BASIC ELECTRICAL AND ELECTRONICS ENGINEERING

2 MARKS1. What is meant by charge?Charge is an electrical property of the atomic particles which matterconsists. The charge of an electron is so small. Charge in motion representscurrent. The unit of charge is coulomb.2. What is meant by Current?The flow of free electrons in a conductor is called current. Unit is ampere(A).I = Q/t3. What is meant by Voltage?The poterntial difference between two points is called as voltage. Unit isVolts (V).V=W/Q , W=work done in joules & Q = charge in coulombs4. State Ohm’s Law.The potential difference across any two ends of a conductor is directlyproportional to the current flowing between the two ends provided thetemperature of the conductor remains constant.5. State Krichoff’s Voltage LawKVL states that the algebraic sum of voltages in a closed path is zero.6. State Krichoff’s current Law.KCL states that the algebraic sum of currents in a node is zero.7. Give notes on Nodal Analysis.KCL is used.No: of equations = n-1, n=no: of nodes8. Give notes on Mesh Analysis.KVL is usedHere mesh currents are found.9. Give short notes on resistor.It is a property of a substance3 which opposes the flow of electrons. It isdenoted by R and its unit is Ohm (_)10. Distinguish between a Branch and a node of a circuit.A pair of network which connects the various points of the network iscalled branchA point at which two or more elements are joined together is called node.11. Distinguish between a mesh and a loop of a circuit.A mesh is a loop that does not contain other loops. All meshes are loop,but all loops are not meshes.A loop is any closed path of branches12. Write down the formula for a star connected network is converted into adelta network?RA=( R1 R2)/( R1 +R2+ R3)RB=( R1 R3)/( R1 +R2+ R3)

RC=( R2 R3)/( R1 +R2+ R3)13. Write down the formula for a delta connected network is converted into astar network?R1=( RARB+RBRC+RCRA)/RC

R2=( RARB+RBRC+RCRA)/RB

R3=( RARB+RBRC+RCRA)/RA

14. Define line currents and phase currents?The currents flowing in the lines are called as line currentsThe currents flowing through phase are called phase currents15. Define line voltage and phase voltage?The voltage across one phase and neutral is called line voltage & thevoltage between two lines is called phase voltage16. Give the phase value & Line value of a star connected system.VL= 3Vph

17. Give the phase value and line valued of a delta connected system.IL= 3Iph

18. What is the power equation for a star connected system?P= 3I V cosΦ L L W19. What is the power equation for a delta connected system?P= 3I V cosΦ L L W20. What is meant by Real power?Real power means the useful power transfer from source to load. Unit iswatts.21. What is meant by apparent power?Apparent power is the product of voltage and current and it is not truepower. Unit is VA22. What is reactive power?If we consider the circuit as purely inductive the output power is reactivepower. Its unit is VAR23. Define Instrument.Instrument is defined as a device for determining the value or magnitudeof a quantity or variable.24. Mention the two main differences between an ammeter and a voltmeter.Ammeter VoltmeterIt is a current measuring device it is a voltage measuring deveiceAlways connected in series with circuit Always connected in parallel with circuitThe resistance is very small The resistance is very high25. Give short notes on resistor.It is a property of a substance3 which opposes the flow of electrons. It isdenoted by R and its unit is Ohm (_)26. What is control system?A system consists of a number of components connected together toperform a specific function . In a system when the output quantity is controlled byvarying the input quantity then the system is called control system.27. What are the two major types of control system?

The two major types of control system are open loop and closed loop28. .Define open loop control system.The control system in which the output quantity has no effect upon theinput quantity are called open loop control system. This means that the output isnot feedback to the input for correction.29. .Define closed loop control system.The control system in which the output has an effect upon the inputquantity so as to maintain the desired output value are called closed loop controlsystem30. Mention the errors in Moving iron instruments.Hysteresis errorTemperature errorStray magnetic field errorFrequency errorEddy current error31. Mention any two precautions to be taken while using an Ammeter.It should never be connected across any source.The polarity must be observed correctly.First use the highest range and then decrease the voltage range untilthe sufficient deflection is obtained.32. Give some applications of DC motor.Shunt : driving constant speed, lathes, centrifugal pumps, machine tools,blowers and fans, reciprocating pumpsSeries : electric locomotives, rapid transit systems, trolley cars, cranes andhoists, conveyorsCompound : elevators, air compressors, rolling mills, heavy planners33. Define slip.S = Ns – Nr

Ns

Where, Ns = synchronous speed in rpm.Nr = rotor speed in rpmS = Slip34. Define synchronous speed.It is given by Ns = 120f / p rpm.Where Ns = synchronous speed, p = no. of stator poles, f = supply frequency inHz35. Why a single phase induction motor does not self start?When a single phase supply is fed to the single phase induction motor. Itsstator winding produces a flux which only alternates along one space axis. It isnot a synchronously revolving field, as in the case of a 2 or 3phase stator winding,fed from 2 or 3 phase supply.36. Is Induction motor runs with synchronous speed or not.

Induction motor never runs with synchronous speed. It will stop if it triesto achieve synchronous speed.37. Define Form factor and Crest factor.Form factor= RMS valueAverage ValueCrest(peak) factor=Maximum ValueRMS value38. . Which type of instrument is called as universal instrument?The moving iron instrument are known as universal instruments, becausethese instruments can be used for AC and DC.39. What are the applications of MI instruments?i) Used as multirange ammeters and voltmeters.ii) Used as in expensive indicators such as charging and dischargingcurrent indicators in automobiles.iii)Extensively used in industries for measurement of AC voltage andcurrent where errors of the order of 5% to 10% are accepetable.40. What is meant by eddy current damping?When the conductor moves in a magnetic field an emf is induced in it andif a closed path is provided ,a current flows known as eddy current. This currentintersect with the magnetic field to produce an electromagnetic torque , whichopposes the deflecting torque.41. .How is electrical power measured?i) Using Voltmeter-ammeter method for DC circuits.ii)Using Watt meters for AC circuits.42. .What do you mean by compensation coil in a wattmeter?By connecting a compensating coil in series with a p[ressure coil ,Theerror caused by the pressure coil flowing in the current coil can be neutralized.43. What are the three types of power used in a a.c circuit?i) Real power or active power P=EI cosii) Reactive power Q=EI siniii) Apparent power,S=EI44. Define average value.The average value of an alternating current is that value of steady directcurrent which transfers the same charge as the alternating current flowing for thesame time.45. Define RMS value.The effective value of an alternating current is that value of steady ,directcurrent which produces the same heat as that produced by the alternating currentwhen passed which produces the same heat as that produced by the alternatingcurrent when passed through the same resistance for the same interval of time.46. Define reactive power.The power consumed by a pure reactance (XL or Xc ) in a a.c circuit is

called reactive power. The unit is VAR. Q=EIsin.47. What is the basic principle of a dc generator?Basic principle of a dc generator is Faraday’s law of electromagneticinduction.That is whenever a conductor is moved in amagnetic field dynamicallyinduced emf is produced in that conductor.48. .What is the purpose of interpoles in modern d.c machine?In modern d.c machines commutating poles or interpoles are provided toimprove commutation.49. What is the use of commutator and brush in a d.c machine?The commutator converts the alternating emf into unidirectional or direct emf.The brushes are mainly used to collect current from the commutator.50. What is a d.c series motor?In a d.c series motor,the field winding is connected in series with thearmature.The field winding should have less number of turns of thick wire.51. Why a series motor cannot be started without any load?Series motor cannot be started without any load because under no loadcondition the starting torque is less and motor rotates at dangerous speed and maybe damaged.52. What is meant by transformer?The transformeris a static piece of apparatus by means of which electricalenergy is transformed from one circuit to another with desired change in voltageand current , without any change in the frequency.It works on the principle ofmutual induction.53. What are the different types of single phase motor?i)Single phase induction motorii)Single phase synchronous motor.iii)Single phase series motor54. What are the two types of rotors of an induction motor?i) Squirrel cage rotorii)Slip ring or wound rotor

6 MARKS1. A 3 φ 4 pole 50 hz induction motor runs at 1460 r.p.m. find its % of slip.SolutionN s = 120f/p= 120*50/4= 1500r.p.m.Running speed of motorn= 1460r.p.m.Slip S=( N s–N)/ N s*100=(1500-1460) x 100 / 1500

= 2.667%2. Explain the working principle of Transformer.A Transformer is a device that transfers electrical energy from one circuit toanother by electromagnetic induction (transformer action). The electrical energy isalways transferred without a change in frequency, but may involve changes inmagnitudes of voltage and current. Because a transformer works on the principle ofelectromagnetic induction, it must be used with an input source voltage that varies inamplitude. There are many types of power that fit this description; for ease of explanationand understanding, transformer action will be explained using an ac voltage as the inputsource.The amount of power used by the load of an electrical circuit is equal to thecurrent in the load times the voltage across the load, or P = EI. If, for example, the load inan electrical circuit requires an input of 2 amperes at 10 volts (20 watts) and the source iscapable of delivering only 1 ampere at 20 volts, the circuit could not normally be usedwith this particular source. However, if a transformer is connected between the sourceand the load, the voltage can be decreased (stepped down) to 10 volts and the currentincreased (stepped up) to 2 amperes. Notice in the above case that the power remains thesame. That is, 20 volts times 1 ampere equals the same power as 10 volts times 2amperes.A Transformer consists of the following partsA primary coil or winding.A secondary coil or winding.A core that supports the coils or windings.The primary winding is connected to a 50 hertz ac voltage source. The magneticfield (flux) builds up (expands) and collapses (contracts) about the primary winding. Theexpanding and contracting magnetic field around the primary winding cuts the secondarywinding and induces an alternating voltage into the winding. This voltage causesalternating current to flow through the load. The voltage may be stepped up or downdepending on the design of the primary and secondary windings.

3. Calculate the amount of resistance (R) in a circuit, given values of voltage (E) andcurrent (I):the amount of resistance (R) offered by the lamp4. calculate the amount of voltage supplied by a battery, given values ofcurrent (I) and resistance (R):the amount of voltage provided by the battery5. Calculate the electric power in the given circuit and discuss the effect ofincreasing the battery voltage.The formula for determining the power in an electric circuit: by multiplying thevoltage in "volts" by the current in "amps" we arrive at an answer in "watts." Let's applythis to the given circuit.In the above circuit, we know we have a battery voltage of 18 volts and alamp resistance of 3 _. Using Ohm's Law to determine current, we get:Now that we know the current, we can take that value and multiply it bythe voltage to determine power:6. What is meant by DEFLECTING TORQUE ?The deflecting torque is produced by making use of one of the magnetic,chemical, electrostatic and electromagnetic induction effects of current or voltageand causes the moving system of the instrument to move from its zero positionwhen the instrument is connected in an electrical circuit to measure the electricalquantity. The method of producing this torque depend upon the type ofinstrument. In attracting the type of instrument, this torque to equal toTd = 1/2 I2 dL/dθWhereas in Pmmc instrumentsTd = BilurWhere B - magnetic densityi - current flowingl - length of coilu - number of turnr - radius of coil7. Find the voltage across each resistors in the following circuit.8. The effective resistance of two resistors connected in series is 100 .When connected in parallel, then effective value in 24 ohm’s.Determine the value of two resistorsSeries R1+R2=100 => R2 =100 - R1R1R2/R1+R2 = 24R1R2/100 = 24R1R2 =2400R1 (100-R1) = 2400100 R1-R1^2-2400 = 0

R1^2-100 R1 + 2400 = 0(R1-60)(R1-40) = 0There Fore R1 = 60; R1 = 40When R1 = 60 ; R2 = 100 – 60 = 40When R1 = 40 ; R2 = 100 - 40 = 609. Find the Req between two points A & B.1/Req = ½+1/3+1/3 = 1.17 (Req = 1/1.17= 0.8547)1/Req = 2+.85+4Req = 7.210. Explain about Krichoffs voltage and current laws.Kirchhoff’s Current LawThe sum of current flowing towards a function is equal to the currentflowing away from it.Consider a function formed by 6 conductors. The current in theseconductors are i1, i2, .i6.Some of these currents are flowing towards a 8other’s away from AAccording to Kirchhoff’s Law,i1+i4+i5+i6 = i2+i3(Flowing towards) (Flowing away from A)Kirchhoff’s Voltage Law (II Law)In a closed circuit, the sum of the potential drops is equal to thesum of the potential resistanceABCDA forms a closed circuit.From A -> B, We have a potential drop of IR1.From D -> A, We have a potential drop of V.Sum of potential drops = IR1+IR2+IR3Potential rise from D -> A =VIR1+IR2+IR3 = V11. Explain the principle of operation of DC Motor.When a current passes through a conductor, lines of magnetic force(flux) are generated around the conductor. The direction of the flux isdependent on the direction of the current flow. In terms of conventionalcurrent flow (positive to negative) then, using your right hand point yourthumb in the direction of the current flow and your fingers will wrap aroundthe conductor in the same direction of the flux lines.On the side of the conductor where the lines of flux oppose each other, themagnetic field will be made weaker. On the side of the conductor where the lines of fluxare not opposing each other, the magnetic field will be made stronger. Because of thestrong field on one side of the conductor and a weak field or, the other side, the conductorwill be pushed into the weaker field.The armature is connected to the commutator which rides along the brushes whichare connected to a DC power source. The current from the DC power source flows from

the positive lead, through the brush labeled A1 through one commutator section, throughthe armature coil, through the other commutator section, through the brush labeled A2and back to the negative lead.This current will generate lines of flux around the armatureand affect the lines of flux in the air gap. On the side of thecoil where the lines of fluxoppose each other, the magnetic field will be made weaker. On the side of the coil wherethe lines of flux are riot opposing each other, the magnetic field is made stronger.Because of the strong field on one side of the coil and the weak field on the other side,the coil will be pushed into the weaker field and, because the armature coil is free torotate, it will rotate.The torque available at the motor shaft (turning effort) is determined by themagnetic force (flux) acting on the armature coil and the distance from the renter ofrotation that force is. The flux is determined by the current flowing through the armaturecoil and strength of the field magnets.12. Explain the working principle of three phase induction motor.In three phase induction motor, the magnetic field generated by the stator rotatesin the ac case. Three electrical phases are introduced through terminals, each phaseenergizing an individual field pole. When each phase reaches its maximum current, themagnetic field at that pole reaches a maximum value. As the current decreases, so doesthe magnetic field. Since each phase reaches its maximum at a different time within acycle of the current, that field pole whose magnetic field is largest is constantly changingbetween the three poles, with the effect that the magnetic field seen by the rotor isrotating. The speed of rotation of the magnetic field, known as the synchronous speed,depends on the frequency of the power supply and the number of poles produced by thestator winding. For a standard 60 Hz supply, as used in the United States, the maximumsynchronous speed is 3,600 rpm.In the three phase induction motor, the windings on the rotor are not connected to

a power supply, but are essentially short circuits. The most common type of rotorwinding, the squirrel cage winding, bears a strong resemblance to the running wheel usedin cages for pet gerbils. When the motor is initially switched on and the rotor isstationary, the rotor conductors experience a changing magnetic field sweeping by at thesynchronous speed.From Faraday's law, this situation results in the induction of currents round therotor windings; the magnitude of this current depends on the impedance of the rotorwindings. Since the conditions for motor action are now fulfilled, that is, current carryingconductors are found in a magnetic field, the rotor experiences a torque and starts to turn.The rotor can never rotate at the synchronous speed because there would be no relativemotion between the magnetic field and the rotor windings and no current could beinduced. The induction motor has a high starting torque.13. Explain the working principle of single phase induction motor.Single phase induction motor has only one stator winding (main winding) andoperates with a single-phase power supply. In all single-phase induction motors, the rotoris the squirrel cage type. The single-phase induction motor is not self-starting. When themotor is connected to a single-phase power supply, the main winding carries analternating current. This current produces a pulsating magnetic field. Due to induction,the rotor is energized. As the main magnetic field is pulsating, the torque necessary forthe motor rotation is not generated. This will cause the rotor to vibrate, but not to rotate.Hence, the single phase induction motor is required to have a starting mechanism that canprovide the starting kick for the motor to rotate.The starting mechanism of the single-phase induction motor is mainly anadditional stator winding (start/ auxiliary winding) as shown in Figure. The start windingcan have a series capacitor and/or a centrifugal switch. When the supply voltage isapplied, current in the main winding lags the supply voltage due to the main windingimpedance. At the same time, current in the start winding leads/lags the supply voltage

depending on the starting mechanism impedance. Interaction between magnetic fieldsgenerated by the main winding and the starting mechanism generates a resultant magneticfield rotating in one direction. The motor starts rotating in the direction of the resultantmagnetic field. Once the motor reaches about 75% of its rated speed, a centrifugal switchdisconnects the start winding. From this point on, the single-phase motor can maintainsufficient torque to operate on its own.14. Explain the working principle of single phase Energy Meter.An electric meter or energy meter is a device that measures the amount ofelectrical energy supplied to or produced by a residence, business or machine.The most common type is a kilowatt hour meter. When used in electricityretailing, the utilities record the values measured by these meters to generate an invoicefor the electricity. They may also record other variables including the time when theelectricity was used.Modern electricity meters operate by continuously measuring the instantaneousvoltage (volts) and current (amperes) and finding the product of these to giveinstantaneous electrical power (watts) which is then integrated against time to giveenergy used (joules, kilowatt-hours etc). The meters fall into two basic categories,electromechanical and electronic.The energy meter operates by counting the revolutions of an aluminium discwhich is made to rotate at a speed proportional to the power. The number of revolutionsis thus proportional to the energy usage. It consumes a small amount of power, typicallyaround 2 watts.The metallic disc is acted upon by two coils. One coil is connected in such a waythat it produces a magnetic flux in proportion to the voltage and the other produces amagnetic flux in proportion to the current. The field of the voltage coil is delayed by 90degrees using a lag coil. This produces eddy currents in the disc and the effect is such thata force is exerted on the disc in proportion to the product of the instantaneous current and

voltage. A permanent magnet exerts an opposing force proportional to the speed ofrotation of the disc - this act as a brake which causes the disc to stop spinning whenpower stops being drawn rather than allowing it to spin faster and faster. This causes thedisc to rotate at a speed proportional to the power being used.The type of meter described above is used on a single-phase AC supply. Differentphase configurations use additional voltage and current coils.The aluminium disc is supported by a spindle which has a worm gear whichdrives the register. The register is a series of dials which record the amount of energyused. The dials may be of the cyclometer type, an odometer-like display that is easy toread where for each dial a single digit is shown through a window in the face of themeter, or of the pointer type where a pointer indicates each digit. It should be noted thatwith the dial pointer type, adjacent pointers generally rotate in opposite directions due tothe gearing mechanism.

10 MARKS1. DETERMINE THE EQUIVALENT RESISTANCE BETWEENTERMINALS A & BSOLUTION50 & 12.5 ARE PARALLEL50*12.5 / 50+12.5 = 10STEP – ISTEP – IIB20*30/20+30 = 12STEP – IIIB35 860A60*20/60+20 = 15STEP – I VSTEP – VRAB = 502. Find the mesh currents in the following networkSolution:The solution of -1 amp for I2 means that our initially assumed direction of current wasincorrect. In actuality, I2 is flowing in a counter-clockwise direction at a value of

(positive) 1 amp:3. Explain the working of a moving iron type instruments.These instruments are widely used in laboratories and switch board atcommercial frequencies because these are cheaper in cost, robust in construction andcan be manufactured with required accuracy.These are generally of two types:-1. The attraction type.2. The repulsion type.The attraction type instrument operate on the principle of attraction of a single piece ofsoft iron into a magnetic field and repulsion type instrument operate on the principle ofrepulsion of two adjacent iron pieces magnified by the same magnetic field. Repulsiontype instrument are more sensitive than attraction type instrument as in repulsion typeinstrument large separating torque is developed by having two iron element positionalclass together inside the field coil where the magnetizing effect is maximum.In both type of these instruments, the current under measurement is passed through a coilof wire. This current carrying coil set up the necessary field depending on the magnitudeof the current to be measured. The coil may be of a few turns of very heavy conductor orof many turns of fine wire. The instrument to be used as an ammeter is provided with acoil of few turns of thick wire in order to have low resistance and carry large current andthat to be used as a voltameter is provided with a coil of large number of turns of wire inorder to have high resistance and draw as small current as possible.4. Derive the expression for torque produced in moving iron instrument.Let L be the self inductance corresponding to a total angular deflection ofq radians and change in inductance be dL correponding to small change indeflection angel dq due to small change in current.The change in energy of magnetic field,dw = Td dθSince change in energy dE = workdone, dwTd dθ = ½ I2dLTd = ½ I2dL/dθwhere I is in amperes, L is in Henry and θ is in Radians.Thus toruqe is proportional to the square of the instrument current and to the rateof change of inductance with deflection.

5. An energy meter revolves 10 revolutions of disc for unit of energy. Findthe number of revolutions made by it during an hour when connectedacross when connected 20A at 210V and 0.8 power factor leading. Ifenergy meter revolves 350 revolutions, find the % error.Answer.Energy consumed in one hour = VI cos φ / 1000= 210 x 20 x 0.8 / 1000= 3.360 kwh.The number of revolution the meter should make it is correct = 3.360 xregistration const in revolution per kwh= 3.360 x 100= 336Number of revolution actually made = 350% error = (350-336) x 100 / 350% error = 0.1466 %6. Explain how following torque are produced in pmmc instrument andattracted type moving iron instruments1. Deflecting torque2. Control torque3. Damping torque1. DEFLECTING TORQUE:- The deflecting torque is produced by makinguse of one of the magnetic, chemical, electrostatic and electromagneticinduction effects of current or voltage and causes the moving system of theinstrument to move from its zero position when the instrument is connected inan electrical circuit to measure the electrical quantity. The method ofproducing this torque depend upon the type of instrument. In attracting thetype of instrument, this torque to equal toTd = 1/2 I2 dL/dθWhereas in Pmmc instrumentsTd = BilurWhere B - magnetic densityi - current flowingl - length of coilu - number of turnr - radius of coil2. CONTROLLING TORQUE:- The magnitude of the movement to themoving system would be somewhat indefinite under the influence ofdeflecting torque unless some controlling torque exist. This torque opposesthe deflecting torque and increases with increase in deflection of the moving

system without controlling system the irrespective magnitude of current andmoreover, once deflected it would not return to its zero position on removingthe current.In attraction type instrument it is produced by spring control and in PMMCtoo it would be produced by spring control.3. DAMPING TORQUE:- This torque is also necessary to avoid oscillationof the moving system about it's final deflected position owing to the inertia ofthe moving parts and to bring the moving system to rest in it's final deflectedposition quickly.7. Briefly discuss the construction and working of a PMMC instrument.How range is extended in moving coil ammeter and voltmeter?Answer.CONSTRUCTION:- In a permanent magnet with soft iron pole pieces, acylindrical iron core is mounted between the two poles of the magnet givingvery narrow air gap in which the sides of a pointted light rectangular coil lies.The rectangular coil is wound of many turns of coil. The purpose of usingcore is to make the field uniform and to reduce the reluctance of the magneticcircuit. A low reluctance helps to retain permeance of magnet for a longerperiod.WORKING OF TORQUE EQUATION:- When the current to be measuredis passed through the coil, say in the direction as shown in fig. (a), deflectingtorque is produced. On account of relation between permanent magnetic fieldand coil magnetic field, the direction of deflecting torque can be determinedby applying Fleming's left hand rule.It is the current in amperes flowing through the coil of turns N and length lmeters B is flux density in test as in air gap:-Then, deflecting force F = BilN newtons.If r is the distance in meters in between the centers of the coil and force F.Then deflecting torque Td = F x r = BilNr Nm

From the above expression it is obvious that if flux density B in the air gap isconstant, thendeflecting torque Td = i , Tc= Td

So, θ = iAnd since q is directly proportional to current, the scale of the basic dcPMMC instrument are usually linearly spaced. Hence scale is linear.8. Discuss the construction and working of an electrodynamic wattmeter

with the help of diagram?Answer.This type of instrument is similar in design and principle to the dynanometertype ammeter and voltameter.WORKING AND CONSTRUCTION:- When the instrument of this type isused as wattmeter, the fixed coil which is divided into two equal portions inorder to provide them uniform field , is employed as current coil and movingcoil is used as pressure coil. The fixed coil which is divided into two equalportion in order to provide them uniform field, is employed as current coil andthe moving coil is used as pressure coil, i.e the fixed coil carries the currentproportional to the voltage across the circuit. A high non inductive resistanceis connected in series with the moving coil in order to limit current. Themagnetic field of the fixed and moving coil react on one another causing themoving coil to turn about it's axis. The movement is controlled by hair springswhich also leads the current into and out of the moving element. Damping isprovided by light aluminium moving in an air dash pot. The pointer is fixed tothe moving coil spindle and moves over a suitable caliberated scale.THEORY:- Let us be the supply voltage, i the load current and R the loadresistance of the moving coil circuit.Current through fixed coil, if = iCurrent through moving coil, im = V/Rdeflecting torque, Td ∝ if im ∝ V/RFor a DC circuit the deflecting torque is thus proportional to the power and forany circuit with fluctuating torque. The instantaneous to the instantaneouspower.9. Compare merits and demerits of moving iron type instruments anddynamometer type instruments. Which one is superior why?Answer.1. TORQUE HEIGHT RATIO:- Dynamometer type instruments have equal smalltorque height ratio.2. FRICTION ERROR:- Dynamometer type instruments have considerablefriction error.3. FRICTION LOSS:- Owing to heavy moving system, dynamometer typeinstruments have more friction losses.4. COST AND SENSITIVITY TO OVERLOAD:- As a result of measures to

reduce the frictional error, the dynamometer type instruments are more sensitiveto overloads and mechanical impactsis in comparison to moving iron typeinstruments.5. SENSTIVITY:- The senstivity of dynamometer instrument is typically verypoor due to poor deflecting torque.6. POWER CONSUMPTION:- Dynamometer type instrument havecomparatively higher power consumption.7. EFFECT OF STRAY MAGNETIC FIELD:- There is no effect of straymagnetic field on moving iron type while dynamometer type are most sensitivetowards it.8. HYSTERISIS AND EDDY CURRENT ERRORS:- Dynamometer typeinstruments are free from these erors while moving iron have these errors.9. EFFECT OF WAVE FORM:- Dynamometer type instruments are very usefulfor accurate measurement of runs voltage while frequency change serious e rrorsin AC measurement in moving iron type instruments.10. CALIBRATION:- Dynamometer type instruments have same calibration forAC and DC measurements while moving iron type have a difference between ACand DC calibration.10. Why shunt is usually used voltmeter and ammeter? A moving coilinstrument has a resistance of 5 _ and gives full deflection of 100mv.Show how the instrument may be used to measure:-1. voltage upto 50V2. current upto 10AAnswer.Shunt is usually used in voltmeter and ammeter to extend the range ofvoltmeter and ammeters.Rm = 5_Vm = 100mvIm = Vm/Rm = 100mv/5_ = 20mA1. For measuring voltage upto 50V.Series resistance is used with the instrument whose resistance isR = V/Im - Rm = 50/(20 x 10-3) - 5R = 2.5 x 10-3 - 5 = R = 2495 _2. Such resistance of resistance Rf is used to be connectedRf = Rm/[I/Im - 1]= 5/[10/20 x 10-3 -1] = 5 x 2/998Rf = 0.01002004 _11. Explain the principle of operation of attraction type moving iron

instruments and explain how the controlling and damping forces areobtained?Answer. The earliest and simplest form of attraction moving iron instrumentsuses a solenoid and moving oval shaped soft iron pinoted eccentrically. Tothis iron a pointer is attached so that it may deflect along with the moving ironover a graduate scale. The iron is made of sheet metal specially shaped to givea scale as nearby uniform as possible. The moving iron is drawn into field ofsolenoid when current flows through it. The movement of the iron alwaysfrom weaker magnetic field outside the coil into the stronger field inside thecoil regardless the direction of flow of current. When the current to bemeasured is passed through the solenoid, a magnetic field is set up inside thesolenoid, which in turn magnetises the iron. Thus the iron is attached into thecoil causing the spindle and the pointer to rotate.So much instruments normally have spring control and pneumatic dampingforces.12. 3-φ 4 pole induction motor is supplied from 3φ 50Hz ac supply. Find(1) synchronous speed(2) rotor speed when slip is 4%(3) the rotor frequency when runs at 600r.p.m.Sulution1) Ns =120f/p=120*50/4= 1500 r.p.m.2) speed when slip is 4% or .04N=Ns (1-s)=1500(1-0.04)= 1440 r.p.m.3) slip when motor runs at 600 r.p.m.S’=(Ns –N)/Ns

=(1500-600)/1500= 0.613. Explain the method of temperature control in open loop and closed loopsystems.Temperature controllers are needed in any situation requiring a giventemperature be kept stable. This can be in a situation where an object is requiredto be heated, cooled or both and to remain at the target temperature (set point),

regardless of the changing environment around it. There are two fundamentaltypes of temperature control; open loop and closed loop control. Open loop is themost basic form and applies continuous heating/cooling with no regard for theactual temperature output. It is analogous to the internal heating system in a car.On a cold day, you may need to turn the heat on to full to warm the car to 75°.However, during warmer weather, the same setting would leave the inside of thecar much warmer than the desired 75°.Open loop control block diagramClosed loop control is far more sophisticated than open loop. In a closedloop application, the output temperature is constantly measured and adjusted tomaintain a constant output at the desired temperature. Closed loop control isalways conscious of the output signal and will feed this back into the controlprocess. Closed loop control is analogous to a car with internal climate control. Ifyou set the car temperature to 75°, the climate control will automatically adjustthe heating (during cold days) or cooling (during warm days) as required tomaintain the target temperature of 75°.A temperature controller is a device used to hold a desired temperature at aspecified value. The simplest example of a temperature controller is a commonthermostat found in homes. For instance, a hot water heater uses a thermostat to controlthe temperature of the water and maintain it at a certain commanded temperature.Temperature controllers are also used in ovens. When a temperature is set for an oven, acontroller monitors the actual temperature inside of the oven. If it falls below the settemperature, it sends a signal to activate the heater to raise the temperature back to the setpoint. Thermostats are also used in refrigerators. So if the temperature gets too high, acontroller initiates an action to bring the temperature down.14. Explain about open loop and closed loop control system.Figure 1 shows an open loop system. A computed force is applied to the system

which is expected to respond based on the specifications. If the system fails to respondcorrectly (because your estimates were off) or an unanticipated disturbance acted on it,then there is no way to correct the course. On the other hand, figure 2 shows a feed-backsystem. The response C(s) is measured using the sensor H(s) and the resultant iscompared with the input R(s). The resultant difference (error) is acted upon by thecontroller which works on the actuator. The actuator then applies the required force onthe system.The closed loop thus contains the sensor dynamics, the controller dynamics, theactuator dynamics in addition to the system we are interested in. It should be noted thatall measurements have to be done or converted if necessary into one unit so thatcomparison with the target signal is possible. Usually, measurements result in currentsand voltages. Hence, this conversion from a mechanical input to an electrical output isalso included in the sensor, controller and actuator dynamics.In designing the full control system the dynamics of all the components need to beaccounted for. If the controller is very slow compared to the system, it will not send theright input at the right time. In this class, we will assume perfect sensor and actuatordynamics, i.e., what goes into the sensor (it is commonly denoted by H(s)) and theactuator comes out unmodified instantaneously. So we replace them with unity transferfunctions.15. Explain the relation between voltage, current and resistanceAn electric circuit is formed when a conductive path is created to allow freeelectrons to continuously move. This continuous movement of free electrons throughthe conductors of a circuit is called a current, and it is often referred to in terms of"flow," just like the flow of a liquid through a hollow pipe.The force motivating electrons to "flow" in a circuit is called voltage. Voltageis a specific measure of potential energy that is always relative between two points.When we speak of a certain amount of voltage being present in a circuit, we are

referring to the measurement of how much potential energy exists to move electronsfrom one particular point in that circuit to another particular point. Without referenceto two particular points, the term "voltage" has no meaning.Free electrons tend to move through conductors with some degree offriction, or opposition to motion. This opposition to motion is more properlycalled resistance. The amount of current in a circuit depends on the amount ofvoltage available to motivate the electrons, and also the amount of resistance inthe circuit to oppose electron flow. Just like voltage, resistance is a quantityrelative between two points. For this reason, the quantities of voltage andresistance are often stated as being "between" or "across" two points in a circuit.To be able to make meaningful statements about these quantities incircuits, we need to be able to describe their quantities in the same way that wemight quantify mass, temperature, volume, length, or any other kind of physicalquantity. For mass we might use the units of "kilogram" or "gram." Fortemperature we might use degrees Fahrenheit or degrees Celsius. Here are thestandard units of measurement for electrical current, voltage, and resistance:16. Explain the construction of DC machine with neat diagram.A D.C. machine consists mainly of two part the stationary part called stator andthe rotating part called stator.The stator consists of main poles used to produce magnetic flux ,commutatingpoles or interpoles in between the main poles to avoid sparking at the commutator but inthe case of small machines sometimes the interpoles are avoided and finally the frame oryoke which forms the supporting structure of the machine.The rotor consist of an armature a cylindrical metallic body or core with slots in itto place armature windings or bars, a commutator and brush gearsThe magnetic flux path in a motor or generator is show below and it is called themagnetic structure of generator or motor.Cross sectional view of a DC MachineFrameFrame is the stationary part of a machine on which the main poles andcommutator poles are bolted and it forms the supporting structure by connecting the

frame to the bed plate. The ring shaped body portion of the frame which makes themagnetic path for the magnetic fluxes from the main poles and interpoles is called Yoke.YokeIn early days Yoke was made up of cast iron but now it is replaced by cast steel.This is because cast iron is saturated by a flux density of 0.8 Wb/sq.m where as saturationwith cast iron steel is about 1.5 Wb/sq.m.So for the same magnetic flux density the crosssection area needed for cast steel is less than cast iron hence the weight of the machinetoo. If we use cast iron there may be chances of blow holes in it while casting. so nowrolled steels are developed and these have consistent magnetic and mechanical properties.End Shields or BearingsIf the armature diameter does not exceed 35 to 45 cm then in addition to poles endshields or frame head with bearing are attached to the frame. If the armature diameter isgreater than 1m pedestral type bearings are mounted on the machine bed plate outsidethe frame. These bearings could be ball or roller type but generally plain pedestralbearings are employed. If the diameter of the armature is large a brush holder yoke isgenerally fixed to the frame.17. Explain the Working of dynamometer type wattmeterThe principle of operation of the electrodynamometer-type wattmeter is the sameas that for dynamo-electric machines. The deflection torque is produced by theinteraction of two magnetic fluxes. One of the fluxes is produced by a fixed coil whichcarries a current proportional to the load current and therefore called the current coil. Theother flux is created by a movable coil which carries a current proportional to the loadvoltage and thus called the voltage or potential coil. A high non-inductive resistance isconnected to the potential coil so that its current is almost in phase with the load voltage.The control torque is provided by a control spring.In a dynamometer type wattmeter the fixed coil (current coil) is connected in

series with the load.This coil is divided in to two parts and they are kept parallel to eachother. The coil is thick in cross section and has less number of turns.The moving coil(pressure coil) is connected across the load. It is thin in cross - section and has hundredsof turns. It has a non - inductive high resistance in series with itThe wattmeter is an electrodynamic instrument for measuring the electric poweror the supply rate of electrical energy of any given circuit. The device consists of a pair offixed coils, known as current coils, and a movable coil known as the potential coil.The current coils are connected in series with the circuit, while the potential coil isconnected in parallel. Also, on analog wattmeters, the potential coil carries a needle thatmoves over a scale to indicate the measurement. A current flowing through the currentcoil generates an electromagnetic field around the coil. The strength of this field isproportional to the line current and in phase with it. The potential coil has, as a generalrule, a high-value resistor connected in series with it to reduce the current that flowsthrough it. The result of this arrangement is that on a dc circuit, the deflection of theneedle is proportional to both the current and the voltage, thus conforming to the equationW=VA or P=EI. On an ac circuit the deflection is proportional to the averageinstantaneous product of voltage and current, thus measuring true power, and possibly(depending on load characteristics) showing a different reading to that obtained by simplymultiplying the readings showing on a stand-alone voltmeter and a stand-alone ammeterin the same circuit. The two circuits of a wattmeter are likely to be damaged by excessivecurrent. The ammeter and voltmeter are both vulnerable to overheating - in case of anoverload, their pointers will be driven off scale - but in the wattmeter, either or even boththe current and potential circuits can overheat without the pointer approaching the end ofthe scale! This is because the position of the pointer depends on the power factor, voltageand current. Thus, a circuit with a low power factor will give a low reading on the

wattmeter, even when both of its circuits are loaded to the maximum safety limit.Therefore, a wattmeter is rated not only in watts, but also in volts and amperes.18. Explain the construction of transformer with neat diagram.A transformer is an electrical device used to convert AC power at a certainvoltage level to AC power at a different voltage, but at the same frequency.The construction of a transformer includes a ferromagnetic core around whichmultiple coils, or windings, of wire are wrapped. The input line connects to the 'primary'coil, while the output lines connect to 'secondary' coils. The alternating current in theprimary coil induces an alternating magnetic flux that 'flows' around the ferromagneticcore, changing direction during each electrical cycle. The alternating flux in the core inturn induces an alternating current in each of the secondary coils. The voltage at each ofthe secondary coils is directly related to the primary voltage by the turns ratio, or thenumber of turns in the primary coil divided by the number turns in the secondary coil.For instance, if the primary coil consists of 100 turns and carries 480 volts and asecondary coil consists of 25 turns, the secondary voltage is then:secondary voltage = (480 volts) * (25/100) = 120 voltsTwo coils of wire (called windings) are wound on some type of core material. Insome cases the coils of wire are wound on a cylindrical or rectangular cardboard form. Ineffect, the core material is air and the transformer is called an AIR-CORETRANSFORMER. Transformers used at low frequencies, such as 50 hertz and 400 hertz,require a core of low-reluctance magnetic material, usually iron. This type of transformeris called an IRON-CORE TRANSFORMER. Most power transformers are of the ironcoretype. The principle parts of a transformer and their functions are:The CORE, which provides a path for the magnetic lines of flux.The PRIMARY WINDING, which receives energy from the ac source.The SECONDARY WINDING, which receives energy from the primarywinding and delivers it to the load.The ENCLOSURE, which protects the above components from dirt,moisture, and mechanical damage.A soft-iron-core transformer is very useful where the transformer must bephysically small, yet efficient. The iron-core transformer provides better power transfer

than does the air-core transformer. A transformer whose core is constructed of laminatedsheets of steel dissipates heat readily; thus it provides for the efficient transfer of power.The majority of transformers you will encounter in Navy equipment contain laminatedsteelcores. These steel laminations are insulated with a non conducting material, such asvarnish, and then formed into a core. It takes about 50 such laminations to make a core aninch thick. The purpose of the laminations is to reduce certain losses.