If welding is done in this area, welders have the opportunity to better tailor the power supply to the job. With lower open circuit voltage, the slope of the curve is less. This allows the welder to regulate the welding current to some degree by varying the arc length. This can assist in weld-pool control, particularly for out-of-position work.

Some welding generators carry this feature beyond the limited steps

described above. Generators that are compound wound with separate and continuous current and voltage controls can provide the operator with a selection of volt-ampere curves at nearly any amperage capability within the total range of machine.

Thus, the welder can set the desired arc voltage with one control and the





arc current with another. This adjusts the generator power source to provide a





static volt-ampere characteristic that can be "tailored" to the job throughout most of its range. The volt-ampere curves that result when each control is changed independently are shown in Figures

Welding power sources are available that produce both constant current and constant voltage. These units are used for field applications where both are needed at the job site and utility power is not available. Also, many new designs use electronic solid-state circuitry to obtain a variety of volt-ampere characteristics.

22. Compare DC welding and AC welding. [ CO1 – L2 - May’13, Apr’14 ] Sl.no Factors D.C welding A.C welding

1. Equipment Motor-generator set or rectifier is required in case of availability of a supply; otherwise oil generator set is required.

Only a transformer is required.

2. Prime Cost Two or three times of transformer.

Comparatively low

3. Operating efficiency

Low High 85%

4. No-Load voltage

Low Too high

5. Power factor High Low

6. Heating Uniform heating Non-uniform heating

7. Arc stability Higher -





8. Arc blow Pronounced Not so pronounced with A.C





23. An insulating material 2cm thick and 150 cm2 in area is to be heated by dielectric heating. The material has relative permittivity of 4 and power factor of 0.04. Power required is 400 W and frequency of 40MHz is to be used. Determine the necessary voltage and the current that will flow through the material. If the voltage were to be limited to 700 V, what will be the frequency to get the same loss? [ CO1 – H2 - Apr’15 ]

Given Data:

Thickness of an insulating material (d) = 2cm

Area of an insulating material (A) = 150cm2

Relative permittivity Kr = 4

Power factor cosΦ = 0.04

Power P = 400w

Frequency f = 40 MHz

Voltage limited V2 = 700V

Current flowing through the material

Heat produced

Assuming absolute permittivity K0 =





24. A furnace consuming 5KW takes 15 minutes to just melt 2.5 kg of

Aluminum, the initial temperature . Find the efficiency of the

furnace when the specific heat of Aluminum is

Fusion is 320

J/gm. [ CO1 – H2 - May’13 ] Given data:

1. Determine the efficiency of a high frequency induction furnace which takes 10 minutes to melt 1.815 Kg of aluminium the input to the furnace being 5 KW and

Heat required to melt 2.5 kg of aluminium =

Converting J to KWh: 1J = 2.78





the initial temperature 15 degree centigrade, specific heat of Aluminium is 0.212 cal/Kg/˚C, melting point is 660˚C and latent heat of fusion of aluminium =

Kcal/Kg. (Apr’14) Given data:

25. Estimate the efficiency of a high frequency induction furnace which takes 12 minutes to melt 1.3 Kg of aluminium. The input to the furnace being 4.5 KW and the initial temperature 15 °C. Take specific heat of Aluminium is 880 J/Kg/˚C, melting point of Al is 660˚C and latent heat of fusion of Al is 32KJ/Kg. [ CO1 – H2 - Nov’14 ]

Given data:








26. A 15 kW, 220V, single phase resistance oven employs circular nickel -

chromium wire for its heating element. The temperature is not to exceed

1230ºC and its temperature of the charge to be 500ºC. Calculate the size

and length of the wire. Assume radiating efficiency = 0.6, emissivity = 0.9,

specific resistance of nickel - chrome wire = 101.6 x 10-6 Ωcm. [CO1 – H2 -

Nov’15 ]

Given Data:

Power P = 15kW

Voltage V = 220V

Temperature T1 = 1230ºC → 273 + 1230 = 1503K

Temperature T2 = 500ºC → 273 + 500 = 773K

Radiating efficiency ηrad = 0.6

Emissivity = 0.9

Specific resistance ρ = 101.6 x 10-6 Ωcm

We know that,






Now,

Total heat dissipated/sec. = Electrical power input

Multiplying

From ,





Unit – IV

Solar Radiation and Solar Energy Collectors

Part – A

1. .What do you meant by Solar Constant? [ CO2 – L1 ] THE SOLAR CONSTANT

The solar constant, Gsc, is the energy from the sun, per unit time, received on a unit area of surface perpendicular to the direction of propagation of the radiation at the earth's mean distance from the sun, if the earth’s the atmosphere is fully transparent. It may be viewed on any unit surface normal to sun’s rays on a sphere of radius equal to the sun-earth mean distance, thus alleviating the difficulty in imagining a fully transparent atmosphere around the earth. The recently reported value of the solar constant is 1367 W/m2 .

2. Define Wien Displacement Law. [ CO2 – L1 ] The peak wavelength of radiation emitted from an object is dependent upon the temperature of the object and can be calculated using the Wien Displacement Law when the temperature of the object is known. (In astronomy these are solid objects such as stars and planets.)

Wien Displacement Law:

maximum = 2897 / T

maximum = The peak wavelength of energy in micrometers T = The temperature of the object radiating energy

Using this law, the peak wavelength of radiation emitted from an object is inversely proportional to the temperature of the object. The irradiance or radiation output of an object can be calculated using the Stefan-Boltzman Law when the temperature is known.

3. Define Stefan-Boltzman Law. [ CO2 – L1 ]

Stefan-Boltzman Law: E = T4

E = Surface Irradiance of the object





* = Emissivity of the object

= Stefan-Boltzman Constant (5.67x10-8W/m2K4 ) T = Temperature of the object

Emissivity is the factor of how well a surface can absorb and emit energy. Emissivity numbers range from 0 to 1. Very black objects such as charcoal have an emissivity near 1 while shiny objects have an emissivity near 0. The Wien Displacement & Stefan-Boltzman laws strictly apply only to black bodies.

4. What is meant by Insolation? [ CO2 – L1 ]

Insolation: Solar Radiation Striking the Surface

I = S cos Z

I= Insolation

S~ 1000 W/m2 (Clear day solar insolation on a surface perpendicular to incoming solar radiation. This value actually varies greatly due to atmospheric variables.) Z = Zenith Angle (Zenith Angle is the angle from the zenith (point directly overhead) to the Sun's position in the sky. The zenith angle is dependent upon latitude, solar declination angle, and time of day.)

Z = cos-1 (sin sin + cos cos cos H)

= Latitude H = = Hour Angle = 15o x (Time - 12) (Angle of radiation due to time of day.

5. What are the main components of Flat plate collector [ CO2 – L1 ]

Flat plate collector: It usually consists of five main components

i. An absorber [metal-galvanized iron, aluminum, copper or plastic].

ii. Tubes or pipes for conducting or directing the heat transfer fluid.

iii. One or more covers[glass of 4 mm thickness]

iv. Insulation [glass wool, aabestos wool] to minimize the downward heat loss from the absorbing plate.

v. Casing [wood or aluminum], encloses all the components of the collector. Generally flat plate collectors are framed sandwich structures, mounted on roofs or





sloping walls. Cost is acceptable for common use.

6. What are the advantages and disadvantages of solar energy. [ CO2 – L1 ]

Advantages of Solar Energy.

Solar energy is free from pollution

They collect solar energy optically and transfer it to a single receiver,thus minimizing thermal-energy transport requirements

They typically achieve concentration ratios of 300 to 1500 and so are highly efficient both in collecting energy and converting it to electricity.

The plant requires little maintenance or help after setup

It is economical Disadvantages of Solar Energy.

1. Available in day time only 2. Need storage facilities 3. It needs a backup power plant 4. Keeping back up plants hot includes an energy cost which includes coal

burning

7. Give the expression for collector efficiency. [ CO2 – L1 ]

where:

= rate of (useful) energy output (W) Aa = aperture area of the collector (m2) Ia = solar irradiance falling on collector aperture (W/m2)

8. Define Concentrating Collectors [ CO2 – L1 ] Concentrating Collectors: Collectors are oriented to track the sun so that the beam radiation will be directed onto the absorbing.

9. Define Surface Collector [ CO2 – L1 ] Surface Collector: Receiver and the concentrator Receiver: Radiation

is absorbed and converted to some other energy form (e.g. heat).





10. Define Surface Collector [ CO2 – L1 ] Concentrator: Collector that directs radiation onto the receiver. The

aperture of the concentrator is the opening through which the solar radiation enters the concentrator.

11. What are the types of Concentrators. [ CO2 – L1 ]

Planar and non-concentrating type - which provides concentration ratios of up to four and are of the flat plate type. Line focusing type - produces a high density of radiation on a line at the focus. Cylindrical parabolic - concentrators are of this type and they could produce concentration ratios of up to ten. Point focusing type - generally produce much higher density of radiation in the vicinity of a point. Paraboloids are examples of point focus concentrators.

12. What are the benefits of Wind Energy. [ CO2 – L1 ] Wind energy is one of the fastest growing sources of new electricity

generation in the world today. These growth trends can be linked to the multi- dimensional benefits associated with wind energy.

Green Power: The electricity produced from wind power is said to be "clean" because its generation produces no pollution or greenhouse gases. As both health and environmental concerns are on the rise, clean energy sources are a growing demand.

Sustainable: Wind is a renewable energy resource, it is inexhaustible and requires no "fuel" besides the wind that blows across the earth. This infinite energy supply is a security that many users view as a stable investment in our energy economy as well as in our children's' future.

Affordable: Wind power is a cost-competitive source of electricity, largely due to technological advancements, as well as economies of scale as more of these machines are manufactured and put online around the world.

Economic Development: As well as being affordable, wind power is a locally- produced source of electricity that enables communities to keep energy dollars in their economy. Job creation (manufacturing, service, construction, and operation) and tax base increase are other economic development benefits for communities utilizing wind energy.





PART B (16 MARKS)

1. Expalin in detail about the solar radiation entering the earth’s surface. [CO2 –L2] SOLAR RADIATION ENTERING EARTH SURFACE:

INTRODUCTION Energy from the Sun reaching the Earth drives almost every known physical and biological cycle in the Earth system. By making solar radiation calculations and examining radiation measurements, students can gain a better understanding of many physical cycles and concepts associated with the Earth system.

Irradiance - The amount of electromagnetic energy incident on a surface per unit time per unit area. In the past this quantity has often been referred to as "flux". * When measuring solar irradiance (via satellite), scientists are measuring the amount of electromagnetic energy incident on a surface perpendicular to the incoming radiation at the top of the Earth's atmosphere, not the output at the solar surface.

Solar Constant - The solar constant is the amount of energy received at the

top of the Earth's atmosphere on a surface oriented perpendicular to the Sun’s rays (at the mean distance of the Earth from the Sun). The generally accepted solar constant of 1368 W/m2 is a satellite measured yearly average.

Insolation - In general, solar radiation is received at the Earth's surface. The rate at which direct solar radiation is incident upon a unit horizontal surface at any point on or above the surface of Earth. *I will refer to insolation as direct solar radiation at the Earth's surface.

The solar constant is an important value for current studies of global radiation

balance & climate models. The problem that faces scientists studying Earth’s radiation budget and climate is that while satellites can “accurately” measure solar irradiance and calculate a solar constant, the surface insolation is much more difficult to assess. When the solar constant is calculated there are four major problems in trying to relate this radiation intensity to its effect on the Earth's surface or surface insolation.

First, the calculation is made for the top of the atmosphere and not for the

surface of the Earth.





Second, the calculation assumes that the surface receiving the radiation is perpendicular to the radiation.

Third, the calculation assumes that the surface receiving the radiation is at a mean Sun-Earth distance.

Fourth, the calculation assumes that radiation emission from the Sun remains constant.

Trying to relate calculations made for the top of the atmosphere to the surface is a

problem because up to 70% of incoming radiation can be blocked by the atmosphere and cloud cover. In attempts to create global energy budget models, scientists must insert estimations for the amount of energy actually reaching the surface.

Assuming that the surface receiving the radiation is perpendicular to the incoming radiation is a problem because this is a rare occasion even at tropical latitudes due to the rotation of the Earth (time of day), tilt of the Earth's axis in relation to the incoming solar radiation (season), and the latitude and orientation of the surface. All of these factors change the angle of the surface receiving the radiation, which changes the intensity of the energy received.

Assuming that the radiation emission of the Sun is constant is a problem because this value fluctuates with cycles in solar activity. NASA satellites have measured incoming radiation since 1978 and have recorded changes in solar irradiance. This data can be accessed on the internet from Goddard Space Flight Center.

SOLAR RADIATION AND THE ELECTROMAGNETIC SPECTRUM

The electromagnetic spectrum consists of the entire range of frequencies and wavelengths at which electromagnetic waves can travel. The electromagnetic spectrum organizes energy types by wavelength and frequency. The peak wavelength of radiation emitted from an object is dependent upon the temperature of the object and can be calculated using the Wien Displacement Law when the temperature of the object is known. (In astronomy these are solid objects such as stars and planets.)

Wien Displacement Law:

maximum = 2897 / T

maximum = The peak wavelength of energy in micrometers T = The temperature of the object radiating energy





Using this law, the peak wavelength of radiation emitted from an object is inversely proportional to the temperature of the object. The irradiance or radiation output of an object can be calculated using the Stefan-Boltzman Law when the temperature is known.

Stefan-Boltzman Law: E = T4

E = Surface Irradiance of the object

* = Emissivity of the object

= Stefan-Boltzman Constant (5.67x10-8W/m2K4 ) T = Temperature of the object

*Emissivity is the factor of how well a surface can absorb and emit energy. Emissivity numbers range from 0 to 1. Very black objects such as charcoal have an emissivity near 1 while shiny objects have an emissivity near 0.

The Wien Displacement & Stefan-Boltzman laws strictly apply only to black

bodies. Black bodies are capable of absorbing and emitting radiation at all wavelengths. Because the Sun & Earth are not perfect black bodies, applying these laws to them only allows approximate values to be obtained. The fact that the Sun is not a perfect black body is especially important when studying solar cycles. The most significant variations in solar radiation during these cycles occur in the UV & X-Ray portions of the solar spectrum. In order to compare solar emissions to black body emissions at the same temperature go to the Solar Spectrum/Black Body Graph. SOLAR RADIATION ENTERING THE EARTH SYSTEM

In order to study the effects of solar radiation on the Earth system, it is necessary to determine the amount of energy reaching the Earth's atmosphere & surface. Once the surface irradiance of the Sun is determined the amount of energy reaching the top of the Earth's atmosphere can be calculated using the Inverse Square Law. The average amount of energy received on a surface perpendicular to incoming radiation at the top of the atmosphere is the solar constant. (*While this calculation can lead to a better student understanding of the Inverse Square Law, the accepted value is a yearly average from NASA satellite measurements.)

Solar Radiation Striking the top of the Earth's Atmosphere The Inverse Square Law is used to calculate the decrease in radiation intensity

due to an increase in distance from the radiation source.

Inverse Square Law: I = E(4 x R2)/(4 x r2)

http://bigmac.civil.mtu.edu/public_html/classes/ce459/lectures/lecture3.html





I = Irradiance at the surface of the outer sphere E = Irradiance at the surface of the object (Sun)

4 x R2 = surface area of the object

4 x r2 = surface area of the outer sphere

In order to calculate the solar constant the following equation is used: So = E(Sun) x (R(Sun) / r)2

So = Solar Constant E= Surface Irradiance of the Sun R= 6.96 x 105 km = Radius of the Sun r = 1.5 x 108 km =Average Sun-Earth Distance

Insolation: Solar Radiation Striking the Surface

I = S cos Z

I= Insolation S~ 1000 W/m2 (Clear day solar insolation on a surface perpendicular to incoming solar radiation. This value actually varies greatly due to atmospheric variables.)

Z = Zenith Angle (Zenith Angle is the angle from the zenith (point directly overhead) to the Sun's position in the sky. The zenith angle is dependent upon latitude, solar declination angle, and time of day.)

Z = cos-1 (sin sin + cos cos cos H)

= Latitude H = = Hour Angle = 15o x (Time - 12) (Angle of radiation due to time of day. Time is given in solar time as the hour of the day from midnight.)

= Solar Declination Angle

Solar Declination Angles for the Northern Hemisphere Vernal Equinox Mar. 21/22 = 0o

Summer Solstice Jun. 21/22 = +23.5o

Autumnal Equinox Sept. 21/22 = 0o

Winter Solstice Dec. 21/22 = -23.5o





2. Explain how the energy is cultivated using solar power and explain its collector system. [CO1 – L2 – APRIL 2014]

Sun is the primary source of energy. The earth receives

units of energy from the sun annually. Three broad categories of possible large scale applications are:

a. Heating and cooling of residential and commercial buildings.

b. Chemical and biological conversion of organic material.

c. Conversion of solar energy to electricity.

Residential cooling and heating: A flat plate collector is located on the roof of a house, which collects the

solar energy. The cooling water is pumped through the tubes of the solar collector. The heat is transferred from the collector to the water and the hot water is stored in a storage tank, located at ground level or in the basement of the house. Hot water is utilized to heat or cool the house by adjusting the automatic valve. Photosynthesis production of energy sources:

Solar energy can be transferred into chemical energy in plants and trees through photosynthesis, which is the basis of the world‟s fossil fuels.

Solar power plant:

Central Receiver Solar Power Plant.





It is known that only a fraction of the energy radiated by the sun reaches the earth. But in the atmosphere radiation will be more. Satellite revolving around the earth will receive energy for all the 24 hours and will not affected by the weather condition. The solar panels are installed on the satellite, may vary in area from 16 to 100 square meter based on plant capacity. The solar cells generates DC electric power and transmit it by means of microwaves [10cm in wavelength, 2-3 GHz in frequency], keeps the losses at minimum and this energy will be converted into high voltage DC or commercial frequency electric power.

Solar power plant The antenna is used for transmission [1km in diameter in sending end and 7- 10km in diameter in receiving end] with 55 to 75% of efficiency. The solar cells operates on the principle of photo electricity that is electrons are liberated from the surface of a body when light is incident on it. This is works on schottky effect.





Distributed (Parabolic) Solar Collector

. Distributed (Parabolic) Trough Solar Power Plant.

The solar cells in space will be used for long duration, hence the cells are actuated by both sun rays and diffuse light. In cold weather, the decreased luminous flux is compensated by higher efficiency. Hence efficiency increases with decrease in temperature. [15-20% in efficiency]. Even though sun energy available is free of cost, the fabrication coat and installation of system is too high, so plastic materials are being used more.

The efficiency of solar cells depends on efficiency of collection of solar energy using working fluid (air, water etc). There are 2 main classes of collectors.

Solar concentrators and ii. Flat plate collector.

Solar concentrators: It is collecting device having high flux on the absorber surface than flux impinging on concentrator surface. Optical concentration is by reflecting, referacting elements, positioned to concentrate the incident flux onto a suitable absorber. Due to apparent motion of the sun, it will be in position to redirect the sun rays onto the absorber if they are stationary, so it needs a tracking device. A solar concentrator consists of

i. Reflecting or refrating surface

ii. An absorber

iii. Fluid flow system to carry the heat

iv. Cover around absorber

v. Insulation for the unirradiated portion of the absorber.

vi. Self supporting structural capacity and well adjusted tracking mechanism.





Flat plate collector: It usually consists of five main components

i. An absorber [metal-galvanized iron, aluminum, copper or plastic].

ii. Tubes or pipes for conducting or directing the heat transfer fluid.

iii. One or more covers[glass of 4 mm thickness]

iv. Insulation [glass wool, aabestos wool] to minimize the downward heat loss from the absorbing plate.

v. Casing [wood or aluminum], encloses all the components of the collector. Generally flat plate collectors are framed sandwich structures, mounted on roofs or sloping walls. Cost is acceptable for common use.

Advantages of Solar Energy.

Solar energy is free from pollution

They collect solar energy optically and transfer it to a single receiver,thus minimizing thermal-energy transport requirements

They typically achieve concentration ratios of 300 to 1500 and so are highly efficient both in collecting energy and converting it to electricity.

The plant requires little maintenance or help after setup

It is economical Disadvantages of Solar Energy.

1. Available in day time only 2. Need storage facilities 3. It needs a backup power plant 4. Keeping back up plants hot includes an energy cost which includes coal

burning

4. Explain the Energy Balance and Efficiency of Flat-Plate and Concentrating Collector in Detail. [ CO2 – L2 ]

Energy Balance The detailed energy balance for a photovoltaic cell in a panel can be written replacing the receiver/absorber temperature term with the temperature of the cell:





wher e:

Tc- temperature of the cell (K) Ac - area of the cell surface (m2)

Collector Efficiency At this point in our discussion of how solar energy is collected, we will define

the basic performance parameter, collector efficiency. We will then describe how this is measured and then how these measurements can be combined into an analytical model to predict collector output. This is done in general terms, applicable to flat-plate and concentrating collectors for either thermal or photovoltaic applications.

The solar energy collection efficiency, of both thermal collectors and

photovoltaic collectors is defined as the ratio of the rate of useful thermal energy leaving the collector, to the useable solar irradiance falling on the aperture area. Simply stated, collector efficiency is:

where:

= rate of (useful) energy output (W) Aa = aperture area of the collector (m2) Ia = solar irradiance falling on collector aperture (W/m2)

This general definition of collector efficiency differs depending on the type of collector. The rate of useful energy output from thermal collectors is the heat addition to a heat transfer fluid as defined by Equation (5.2) whereas the useful energy output of a photovoltaic collector is electrical power defined in Equation (5.11). The incoming solar irradiance falling on the collector aperture, Ia, multiplied by the collector aperture area represents the maximum amount of solar energy that could be captured by that collector. Optical Efficiency

In some of the development that follows, we will use the concept of optical efficiency. The optical efficiency of a solar collector is defined as the rate of optical (short wavelength) energy reaching the absorber or receiver, divided by the appropriate solar resource. Dividing Equation (4.3) by Equation (4.4), we have for optical efficiency;

http://www.powerfromthesun.net/Book/chapter05/chapter05.html#ZEqnNum212757

http://www.powerfromthesun.net/Book/chapter05/chapter05.html#ZEqnNum208166





This term is often used in separating out the non-thermal performance of a solar collector. It also forms the maximum limit for collection efficiency as will be seen in our discussions below.

Flat-plate Collectors

Since flat-plate collectors (both thermal and photovoltaic) are capable of absorbing both direct (beam) and diffuse solar irradiance, the appropriate aperture irradiance is the global (total) irradiance falling on the collector aperture.

where It,a is the global irradiance on a collector aperture.

Adding the appropriate useful energy term, we have for thermal and photovoltaic flat- plate collectors, the following definitions of collector efficiency

Flat-plate thermal collectors:

Flat-plate photovoltaic collectors:

Concentrating Collectors Concentrating collectors on the other hand can only concentrate direct (beam)

solar irradiance and therefore the appropriate irradiance term is direct (beam) normal solar irradiance, reduced by the cosine of the angle of incidence. For two-axis tracking collectors, the angle of incidence is zero.





where Ib,a is the direct (beam) irradiance on a collector aperture.

Adding the appropriate useful energy term , we have for thermal and photovoltaic concentrating collectors, the following definitions of collector efficiency:

Concentrating thermal collectors:

and

Concentrating photovoltaic

collectors:

Non-imaging Concentrators One further caveat must be mentioned for non-imaging concentrators such as vee- troughs, conical concentrators and compound parabolic concentrators (CPC). Since these accept some diffuse solar irradiance, the appropriate Ia would be the direct (beam) normal, reduced by the cosine of the angle of incidence plus the circumsolar diffuse solar irradiance falling within the acceptance angle of that specific concentrator.

where the terms Id,aa and Ir,aa are the sky diffuse and reflected diffuse energy that are available within the acceptance angle of the non-imaging concentrator. The definition of these terms is beyond the scope of this book. However, this solar irradiance may





be easily measured by reducing the tube length of a pyrheliometer and attaching this to the aperture of the non-imaging concentrator.

5. Give the detailed performance analysis of Concentrating Collector. [ CO2 – H3 ]

Models of collector performance that will predict collector output under varying solar irradiance, operating temperature and weather conditions, are important to the system designer. This type of model can be used in system performance prediction programs such as SIMPLES, to predict the output of the collector field.

The most prevalent solar collector performance model is called the ‘delta-

T over I curve’ and permits prediction of useful energy out under varying solar irradiance, ambient temperature and system operating temperature. Although its required simplifications work well for low- temperature flat-plate collector systems, it has been modified for use with higher temperature systems as described below. This model has no applicability to photovoltaic collectors.

Figure summarizes the performance of a typical flat-plate collector. Plots a)

and b) show that, the rate of useful energy produced decreases as the temperature of the fluid entering the collector increases. As expected the higher the solar irradiance level, the higher the rate of useful energy produced. Curve b) shows the same data plotted in inverse order. Plots c) and d) show collector efficiency rather than useful energy produced. Curve c) shows that efficiency is independent of solar irradiance when fluid inlet temperature equals ambient temperature. This point defines the optical efficiency. Curves c) and d) can be combined into a single curve e) defining the relationship between collector efficiency, solar irradiance and fluid inlet temperature as described below.

Flat-plate Collectors, The Curve

The following simplifications can be made for a flat-plate collector operating at low temperatures (below approximately 90oC):

There is no reflection of incoming solar irradiance and therefore Γ and ρ are

eliminated. The aperture area is the same as the absorber area (Aa = Ar)

Radiation heat loss can be combined with convection and conduction into a single overall heat loss coefficient, UL (W/m2K). This coefficient is a constant for the particular collector being modeled. (Temperature dependence of this term due to natural convection and radiation causes non-linear affects to occur and will be





discussed below).





If it is further assumed that FR, τ, α, UL are constants for a given collector and flow rate, then the efficiency is a linear function of the three parameters defining the operating condition; solar irradiance, fluid inlet temperature and ambient air temperature. This is the single line shown on curve e) in Figure

The slope of this line represents the rate of heat loss from the collector. For example, collectors with cover sheets will have less of a slope than those without cover sheets. The intercept of the line on the efficiency axis is sometimes called the optical efficiency. Most low-temperature solar collector performance data are presented in terms Equation (4.21).

It should be remembered that both of these terms are multiplied by the heat removal factor, FR making them a function of flow rate. Therefore the flow rate for

any curve should be specified.





Curve a)

Curve b)





Curve c)





Figure shows Performance of a typical flat-plate thermal collector (one glass cover, black-painted absorber, water transfer fluid and ambient temperature 25oC). Curve a)

shows the performance as a function of the variable described above. Curve b) and c) for the same collector, show how the output varies with fluid temperature and irradiance. Curves b) and c) are derived from curve a).

There are two interesting operating points on Figure, curve a). The first is the

maximum collection efficiency, called the optical efficiency. This occurs when the fluid

inlet temperature equals ambient temperature. For this condition, the value is zero. This is the test point described above as part of a collector performance measurement procedures.

The other point of interest on Figure curve a) is the intercept with the axis. This point of operation can be reached when useful energy is no longer removed from the collector, a condition that can happen if fluid flow through the collector stops (power failure). In this case, the optical energy coming in must equal the heat loss, requiring that the temperature of the absorber increase until this balance occurs. This maximum temperature difference or ‘stagnation temperature’ is defined by this point. For well-insulated collectors or concentrating collectors the stagnation temperature can reach very high levels causing fluid boiling and, in the case of concentrating collectors, the absorber surface can melt.

Curves b) and c) of Figure are derived from curve a); the curve. Curve b) shows the dependence of collector output (efficiency) as a function of the water inlet temperature for different levels of irradiance. Note that more energy is derived from a collector when the water temperature entering the collector is low. It is important in solar energy system design to only heat the water or heat transfer fluid to the lowest temperature consistent with system output requirements.

Curve c) of Figure shows the relationship of system output to the irradiance level. It shows that systems operating at low temperature levels (such as swimming pool heating systems) can derive heat from the sun at very low levels of irradiance. Higher temperature operating systems such as domestic hot water systems only derive energy from the sun when the irradiance level is high. Further study of curves b) and c) will reveal most of the important aspects of the thermal design of solar energy systems.





Parabolic Troughs, The Non-Linear Curve

For parabolic trough collectors operating at higher temperatures, the above assumptions must be modified. In order to better approximate the temperature at which heat loss takes place, an average receiver temperature, is calculated as the average between the inlet and outlet fluid temperatures:

The addition of the term to Equation is a simplified attempt to account for the fact that UL is, in fact, not independent of temperature. In fact, the temperature dependence of UL is quite complex. The attempt here is to utilize available test data by using a curve fitting technique. Figure shows typical values and trends for these two concentrators.





Collector efficiency for a typical parabolic trough and a typical parabolic dish collector.

The largest source of error arises from the fact that, in using a simple model,

one is usually extrapolating along the curve, not interpolating. As discussed

earlier, the test data from which a curve is generated is obtained at a constant value of Ia (usually near 1,000 W/m2). This is about as large as Ia becomes. Thus, as Ia decreases in the course of an analysis due to either poor solar irradiance or at

large incidence angles, the quantity increases, usually to an extent much larger than the range of test data.

Now if the curve is indeed close to a straight line, extrapolation may not introduce much error. If there is significant curvature, however, extrapolation can lead to large errors and simple models should be used with caution. Experience has

indicated that, if the curve is reasonably straight, the computed collector performance agrees fairly well with all-day test results, which include significant incidence angle effects. It is noteworthy that in some of the literature, non-linearity on

the test data is fit with a polynomial curve having a 2 term. Although this expression may indeed produce a reasonable fit to the data, there is really no physical reason to introduce an I 2 functional dependence.





6. Explain in detail about the estimation of average solar radiation. [ CO2 – L2 ]

To use a collector performance model data with some degree of confidence, it is important to understand the way in which such data are obtained and what limitations this might impose on how the data can be used. Below we describe the three most common methods of measuring collector performance. There are collector test standards available that specify both the experimental setup and the testing procedure. The most commonly accepted standards are listed in the References and Bibliography at the end of this chapter.

Testing is performed only on clear days when the solar irradiance level is high and constant. Prior to taking measurements, hot heat-transfer fluid is circulated through the absorber or receiver to bring it up to the test temperature. For a flat-plate collector, the test flow rate is generally specified by the test procedure in use. In the case of parabolic trough testing, turbulent flow is maintained within the receiver tube to ensure good heat transfer between the fluid and the wall of the receiver tube. A measurement is made only when the collector is at steady state, which is indicated by a constant rise in heat transfer fluid as it flows through the receiver. Thermal Performance Measurements

In a typical test to determine solar collector efficiency, the collector aperture is aligned as close as possible to normal to the incident direct (beam) solar irradiance. This is done to eliminate any uncertainties due to off-normal incidence angle effects. Figure is a photograph of a turntable at Sandia National Laboratory, Albuquerque used to align the aperture of a parabolic trough collector normal to the incident direct (beam) solar irradiance. Once data are obtained with the aperture normal to the sun, testing is repeated, usually only at one temperature, to determine the effect of varying angles of incidence on collector performance.





Three procedures for measuring the performance of thermal collectors are used, depending on the type of collector system being tested and the experimental apparatus available. We shall briefly describe each method; collector balance, system balance and heat loss measurement methods below. Figure depicts the test setup used in these tests graphically.





Collector Balance - The most common test method used for flat-plate and parabolic trough collectors is that depicted in Figure A. This flow loop must incorporate accurate measurement capabilities for the fluid inlet and outlet temperatures, and the mass flow rate of the heat transfer fluid. It must also include a temperature controller connected to auxiliary heating and cooling devices so that a constant fluid inlet temperature can be maintained. The rate of energy being transferred to the heat transfer fluid under steady state conditions is given in Equation Single collector modules are typically used in these tests, the temperature increase across a single module, especially a parabolic trough module, can sometimes be small (e.g., 1 to 5ºC in a parabolic trough) since these modules are designed to be connected in series in applications. Therefore, extreme care must be taken in making accurate temperature rise measurements. Dudley et al. (1982) describe, in general, the test procedures developed at Sandia National Laboratories for testing parabolic trough collectors. Flow System Balance - In some cases, it is not easy to accurately measure the fluid inlet or outlet temperatures or the mass flow rate. Instead of performing the energy balance on the collector module itself, an insulated flow loop and tank can be used as illustrated in Figure B). The energy balance for the system using this apparatus would be:

For this method to work, the amount of fluid must be large enough so that the bulk temperature of the fluid (and therefore the collector fluid inlet temperature) does not increase too rapidly in order to obtain an approximate steady state temperature condition. Heat Loss Measurement - A third method is often used on concentrating collector systems and involves two tests to obtain a single performance data point. This procedure is illustrated schematically in Figure C). First the rate of optical energy collected defined in Equation is measured. Operating the collector with heat transfer fluid close to ambient temperature does this. Since heat loss from the collector, as shown in Equation is proportional to the difference between fluid and ambient





temperature, there should be no heat loss when they are the same temperatures.





The second test is to operate the collector at normal operating temperatures (a heater is required) without solar input. Usually defocusing the concentrator attains this test condition. Under these conditions, heat will be lost from the receiver because of the temperature difference between it and ambient. If this test is performed at different temperatures, the non-linear relationship between heat loss and temperature difference can be determined. The rate of useful energy out of the collector at any given temperature then is:

As with any test method, there are a number of inaccuracies that can affect the results obtained with this test procedure. Probably the most important discrepancy is from the fact that, due to the temperature difference needed to conduct heat through the absorber wall and transfer it into the working fluid, the absorber surface will be hotter than the heat transfer fluid, under normal operating conditions. However, when heat is being lost from a defocused or shaded collector during the second test above, the absorber surface will be cooler than the heat transfer fluid. This will result in the test measurement indicating heat losses that are lower than actually occur during normal operation.

Incident Angle Modifier - Most solar collector testing is performed using a two- axis tracking device that places the collector aperture normal to the sun. In actual installations, flat-plate collectors are usually mounted in a fixed position with the sun making different incident angles to the collector aperture over the day .

Parabolic troughs likewise usually track about a single axis and have incident angles. A non-zero angle of incidence not only changes the amount of irradiation incident on the collector aperture (already accounted for in the definitions of aperture solar irradiance), but also changes collector performance due to, among other things, variation of surface properties with incident angle and internal shading.

If total energy recovery from a collector field over a period of time is to be estimated from collector performance data, definition of the collector’s performance at other than zero incident angles is necessary. The ratio of collector efficiency at any angle of incidence, to that at normal incidence is called the ‘incident angle modifier’,

Ki. It is measured experimentally by varying the angle of incidence under

noontime solar irradiance conditions with ambient temperature heat transfer fluid passing through the collector. Changes in this optical efficiency measurement give the incident angle modifier. Usually it is expressed as a polynomial curve fit as:





where a and b are coefficients from a polynomial curve fit. Photovoltaic Performance Measurements

Due to the rapid response rate of the photovoltaic cell, testing of photovoltaic panels and concentrating photovoltaic collectors does not have many of the difficulties addressed above for thermal collectors. A detailed outline of performance testing procedures for photovoltaic panels is contained in ASTM 1036.

Generally all that is required is a variable load resistance and measurements of current through the load and voltage across the panel. A thermocouple should be placed adjacent to the photovoltaic cells to ascertain their temperature. An alternative to using a variable resistance is to connect the photovoltaic panel across a large capacitor. Taking simultaneous transient data of both voltage and current will provide the data necessary to plot an I-V curve.

Significantly more equipment is involved to determine the output at different cell temperatures and air mass.

7. Explain in detail about Concentrating Collector. [ CO2 – L2 ] Concentrating Collectors: Collectors are oriented to track the sun so that the beam radiation will be directed onto the absorbing. Surface Collector: Receiver and the concentrator Receiver: Radiation is absorbed and converted to some other energy form (e.g. heat). Concentrator: Collector that directs radiation onto the receiver. The aperture of the

concentrator is the opening through which the solar radiation enters the concentrator





Collector Configurations:

Concentrating Collectors: Fresnel Lens: An optical device for concentrating light that is made of concentric rings that are faced at different angles so that light falling on any ring is focused to the same point. Parabolic trough collector: A high-temperature (above 360K) solar thermal concentrator with the capacity for tracking the sun using one axis of rotation. It uses a trough covered with a highly reflective surface to focus sunlight onto a linear absorber containing a working fluid that can be used for medium temperature space or process heat or to operate a steam turbine for power or electricity generation. Central Receiver: Also known as a power tower, a solar power facility that uses a field of two-axis tracking mirrors known as heliostat (A device that tracks the movement of the sun). Each heliostat is individually positioned by a computer control system to reflect the sun's rays to a tower-mounted thermal receiver. The effect of many heliostats reflecting to a common point creates the combined energy of thousands of suns, which produces high-temperature thermal energy. In the receiver, molten nitrate salts absorb the heat energy. The hot salt is then used to boil water to





steam, which is sent to a conventional steam turbine generator to produce electricity. Concentration Types: Planar and non-concentrating type which provides concentration ratios of up to four and are of the flat plate type. Line focusing type produces a high density of radiation on a line at the focus.

Cylindrical parabolic concentrators are of this type and they could produce concentration ratios of up to ten. Point focusing type generally produce much higher density of radiation in the vicinity

of a point. Paraboloids are examples of point focus concentrators. Concentration Ratio:

Radiative Heat Exchange Between the Sun and the Receiver: The sun is assumed to be a blackbody at Ts and the radiation from the sun on the aperture/receiver is the fraction of the radiation emitted by the sun which is intercepted by the aperture.

A perfect receiver, such as a blackbody, radiates energy equal to ArTr4 and a fraction





of this reaches the sun

Maximum Concentration Ratio: When Tr=Ts, the second law requires that

With θs = 0.27o, the maximum possible concentration ratio for circular concentrators is 45,000 and for linear concentrators, it is 212. Concentration Ratio vs Receiver Temperature:

Thermal Performance:

The generalized thermal analysis of a concentrating collector is similar to that of a flat-plate collector. The expressions for collector efficiency factor F`, the loss coefficient UL, and the collector heat removal factor FR need to derived for a specific configuration. With FR and UL known, the collector useful gain can be calculated from an expression that is similar to that of a flat-plate collector. For a linear concentrator, with no temperature gradients around the receiver tube,

the thermal loss coefficient is





where T is the mean radiation temperature ε is the remittance of the absorbing surface, V is the wind speed and L is the characteristic length. We will use the same terminology used in flat plate collector analysis and consider a cylindrical absorbing tube with a linear concentrator. The thermal loss coefficient UL is given by:

The overall heat transfer coefficient from the surroundings to the fluid in the tube is

Where Do and Di are the outside and inside tube diameters, hfi is the heat transfer coefficient inside the tube and k is the thermal conductivity of the tube. The useful energy gain per unit of collector length:

Where Aa is the unshaded area of the concentrator aperture and Ar is the area of the receiver (πDoL for a cylindrical absorber), S is the absorbed solar radiation per unit of aperture area, Tf is the local fluid temperature and F’ is the collector efficiency factor given by Uo/UL. The actual useful energy gain:





Where Aa is the unshaded area of the concentrator aperture and Ar is the area of the receiver, S is the absorbed solar radiation per unit of aperture area, Ti is the inlet fluid temperature and FR is the collector heat removal factor.





UNIT V

Wind Energy

PART A

1. Give the expression for power available in the wind. [ CO2 – L1 ] Power Available in the Wind

The total quantum of wind energy is enormous. However, a very small percentage is available for practical use. Efficiency of wind-turbine energy conversion plants is only about 30 percent

The power in the wind is proportional to the wind speed cubed; general formula for power in the wind is:

Power = density of air x swept area x velocity cubed 2

P = ½.ρ.A.v³

If the velocity (v) is in m/s, then at sea level (where the density of air is 1.2 kg/m³) the power in the wind is:

Power = 0.6 x v³ Watts per m² of rotor swept area

2. Sketch the block diagram of wind energy conversion system. [ CO2 – L1 ]





3. What are the factors to be considered for site selection? [ CO2 – L1 ] Technical consideration Economic consideration Environmental consideration Social consideration

4. What are the factors to be considered in Technical consideration. [ CO2 –

L1 ] Wind Speed Land topography and geology Grid structure and distance Turbine size

5. What are the factors to be considered in Economic consideration. [ CO2 – L1 ]

Capital cost Land cost Operational and management cost Electricity market

6. What are the factors to be considered in Environmental consideration. [ CO2 – L1 ]

Visual impact

Wild life & endangered species

Electromagnetic interference

Noise impact

7. What are the factors to be considered in Social consideration. [CO2 – L1 ] Regulatory boundaries Public acceptance Land use Distance from the residential area

8 . What are the types of Turbine? [ CO2 – L1 ] There are two basic configurations, namely vertical axis wind turbines (VAWT) and, horizontal axis wind turbines (HAWT).





9. What are the process involved in energy conversion system. [ CO2 – L1 ]

The energy conversion chain is organised into four subsystems: 1. Aerodynamic subsystem, consisting mainly of the turbine rotor, which is

composed of blades, and turbine hub, which is the support for blades; 2. Drive train, generally composed of: low-speed shaft – coupled with the turbine

hub, speed multiplier and high-speed shaft – driving the electrical generator; 3. Electromagnetic subsystem, consisting mainly of the electric generator; 4. Electric subsystem, including the elements for grid connection and local grid.





PART B (16 MARKS)

1. State the principle of wind energy in detail. [ CO2 – L1]

PRINCIPLES OF WIND ENERGY CONVERSION 1. Power Available in the Wind

The total quantum of wind energy is enormous. However, a very small percentage is available for practical use. Efficiency of wind-turbine energy conversion plants is only about 30 percent

The power in the wind is proportional to the wind speed cubed; general formula for power in the wind is:

Power = density of air x swept area x velocity cubed

2 P = ½.ρ.A.v³

If the velocity (v) is in m/s, then at sea level (where the density of air is

1.2 kg/m³) the power in the wind is:

Power = 0.6 x v³ Watts per m² of rotor swept area

Because of this cubic relationship, the power availability is extremely sensitive to wind speed; doubling the wind speed increases the power availability by a factor of eight;

This means that the power density in the wind will range from 10W/m² at 2.5m/s (a light breeze) to 41,000W/m² at 40m/s (a hurricane). This variability of the wind power resource strongly influences virtually all aspects of wind energy conversion systems design, construction, siting, use and economy.

2. Energy Available in the Wind

Because the speed of the wind constantly fluctuates, its power also varies to a proportionately greater extent because of the cube law. The energy available is the summed total of the power over a given time period. The usual starting point to estimate the energy available in the wind at a specific location is some knowledge of the mean or average wind speed over some predefined time period; typically monthly means may be used.The most important point of general interest is that the actual energy available from the wind during a certain period is considerably more than if you take the energy that would be produced if the wind blew at its mean speed without variation for the same period. Typically the energy





available will be about double the value obtained simply by multiplying the





instantaneous power in the wind that would correspond to the mean wind speed blowing continuously, by the time interval. This is because the fluctuations in wind speed result in the average power being about double that which occurs instantaneously at the mean wind speed. The actual factor by which the average power exceeds the instantaneous power corresponding to the mean wind speed can vary from around 1.5 to 3 and depends on the local wind regime's actual variability. The greater the variability the greater this factor.

However, for any specific wind regime, the energy available will still

generally be proportional to the mean wind speed cubed. We shall discuss later in this section how to determine the useful energy that can be obtained from a wind regime with respect to a particular windmill.

3. Converting Wind Power to Shaft Power

There are two main mechanisms for converting the kinetic energy of the wind into mechanical work; both depend on slowing the wind and thereby extracting kinetic energy. The crudest and least efficient technique is to use drag; drag is developed simply by obstructing the wind and creating turbulence and the drag force acts in the same direction as the wind.

The other method, used for all the more efficient types of windmill, is to produce lift. Lift is produced when a sail or a flat surface is mounted at a small angle to the wind; this slightly deflects the wind and produces a large force perpendicular to the direction of the wind with a much smaller drag force. Lift mainly deflects the wind and extracts kinetic energy with little turbulence, so it is therefore a more efficient method of extracting energy from the wind than drag.

It should be noted that the theoretical maximum fraction of the kinetic energy in the wind that could be utilized by a "perfect" wind turbine is approximately 60%. This is because it is impossible to stop the wind completely, which limits the percentage of kinetic energy that can be extracted.

4. Drag Devices If an object is set up perpendicularly to the wind, the wind exerts a

force FD on the object. The wind speed v, the effective object area A and the drag coefficient CD, which depends on the object shape, define the drag force.

With PD = FD.v, the power to counteract the force The ratio of the circumferential speed u to the wind speed v is called

the tip speed ratio l The tip speed ratio of drag devices is always smaller than one.





The optimal power coefficient cP,opt,D of a drag device can be calculated using

u/v = 1/3 as well as the maximum drag coefficient of cD,max = 1.3 This value is also much below the ideal value of 0.593. Therefore, modern wind turbines are lift devices, rather than drag devices, and these achieve much better power coefficients.

5. Lift Devices If wind, which circulates around a body, develops higher flow speeds

along the upper surface than along the lower, an overpressure emerges at the upper surface and an under pressure at the lower.The result is a buoyancy force, according to Bernoulli:

The buoyancy force is calculated using the lift coefficient cL, the air density ρ, the apparent wind speed vA and the projected body area AP. Rotor blades of modern wind generators usually make use of the buoyancy force. The projected area of a rotor blade is defined by the chord t and span that is approximately equal to the rotor radius r.

Drag forces, which have been described in the section about drag devices also, have effects on lift devices:

However, the buoyancy force on a drag device is much higher than the drag force. The ratio of both forces is called the lift-drag ratio ε:

Good rotor profiles can reach lift-drag ratios of up to 400.

The apparent wind speed: Used in the equations above is calculated from the real wind speed vw and the

circumferential speed u. With the tip speed ratio λ = u/vw

The ratio of the drag force FD to the buoyancy force FL. Vector addition of both forces provides the resultant force:

FR = FD + FL

The resultant force can be subdivided into an axial component FRAand a tangential component FRT. The tangential component FRT of the resultant force causes the rotor to turn.





2. Sketch the Functional Block diagram of wind energy system . [ CO2 – L2 ]

FUNCTIONAL STRUCTURES OF WIND ENERGY SYSTEM Clean energy fever is being fuelled by three things: high oil prices, fears over

energy security and a growing concern about global warming. The provision of energy, industry’s cheer leaders say, will change radically over the coming decades. Polluting coal and gas fire power station will make way to cleaner alternatives such as solar and wind. Wind energy is basically caused by the solar energy radiating from the earth. Windmill generation cost is lower than that of diesel power and almost equal to thermal power cost. Wind energy is conversion of kinetic energy (i.e. energy of motion of the wind) into mechanical energy that can be utilized to generate electricity. The wind blows against the blades and they rotate about the axis. The rotational motion is converted to energy by wind turbines because wind turbines produce rotational motion. Wind-energy is readily converted into electrical energy by converting the turbine into an electrical generator. Figure dealing with the simplified block diagram for entire wind energy conversion system helps us to understand how the principal components of wind energy converters work and interact

3. Explain in detail about the various factors involved for site selection of wind turbine installation. [ CO2 – L2 ] SITE SELECTION

Although wind power is a never ending green resource, assessment of environmental risks and impacts- which comprise the backbone of environmental policy- in the context of specific projects or sites often are necessary to explicate and





weigh the environmental trade-offs that are involved. In the case of wind farms, a





number of turbines (ranging from about 250 kW to 750 kW) are connected together to generate large amounts of power. Apart from the constraints resulting from the number of turbines, any site selection should think over the technical, economic, social, environmental and political aspects

I - Technical Considerations

Many technical factors affect the decision making on site selection including wind speed, land topography and geology, grid structure and distance and turbine size. These technical factors must be understood in order to give pair-wise scores to sub-factors.

Wind Speed The viability of wind power in a given site depends on having sufficient

wind speed available at the height at which the turbine is to be installed. Any choice of wind turbine design must be based on the average wind velocity at the selected wind turbine construction site

Land topography and geology

Wind farms typically need large lands. Topography and prevailing wind conditions determine turbine placement and spacing within a wind farm. In flat areas where there is nothing to interfere with wind flow, at least 2600-6000 m2/MW may be required. Wind turbines are usually sited on farms that have slope smaller than 10- 20%.

Grid structure and distance The connection of wind turbines to an electricity grid can potentially

affect reliability of supply and power quality, due to the unpredictable fluctuations in wind power output.

Turbine size Required height for the installation of turbine above ground is one of

the important factors that affect the annual energy generation. Turbine size is related with the energy output, because the bigger the turbine size is, the more wind it is exposed to.

II - Economic Considerations

The economic sub factors that affect the site selection include capital cost, land cost and operational and management costs. It is important to make





economical evaluations by considering time value of money due to long periods of





service life of wind farm projects.

Capital cost Construction, electrical connection, grid connection, planning, wind

turbines, approvals, utilities and management are the main components of capital cost for wind farm projects.

Land cost For the site selection, main economic factor is the cost of the land

where the wind farm is constructed; because, the cost of land primarily depends on the region, soil condition and the distance from the residential area.

Operational and management cost

There will be control functions such as supervisory control and data acquisition (SCADA) which will provide control of each wind turbine in O&M facilities. Business rates, maintenance expenses, rents, staff payments are main components of O&M costs.

Electricity market

Existing of an electricity market for the energy generated is an important factor affecting the economic benefits of the project. There should be energy demand in regions close to wind farms.

III - Environmental Considerations The environmental sub factors that affect the site selection of a wind

farm include visual impact, electromagnetic interference, wild life and endangered species and noise impact.

Visual impact Wind turbines are located in windy places, and most of the time, those

places are highly visible. To many people, those big towers with 2 or 3 blades create visual pollution. To minimize the impacts of visual pollution, many investors implement the actions.

Wild life & endangered species

Wind farms affect birds mainly through collision with turbines and associated power lines, disturbance leading to displacement including barriers to movement, and loss of habitat resulting from wind turbines. To minimise the risk of bird collision, site selection should be done precisely.





Electromagnetic interference Electromagnetic interference is an electromagnetic disturbance that

interrupts, obstructs, or degrades the effective performance of electronics or electrical equipment. Wind turbines may reflect, scatter or diffract the electromagnetic waves which in turn interfere with the original signal arriving at the receiver.

Noise impact Noise can generally be classified according to its two main sources:

aerodynamic and mechanical. Aerodynamic noise is produced when the turbine blades interact with eddies caused by atmospheric turbulence. Mechanical noise is generated by the rotor machinery such as the gearbox and generator. Noise could be reduced by better designed turbine blade geometry and by selection of proper operating conditions.

IV - Social Considerations

Social factors that affect the selection of a site include public acceptance, distance from residential area and alternative land use options of candidate wind farm site. Public may oppose projects because of possible environmental or social effects. Distance from residential area gain importance not to interfere with social life during wind farm construction or operation.

Regulatory boundaries There may be some national or international level regulation related

with the construction and operation of wind farms. These regulations must be explored before evaluating the socio-political position of a wind farm project. Most of them probably change from region to region.

Public acceptance Public is the most vital component of a region and their opposition to

issues can lead to abolish proposed projects. Support of public for wind energy generation is expected to be high in general but proposed wind farms have often been met with strong local opposition

Land use Land use affects the decision of wind farm siting from two points of

view. Firstly, there are some cases where no wind farms can be built although sufficient wind speed was detected. These cases are mainly related with land use or condition. Land related constraints include forest area, Wetlands, Land of high productivity, Archaeological sites, Aviation zones, Military zones etc





Distance from the residential area Noise and vibration stemming from the wind turbines may cause

residents to suffer from sleep disturbance, headaches, visual blurring. Those types of complaints can be avoided if the wind turbines are sited a considerable distance from the residential area

4. Explain the parts of the wind turbine in detail. [ CO2 – L2 ]

Parts of a Wind Turbine Although we talk about “wind turbines,” the turbine is only one of the three main

parts inside these giant machines. 1. Turbines – The first part of course, is the turbine. The giant blades and the

rotor (hub) together make up the “turbine”. As wind passes by, it makes the blades spin around. These blades have an aerodynamic curved shape so as to capture as much energy from the wind as possible. The blades are attached to a hub, which spins as the blades turn. As the rotor turns, it spins a drive shaft which is connected to a generator inside the housing at the top of the tower.

2. Shaft – The second part, the shaft, is actually a gearbox which increases the speed of the spinning blades enough to power the electricity generator.

3. Generator – The third part is the generator, which converts mechanical energy of the moving wind into electrical energy, with the help of the spinning shaft.

ENERGY STORAGE Wind power turbines have operational limitations over very high and very low

speeds. When the power generated exceeds the demand, excess energy can be stored to be used at other times. Excess energy can be conveniently stored in

storage batteries in the form of chemical energy. Excess energy can also be stored

in water power storage in the form of mechanical energy. Wind power plant (WPP) along with Hydroelectric power plant (HPP), when generated power (Pg) exceeds the power demand (Pd), helps to partly divert hydro power plant output to Pumping motor (PM) to pump water from an auxiliary reservoir at the bottom of the dam to main reservoir.





Excess energy can also be stored in the form of compressed air.

When wind is not blowing, energy stored in compressed air could be used to drive wind turbine whose shaft would then drive a generator, thus supplying the needed power

SAFETY INTERLOCKS 1. Modern wind turbines are controlled by computers. If it shows any error in operational parameters, then wind turbine is stopped. 2. Emergency stop – During unfavourable conditions for wind turbines, it can be immediately stopped using emergency stop. 3. Wind velocity is measured and if gusts of wind are too strong or if the average

speed is too high, wind turbine is stopped. 4. To prevent rotor from racing, two revolution counters are mounted on the shaft. If wind turbine speed exceeds 24 rpm, it activates the emergency stop system. 5. If the wind turbine speed exceeds 28 rpm, a parachute attached to the blade tip is

pulled out and thereby speed of the wind turbine decreases. 6. The three blades and wind turbine cap are grounded through lightening rods to

protect them from lightning.

Types of generators used:- 1. For Small rating systems - P.M.type d.c. generators 2. Medium rating systems - P.M.type d.c. generators , Induction generators

Synchronous Generators





3. Large rating systems - Induction generators (3-phase ) Synchronous Generators (3 phase)

4. Controller(C)-Senses wind direction, wind speed generator output and temperature and initiates appropriate control signals to take control action.

5. Yaw motor gear- The area of the wind stream swept by the wind turbine is maximum when blades face into the wind. Alignment of the blade angle with respect to the wind direction to get maximum wind energy can be achieved with the help of yaw control that rotates wind turbine about the vertical axis. In smaller wind turbines, yaw action is controlled by tail vane whereas, in larger turbines, it is operated by servomechanism.

Wind Electric conversion System

Cross-sectional View Apart from the above components, protective schemes for excessive temperature rise of generator, against electrical faults and turbulent wind conditions





are also provided in the system. Practically, Wind power generating system ratings are divided into three groups:-

Small up to 1KW

Medium 1 KW to 50 KW

Large 200KW to Megawatts

5. Explain the various schemes of power generation based on speed & frequency.

[CO2–L2] SCHEMES FOR WIND POWER GENERATION

Based on the speed and frequency, generally following schemes are identified:

I. CSCFS (Constant Speed Constant Frequency Scheme):- Constant speed drives are used for large generators that feed the generated

power to the grid. Commonly synchronous generators or induction generators are used for power generation.

If the stator of an induction machine is connected to the power grid and if the rotor is driven above Synchronous speed, Ns, the machine delivers a constant line frequency (f=PNs/120) power to the grid. The slip of the generators is between 0 and 0.05.The torque of the machine should not exceed max. torque to prevent ‘run away’(speed continues to increase unchecked).

Compared to synchronous generator, Induction generators are preferred because they are simpler, economical, easier to operate, control and maintain and have no synchronization problem. However, Capacitors have to be used to avoid reactive volt ampere burden on the grid.

II. DSCFS (Dual Speed Constant Frequency Scheme):-

In this scheme a dual speed wind turbine is coupled to double winding Induction generator that is specially fabricated with 2 stator windings wound with different number of poles P1& P2 (P1 > P2). When wind speed is low, winding with P1 poles gets connected and power is generated with grid





frequency. Similarly, when wind speed is high, winding with P2 poles gets connected and feed the power to grid at the same frequency. It is Important to note that the difference in speed should be small. Reactive power required by the Induction Generator can be supplied by installing the Capacitor bank.

III. VSCFS (Variable speed constant frequency scheme):-

In this scheme output of three phase alternator (synchronous generator) is rectified by bridge rectifier. The DC output is transmitted through DC transmission lines and then converted back to AC using synchronous inverters and fed to grid system.

This scheme, involving small wind generators is commonly used in autonomous applications such as street lighting. Due to variable speed operation, it yields higher power for both low and high wind speeds. Both horizontal axis and vertical axis turbines are suitable.

IV. Variable speed constant frequency with double output (VSCF with DO):- In this scheme, to increase the power generating capacity of the system, squirrel cage induction generators are replaced by slip ring Induction generator. Rotor power output at slip frequency is converted to line frequency power using rectifier. Output power is obtained both from stator and rotor. Rotor output power increases with increase in slip and speeds. Therefore, operating speed range is Ns to 2Ns i.e. slip ranging from 0 to 1.





V. (VSVFS) Variable speed variable frequency schemes :-

Depending upon the wind speed, squirrel cage Induction Generator generates power at variable frequency. Such generators are excited by Capacitor-bank. The magnitude and frequency of the generated emf depends upon the wind turbine speed, excitation capacitance and load impedance. If load requires constant dc voltage, output of generators is converted into d.c. using chopper controlled rectifiers. Feedback system can be used to monitor and control to get desired performance.

This scheme is suitable for loads that are frequency insensitive such as heating load.

6. What are the types of wind turbines? [ CO2 – L1 ] Types of wind turbines:

A WECS is a structure that transforms the kinetic energy of the incoming air

stream into electrical energy. This conversion takes place in two steps, as follows. The extraction device, named wind turbine rotor turns under the wind stream

action, thus harvesting a mechanical power. The rotor drives a rotating electrical machine, the generator, which outputs electrical power. Several wind turbine





concepts have been proposed over the years. A historical survey of wind turbine technology is beyond the scope here, but someone interested can find that in Ackermann (2005). There are two basic configurations, namely vertical axis wind turbines (VAWT) and, horizontal axis wind turbines (HAWT).

Today, the vast majority of manufactured wind turbines are horizontal axis, with either two or three blades. HAWT is comprised of the tower and the nacelle, mounted on the top of the tower (Figure 2.4). Except for the energy conversion chain elements, the nacelle contains some control subsystems and some auxiliary elements (e.g., cooling and braking systems, etc.). The energy conversion chain is organised into four subsystems:

5. aerodynamic subsystem, consisting mainly of the turbine rotor, which is composed of blades, and turbine hub, which is the support for blades;

6. drive train, generally composed of: low-speed shaft – coupled with the turbine hub, speed multiplier and high-speed shaft – driving the electrical generator;

7. electromagnetic subsystem, consisting mainly of the electric generator; 8. electric subsystem, including the elements for grid connection and local grid.

7. Explain in detail about the Wind Turbine Aerodynamics. [ CO2 – L2 ] Wind Turbine Aerodynamics

The wind turbine rotor interacts with the wind stream, resulting in a behaviour named aerodynamics, which greatly depends on the blade profile.

Actuator Disc Concept

The analysis of the aerodynamic behaviour of a wind turbine can be done, in a generic manner, by considering the extraction process (Burton et al. 2001).

Consider an actuator disc and an air mass passing across, creating a stream- tube.

The conditions (velocity and pressure) in front of the actuator disc are denoted with subscript u, the ones at the disc are denoted with 0 and, finally, the conditions behind the disc are denoted with w.





Wind Turbine Performance: A wind turbine is a power extracting device. Thus, the performance of a wind

turbine is primarily characterized by the manner in which the main indicator – power – varies with wind speed. Besides that, other indicators like torque and thrust are important when the performances of a wind turbine are assessed. The generally accepted way to characterize the performances of a wind turbine is by expressing them by means of non-dimensional characteristic performance curves (Burton et al. 2001). The tip speed ratio of a wind turbine is a variable expressing the ratio between the peripheral blade speed and the wind speed. It is denoted by λ and computed as





Therefore, for safety reasons, above the rated wind speed the captured power

is prevented from increasing further by using an aerodynamic power control subsystem. This modifies the aerodynamic properties of the rotor by severely decreasing its power coefficient, Cp. To this end, multiple power control solutions are usually employed in WECS. Some of them are passive (e.g., stall control), using blade profile properties; others are active (e.g., pitch control), changing blades position relative to the rotating plane.The blades can be turned into the wind (upwind) or away from the wind (downwind). Some control solutions aim at turning the entire rotor away from the wind in order to diminish the aerodynamic efficiency.

EE6801 ELECTRIC ENERGY GENERATION, UTILIZATION AND ...vel tech high tech dr. rangarajan...

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