FINAL EM-I SAP

7/30/2019 FINAL EM-I SAP

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EX- 404ELECTRICAL MACHINEI

Unit-I

Transformer-I

Working principle, e.mf. equation, construction, phasor diagrams, equivalent circuit, voltage regulation, losses,separation of hysteresis and eddy current losses, efficiency, tests: open circuit and short circuit, load, Sumnerstest, Condition for maximum efficiency and regulation, Power and distribution transformer, all-day efficiency,

Excitation phenomenon, Autotransformer: working, Advantages, its equivalent circuit and phasor diagram.

Unit II

Transformer-II

Three phase transformer: its construction, groups and connections, their working and applications; Scottconnection; Parallel operation of Transformers: application, advantages, requirement and load sharing; Tap

changers, cooling, conservator and breather. Pulse and high frequency transformers.

Unit III

Three phase Induction Motor- I

Working principle, construction, comparison of slip ring and squirrel cage motors, steady state analysis, phasor

diagram and equivalent circuit, power flow diagram, torque-speed and power-speed characteristics, Losses andefficiency, No load and block rotor test, circle diagram

Unit IV

Three phase Induction Motor-II

Starting of squirrel cage and slip ring motors, power factor control, Cogging & Crawling, Double cage &Deep

bar Indication Motor, impact of unbalanced supply and harmonics on performance, speed control, braking,

Induction Generator. Applications

Unit VSingle Phase Motors:

Single Phase Induction motor; double revolving field theory, equivalent circuit and its determination,

performance calculation, starting methods and types of single phase Induction motors: their working Principle

and applications, comparison with three phases Induction Motor. Single phase A.C. series motor, Servo

motors, Linear Induction Motor


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Truba College of Science & Technology, Bhopal

Department: Electrical &

Electronics

Session: Jan-2013

Name of Faculty: Priyank nema Semester: four

Subject: Electrical Machine-I Sub Code: EX-404

Time Schedule : Total expected period 50 Extra Periods (if required)-Day Mon Tue Wed Thu Fri Sat Max. Available

No. of

Period

Lect. Lecture to be Covered Planned date of

Completion

REFERENCES

UNIT I

1 Working principle, e.mf. equation R1-(4-9)

2 construction, phasor diagrams, R1-(2-4 ,10-14)

3 equivalent circuit, voltage regulation R1-(20-25 ,40-44)

4 losses, separation of hysteresis and eddy current losses R1-(49-64)

5 efficiency, R1-(50-52)

6 tests: open circuit and short circuit, load, Sumpners test R1-(29-32 ,66-68)

7 Condition for maximum efficiency and regulation R1-(51, 43)

8 Power and distribution transformer R1-(1.45,1.46)

9 all-day efficiency, Excitation phenomenon R1-(53)

10 Autotransformer: working, advantages R1-(70-77)

11 equivalent circuit and phasor diagram R1-(74-76)Unit II

12 Three phase transformer R2-(83)

13 Construction

R2-(84-86)

14 groups and connections R2-(86-100)

15 working and applications R2-(95-100)

16 Scott connection; R2-(101-107)

17 Parallel operation of Transformers R1-(84)

18 application, advantages R2-(149)

19 requirement and load sharing R1-()

20 Tap changers R1-(100-105)

21 cooling, R2-(160)

22 Conservator R2-(161)

23 Breather R2-(161)

24 Pulse and high frequency transformers R1-(123,128)

25 Pulse and high frequency transformers R1-(123,128)

UNIT III


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26 Working principle construction, R1-(705)

27 comparison of slip ring and squirrel cage motors R2-(309)

28 steady state analysis R1-(724)

29 phasor diagram and equivalent circuit R1-(720,721)

30 power flow diagram R1-(716)

31 torque-speed and power-speed characteristics, R1-(728)

32 Losses and efficiency, R1-(716)

33 No load and block rotor test R1-(767-777)

34 circle diagram R1-(775-781)

UNIT IV

35 Starting of squirrel cage and slip ring motors R1-(791-796)

36 power factor control, Cogging & Crawling R1-(786-788),R2(393-396)

37 Double cage &Deep bar Indication Motor R2-(383-385)

38 impact of unbalanced supply R2-()

39 harmonics on performance R2-(393)

40 Speed control, R2-(396-407)

41 Braking R2-()42 Induction Generator. Applications R2-(408-411)

UNIT V

43 Single Phase Induction motor; double revolving field

theory,

R2-(550-552)

44 equivalent circuit and its determination R2-(554-557)

45 performance calculation R2-(554-557)

46 starting methods and types of single phase Induction

motors

R2-(563-571)

47 Working principle and applications R2-(563-571)

48 Comparison with three phases Induction Motor. R2-(571)

49 Single phase A.C. series motor, R2-(574)

50 Servo motors, Linear Induction Motor R2-(584-589)

REFRENCES:

S.N. Title Author publisher

1 Electrical machine Dr. P.S.bhimbra Khanna

2 Electrical Machines Ashfaq Hussain Dhanpat Rai

3 Electrical Machines Nagrath and Kothari TMH

Prepared by Approved by (HOD)

Mr.Priyank Nema


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List of Experiments

1. Perform turn ratio and polarity test on 1-phase transformer

2. Perform load test on a 1-phase transformer and plot its load characteristic

3. Perform OC and SC tests on a 1-phase transformer and determine its equivalent circuit. Also find

Its efficiency and regulation at different load and power factor.

4. Perform OC and SC tests on a 3-phase transformer and determine its equivalent circuit. Also find

Its efficiency and regulation at different load and power factor.

5. Perform Sumpners test on two 1-phase transformer and determine its efficiency at various load.

6. Perform No-load and block rotor test on a 3- phase IM and determine its equivalent circuit.

7. Perform load test on a 3- phase IM and plot its performance characteristics.

8. Study various types of starters used for 3- IMs.

9. Perform No-load and block rotor test on a 1- phase IM and determine its equivalent circuit.


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Points of PonderUnitI

A transformer operates on the principle of mutual induction between two (or more)

inductively coupled coils or circuits, called primary and secondary. Energy is transferred

from the primary to secondary at different voltages and currents keeping frequency constant.

A transformer can be wound with either of the following two types of windings

1. Core-type windings wound around the two limbs of a rectangular iron core.2. Shell-type windings wound over the central limb of the rectangular iron core.

The core of the transformer is made of CRGO silicon steel to reduce the eddy current losses.

The oil in the transformer performs an important function of serving as an insulating medium

between the main tank of the transformer and the windings on the core of the transformer.

A transformer can be classified on the basis of the services rendered as

1. Distribution Transformer.2. Power Transformer.The change in terminal voltage of transformer from no-load to full load with respect to the

rated terminal voltage at full load is termed as voltage regulation, and in percentage form it is

called as percentage voltage regulation.

The polarity test on transformer is performed to check whether the polarities of the

transformer in accordance with dot convention are in order.

The open circuit on transformer is performed to determine magnetizing reactance Xm and

equivalent resistance rh,e due to iron loss.

The short circuit test on transformer is performed to determine the equivalent resistance and

leakage reactance either referred to the primary side or referred to the secondary side.

With the help of Sumpners test both the open circuit and short circuit tests can be performed

simultaneously. The only need of Sumpners test is that another identical transformer isrequired.

The transformer efficiency is maximum when its copper losses equal the iron losses.

The All-day efficiency of a transformer is the ratio of energy output from the transformer to

the energy input for a period of 24 hours.

The core or iron losses in a transformer remain substantially constant from no load to full

load.

The transformers are rated in volt-ampere rating rather than in real power.

An auto-transformer is a special kind of single winding transformer in which a part of

winding is common to both primary and secondary and the two windings are not electrically

isolated.


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UnitII

Modern Large transformers are of three phase type. Both primary and secondary have three

phase windings. Sometimes, large transformers, besides primary and secondary, have a third

winding known as tertiary winding which helps in allowing the third harmonic current to

flow, thus reducing the third harmonic voltage.

A three phase transformer can have several vector groupings such as

1. Yd12. Yd113. Dy14. Dy115. Yy0 etc.Depending upon the application, the various methods of connection of three phase

transformers are

1. Star/Star Connection.2. Star/Delta Connection.3. Delta/Star Connection.4. Delta/delta Connection.5. Open Delta Connection etc.A three phase supply can be converted to a two phase supply by connecting two single phase

transformers of a suitable ratio in what is known as Scott Connection. The Scott connection

of transformers can also convert a three phase supply to single phase supply.

In the delta/delta connection, if one of the phases of both the primary and secondary windings

is disconnected, the transformer is then termed as open-delta connected transformer. The

available rating of then resulted open delta connection is 58% of the normal delta/delta

connection.To meet increased power demand, single-phase transformers may be connected in parallel.

Such transformers connected in parallel need to fulfill certain conditions.

When two single phase transformers of different KVA ratings (but of equal voltage ratios)

are connected in parallel, they divide the total load in proportion to their respective KVA

ratings.

When two transformers with unequal voltage ratios share a load, a circulating current is

created in transformer secondarys.Regulating the voltage of a transformer is a requirement that often arises in a power

application or power system. In an application it may be needed

1. To supply a desired voltage to the load.2. To counter the voltage drops due to loads.

3. To counter the input supply voltage changes on load.

Several arrangements to cool the hot oil of the transformer have been designed

(a) ONAN (b) ONAF (c) OFAN (d) OFAF etc.


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Buchholz relay is a safety arrangement incorporated in a transformer to give an alarm in caseof an incipient fault, or to trip the circuit breaker if the fault is not noticed and is allowed to

develop to a greater magnitude.

UnitIII

A rotating and constant resultant magnetic field rotating at the angular frequency of the

supply is produced by a three phase group of stator windings displaced physically in space

displaced by 1200 from each other, if the currents flowing through the windings are also

displaced in time by 1200.

A three phase induction motor, in order to produce a constant rotating magnetic field,

requires three identical and individual windings, displaced physically on the stator by 1200,

and carrying currents also displaced by 1200 in time.

The squirrel cage and slip ring type of induction motors are the two types of three phase

induction motors. The squirrel cage three phase induction motor is simpler to construct, run

and operate than the slip ring induction motor.

A three phase induction motor is called as asynchronous motor because it never attains speed

equal to synchronous speed.

The voltage equation of three phase induction motor with distributed and short-pitched coils

is given by

pbmphphkkNE 44.4

Where the symbols have their usual meanings.

The slip of three phase induction motor is given by

%100

s

rs

N

NNs

At standstill (i.e.) when Nr = 0; s = 1p.u

And when Nr = Ns; s = 0 p.u.

The performance of three phase induction motor at different stages of its operation can be

explained by its equivalent circuits drawn on per phase basis.

The various operating mode of three phase induction is given as

1. Motor mode (1 < s < 0)2. Generator mode (s < 0)3. Braking mode (s > 1).The torque-speed characteristics of three phase induction motor, with different values of rotorcircuit resistance, show that it is possible to increase the starting torque by increasing the

rotor circuit resistance.

The no-load test and the blocked rotor test are performed to determine the parameters of the

three phase induction motor. The rotational loss can be separated out from the iron loss by

conducting the no load test.


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The performance characteristics of both cage rotor and slip ring rotor induction motors can bedetermined approximately by a graphical method known as the circle diagram. This method

is, however, not applicable to double cage induction motor.

In an induction motor, there is an air gap and the stator and rotor windings are distributed

along the periphery of the air gap rather than concentrated on a core as in a transformer.

Therefore, the leakage reactance of stator and rotor windings are quite large compared to thatof a transformer.

UnitIV

There are several methods of starting a three phase induction motor such as

Direct-on-line starting. Reduced voltage starting.

Stator impedance starting Autotransformer starting Star-delta starting

The star-delta starting is a specific case of autotransformer method where the value of x (i.e.

the tap of autotransformer) is 1/3.In case of three phase slip ring induction motor, the starting torque can be enhanced by

providing an external resistance in the rotor circuit.

The deep bar squirrel cage motor is a special type of construction where a high starting

torque and a low starting current can be developed.

There is another special kind of construction known as double-cage rotor which provides a

high-starting torque, and sufficient speed at the normal running condition. The combined

effect of both these features provides excellent torque-speed characteristics.

Crawling is a peculiar phenomenon of three phase induction motor which causes the machine

to run stably at low speed, due to seventh and fifth harmonics of winding mmf developed on

account of certain combinations of the number of stator slots and the number of rotor slots. If

the numbers of stator slots become equal to the number of rotor slots or an integral multiple

of the rotor slots, a phenomenon called cogging occurs.

Some slot combinations develop higher harmonic effects such as locking and stalling. To

overcome these effects, designers have prescribed certain rules for the choice of rotor slots of

squirrel cage induction motors.

Several methods of speed control of induction motors are available as follows:

1. Voltage control.2. Rotor resistance control.3. Pole changing.4. Frequency control.5. Cascade arrangement.6. Injecting voltage in the rotor circuit.


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The voltage control method and the rotor resistance control do not give a wide range of speedcontrol. These methods are therefore not used for industrial applications. They are suitable

only for small appliances.

When the frequency control method of speed control, the maximum torque remains

independent of frequency and the slip at maximum torque decreases with the increase in

frequency. The starting torque too decreases with increase in frequency.When the torque of the induction motor is reversed, it becomes the generator. The value of

the slip becomes negative. The induction generator therefore needs to be run by a prime-

mover whose speed is more than the synchronous speed of the induction machine.

UnitV

At starting when the slip is equal to 1, the starting torque of the single phase induction

motor is zero due to two oppositely directed torques. Therefore it means that the single

phase induction motor is not self starting. But if by any means the rotor is moved, then

the motor will start running due to the net torque resulting from the two oppositely

rotating field. This phenomenon can be explained by the double-revolving field theory,

which treats the equivalent circuit of the single-phase induction motor as the combination

of two rotors-one moving in the forward direction and the other in the backward

direction.

The parameters of the equivalent circuit of single phase induction motor can be

calculated from no load test and blocked rotor test.

The auxiliary winding has a smaller number of turns of thin wire than the main winding.

The auxiliary winding, therefore, has a high resistance and low reactance. The main

winding made of larger number of turns of thick wire has a low resistance and high

reactance. But because of lower total impedance of the main winding, the current in it is

usually higher than the current in the auxiliary winding.

The various methods of starting a single phase induction motor are categorized as

1. Split-phase, resistance-start motor.2. Split-phase, capacitor-start motor.

Single value capacitor motor. Two value capacitor motor.

3. Shaded pole motor.A stepper motor is an electromechanical drive which is used as a control device for

incremental motion purposes.

Servomotors are power devices used in electrical control systems. Servomotors may be of dc

or ac type. There are two methods of controlling a dc servomotor:

1. Field control and2. Armature Control.


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An ac servomotor is a squirrel cage or drag cup induction motor. AC servomotors aregenerally of two-phase type for low power applications. Three-phase ac servomotors are used

where large power is required.

Hysteresis motor is a type of synchronous motor that does not employ dc field excitation. Its

operation depends on the permanently magnetized rotor which provides continuously

revolving magnetic flux. The torque remains practically constant from the locked rotorposition to the synchronous speed.


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Interview /viva questions

Unit-1

1.

What are the functions of transformer?2. Is it possible for any voltage winding in a transformer to serve as its

primary?

3. Why is it that the emfs induced in the primary and secondary windings of atransformer are in phase with each other?

4. How can the iron loss be minimized in a transformer?5. How can core loss be measured in a transformer?6. Which equivalent circuit parameter can be determined from the open circuit

test on a transformer?

7. Which equivalent circuit parameter can be obtained from short circuit test ona transformer?

8. What is the order of magnitude of n load current in a transformer?9. How are the transformer losses affected if power factor of a given load is

varied?

10.How is the voltage regulation of a transformer affected by a change in itsoperating frequency?

Unit-2

1. What is achieved by Scott connection of a transformer?2. How many transformers are needed for Scott connection?3. What tapping should be provided on the main and Teasure transformer?4. What is the phasor difference between the output voltages of Scott

connection?

5. In case the load on two secondaries of Scott connected transformers aredifferent, what will be the position of current in the primary winding?

6. Why parallel operation of transformers is needed in practical case?7. How many transformers are at least needed to conduct this experiment?8. What are the essential conditions for operating two single phase transformer

in parallel?

9. What is the maximum limit of circulating current that can be permitted forsatisfactory parallel operation?


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10.What is the effect of wrong polarity on the parallel connected transformers?Unit-3

1. Why three phase induction motors are very popular as drives for industrialapplication?

2. Why the stator of an induction motor is limited?3. What is the normally the thickness of lamination of stator core?4. What is the shape of slots used for the stator of induction motor?5. What are advantages of using semi-closed slots for the stator?6. What is the frequency of rotor flux in case of 3-phase induction motor?7. What are the various types of 3-phase induction motors as per the rotor

construction?

8. How the starting torque is increased in slip ring induction motor?9. How the starting torque is increased in squirrel cage motor?10.In double cage rotor which of the cage is active during starting?

Unit-4

1. Why the starting current drawn by the motor is so high?2. How the starting current can be reduced to permissible value?3. What are the various types of starter used for starting of 3-phase induction

motor?4. How the starting current is reduced in rotor resistance starter?5. How high starting torque is obtained in slip ring induction motor?6. What is the basic aim in conducting no load and block rotor test on 3-phase

induction motor?

7. What is the various information that is needed to draw the circle diagram?8. Under what condition no load test is performed on the motor?9. What is the approximate value of no load power factor of 3-phase induction

motor?

10.Why the no load power factor of motor is quite low?Unit-5

1. What information is obtained from no loads and block rotor test on singlephase induction motor?


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2. What is the slip of forward and backward rotor?3. How many windings are provided on the stator of split phase induction

motor?

4. How the phase splitting between two windings can be increased?5. At what speed of the motor starting winding is disconnected from the circuit

of the main winding?

6. What are the various parameters of the equivalent circuit that can becalculated from block rotor test?

7. How much is the phase splitting between the two windings?8. State the condition under which no load test is performed?9. Under what condition, block rotor test is performed?10.What are the different types of servo motor?


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FINAL EM-I SAP

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