LATHA MATHAVAN ENGINEERING COLLEGELATHA MATHAVAN NAGAR

Madurai

EE 6504- ELECTRICAL MACHINES IIIMPORTANT 2 MARKS

QUESTION WITH ANSWER

PREPARED BY,

Mr.A.LAWRANCE SAHAYA SUNDAR, AP/EEE

[DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING]

LATHA MATHAVAN ENGINEERING COLLEGE Latha Mathavan nagar, Kidaripatti, Madurai – 625 301

Subject Name: ELECTRICAL MACHINES II Branch : EEE

Subject Code: EE6504 Semester : VName of the faculty: A.LAWRANCE SAHAYA SUNDAR

Unit –I

SYNCHRONOUS GENERATOR

1. What do you mean by the salient-pole type rotor?

Salient - pole type rotor means a low and moderate speed rotor having large diameter and small axial length with projected poles coming out of the rotor frame the outer surface of which almost follows the inner cylindrical surface of the stator frame.

2. Define voltage regulation of an alternator?

The voltage regulation of an alternator is defined as the increase in terminal voltage when full load is thrown off, assuming field current and speed remaining the same.

Percentage regulation = E0 - V x 100E0 = No load terminal voltageV = Full load rated terminal voltage.

3. What are the advantages of having rotating field system?

1. Better insulation2. Ease of current collection3. Increased armature tooth strength.4. More rigid construction5. Reduced armature leakage reactance.6. Lesser number of sliprings.

4.Why is EMF method called Pessimistic method?The value of voltage regulation obtained by EMF method is always more than the

actual value, therefore it is called Pessimistic method.

5.Why is MMF method called Optimistic method?

The value of voltage regulation obtained by MMF method is less than the actual value, therefore it is called Optimistic method.

6.Compare salient pole rotor & smooth cylindrical rotor.

Salient Pole Rotor Cylindrical Rotor

1. Large diameter and short axial length 1 . Small diameter and long axial length

2. Used for lowspeed alternators 2. Used for high - speed turboalternators

3.Has projecting poles 3. No projecting poles

4. Needs damper windings 4. Does not need damper windings.

5.Windage loss is more 5. Windage loss is less

7. How is the armature winding in alternators different from those used in dc machines?

The armature winding of the alternator is placed in the stator, but the in case of dc machines, armature winding is placed in rotor.

8. What are the methods by using zero p.f. lagging curve can be obtained?

Zero power factor characteristic of an alternator gives the variation of terminal voltage with field current, when the alternator is delivering its full rated current to a zero power factor (lagging)load. This characteristic is obtained by running the machine at synchronous speed and connecting a purely inductive 3phase load to its terminals. The load is varied in steps and at each step the field current is adjusted, so that the armature current is equal to its rated value.

9. What are squirrel-cage windings of alternators? How and why are they used?

Damper windings are squirrel cage windings of the alternators. This winding is placed in rotor pole shoes.

10. Write down the equation for frequency of emf induced in an Altenator.

Frequency of emf induced in an Alternator,f ,expressed in cycles per second or Hz, is given by the following equation,

f = (PN)/120 HzWhere,

P-Numberofpoles N-Speed in rpm

11. What are the advantages of salient pole type construction usedfor Synchronous machines?

Advantages of salient-pole type construction are : They allow better ventilation

The pole faces are so shaped that the radial air gap length increases from the pole center to the pole tips so that the flux distribution in the air-gap is sinusoidal in shape which will help the machine to generate sinusoidal emf

Due to the variable reluctance the machine develops additional reluctance power which is independent of excitation

12. Name the types of Alternator based on their rotor construction.

Alternators can be classified into the following two types according to its rotor construction,

Smooth cylindrical type alternator Salient pole alternator

13.Why is short pitch winding preferred over full-pitch winding ?

Advantages, Waveform of the emf can be approximately made to a sine wave and

distorting harmonics can be reduced or totally eliminated. Conductor material , copper , is saved in the back and front end Connections due to less coil-span. Fractional slot winding with fractional number of lots/phase can be

used which in turn reduces the tooth ripples. Mechanical strength of the coil is increased

14. Define winding factor.

The winding factor Kd is defined as the ratio of phasor addition of emf induced in all the coils belonging to each phase winding to their arithmetic addition.

15. Why are Alternators rated in kVA and not in kW?

The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class. Apart from the constant loss incurred in Alternators is the copper loss, occurring in the 3 -phase winding which depends on I2 R, the square of the current delivered by the generator. As the current is directly related to apparent - power delivered by the generator , the Alternators have only their apparent power in VA/kVA/MVA as their power rating.

16.What is the necessity for predetermination of voltage regulation?

Most of the Alternators are manufactured with large power rating , hundreds of kW or MW, and also with large voltage rating upto 33kV. For Alternators of such power and voltage ratings conducting load test is not possible. Hence other indirect methods of testing are used and the performance like voltage regulation then can be predetermined at any desired load currents and power factors.17.Name the various methods for predetermining the voltage regulation of

3-phase Alternator.

The following are the three methods which are used to predetermine the voltage regulation of smooth cylindrical type Alternators

Synchronous impedance / EMF method

Ampere-turn / MMF method

Potier / ZPF method

18.What are the advantages and disadvantages of estimating the voltage regulation of an Alternator by EMF method?

Advantages:

Simple no load tests (for obtaining OCC and SCC) are to be conducted Calculation procedure is much simpler

Disadvantages:

The value of voltage regulation obtained by this method is always higher than the actual value.

19.What are the test data required for predetermining the voltage regulation of an Alternator by MMF method?

Data required for MMF method are :

Effective resistance per phase of the 3-phase winding (R) Open circuit characteristic (OCC) at rated speed/frequency Short circuit characteristic (SCC) at rated speed/frequency

20. Why is the MMF method of estimating the voltage regulation considered as the optimistic method?

Compared to the EMF method, MMF method, involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes care of saturation effect. As this method require more effort, the final result is very close to the actual value. Hence this method is called optimistic method.

21. State the condition to be satisfied before connecting two alternators in parallel? The following are the three conditions to be satisfied by synchronizing the

additional Alternator with the existing one or the common bus-bars. The terminal voltage magnitude of the incoming alternator must be made

equal to the existing Alternator or the bus-bar voltage magnitude. The phase sequence of the incoming Alternator voltage must be similar to the

bus-bar voltage. The frequency of the incoming Alternator voltage must be the same as

the bus-bar voltage.

22. List the factors that affect the load sharing in parallel operating generators?

The total active and reactive power delivered to the load, connected across the common bus-bars, are shared among Synchronous generators, operating in parallel, based on the following three factors,

Prime-mover characteristic/input Excitation level and Percentage synchronous impedance and its R/X ratio

23. How are alternators classified? According to type of field system

Stationary field system type Rotating field system type

According to shape of field system Salient pole type Smooth cylindrical type

24. Which type of Synchronous generators are used in Hydro-electric plants and why?

As the speed of operation is low for hydro turbines use din Hydro-electric plants, salient pole type Synchronous generators are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor.

25. Why almost all large size Synchronous machines are constructed with rotating field system type?

The following are the principal advantages of the rotating field system type construction of Synchronous machines:

The relatively small amount of power, about 2%, required for field system via slip-rings and brushes. For the same air gap dimensions, which is normally decided by the kVA rating, more space is available in the stator part of the machine for providingmore insulation to the system of conductors, especially for machines rated for 11kV or above. Insulation to stationary system of conductors is not subjected to mechanicalstresses due to centrifugal action. Stationary system of conductors can easily be braced to prevent

deformation.

It is easy to provide cooling arrangement for a stationary system of conductors.

Firm stationary connection between external circuit and system of conductors enable he machine to handle large amount of volt-ampere as high as 500MVA.

Unit –II

SYNCHRONOUS MOTOR

1. What is hunting? How is hunting minimized?

When a synchronous motor is used for driving a fluctuating load, the rotor starts oscillating about its new position of equilibrium corresponding to the new load. This is called hunting or phase swinging. To prevent hunting, dampers (or) damping grids are employed. Damper windings are short circuited, copper bars are embedded in the faces of the field poles of the motor.

2.When is a synchronous motor said to receive 100%

excitation? When Eb = V, synchronous motor receive 100% excitation.

3. What is a synchronous capacitor?

An over excited synchronous motor, running without any mechanical load, used specifically for power factor correction is known as synchronous capacitor.

4. When is a synchronous motor said to be under - excited? What will be the p.f at this condition?

Excitation emf Eb less than supply voltage Eb < V. At this condition the power factor is Lagging power factor.

5. What are the inherent disadvantages of synchronous motor?Disadvantages of synchronous motor are,

i) Higher costii) Necessity of a dc excitation sourceiii) Greater initial costiv) High maintenance costv)

6. Mention four applications of synchronous motor?

Applications of synchronous motor are

i) Power factor correctionii) Constant speed, constant load drivesiii) Voltage regulation of transmission lines.

7. What is the role of synchronous motor in a transmission line? How?

Synchronous motor acts as a voltage regulator in a transmission line. When line voltage decreases due to inductive load, motor excitation is increased thereby increasing its power factor which compensates for the line voltage drop. When the line voltage increases due to line capacitive effect, synchronous motor excitation is decreased, thereby making its power factor lagging which helps to the maintain the transmission line voltage at its normal value.

8. Enlist the advantages of synchronous motor.

Advantages of Synchronous Motors:

1. The speed is constant and independent of load.

2. These motors usually operate at higher efficiencies.

3. Electro magnetic power varies linearly with the voltage.

4. These motors can be constructed with wider air gaps than induction motors, which make them better mechanically.

5. An Over excited synchronous motor having a leading power factor can be

operated in parallel with induction motors.

9. Define pullout torque in synchronous motor?

The maximum torque which the motor can develop without pulling out of step or synchronism is called the pull out torque.10. What is synchronous condenser?

Synchronous motor is operating at an over excited condition is called synchronous condenser. The synchronous condensers having leading power factor are widely used for improving power factor of those power systems which employ a large number of induction motors and other lagging power factor loads.

11. Define pull in torque in synchronous motor?

It pertains to the ability of the machine to pull into synchronism when changing from induction to synchronous motor operation.12. What is meant by V curves of synchronous motor?

The V-curves show the relation that exists between the armature current and field current for different constant power input.

13. Name the important characteristics of a synchronous motor not found in an induction motor?

Essential features of synchronous machine are

i) The rotor speed is synchronous with stator rotating field,ii) The power factor can be easily varied by varying its field current,iii) It is used for constant speed operation.

15. What is the common starting method used for synchronous motor?

i) Starting with the help of damper winding.

ii) Starting with the help of separate small induction motor.

iii) Starting by using an ac motor coupled to the synchronous, motor

16. Why does the synchronous motor always run at synchronous speed?

A synchronous motor always runs at synchronous speed because of the magnetic locking between the stator and rotor poles.

17. Enlist the disadvantages of synchronous motor.

Disadvantages of Synchronous Motor are,

1. It cannot be started under load.2. It requires dc excitation which must be supplied from external source.3. It has a tendency to hunt.4. It cannot be used for variable speed jobs as there is no possibility of speed adjustment5. Collector rings and brushes are required.

18. State the characteristic features of synchronous motor.

a. the motor is not inherently self startingb. The speed of operation is always in synchronous with the supply frequency irrespective of load conditionsc. The motor is capable of operating at any power factor.

19. In what way synchronous motor is different from other motors?

All dc and ac motors work on the same principle. Synchronous motor operates due to magnetic locking taking place between stator and rotor magnetic fields.

20. What is the effect on speed if the load is increased on a 3 phase synchronous motor?

The speed of operation remains constant from no load to maximum load in the motor operating at constant frequency bus bars.

21. Why a synchronous motor is a constant speed motor?

Synchronous motor work on the principle of force developed due to the magnetic attraction established between the rotating magnetic field and the main pole feed. Since the speed of rotating magnetic field is directly proportional to frequency the motor operates at constant speed.

22. What is the phasor relation between induced emf and terminal voltage of a 3 phase synchronous motor?

The rotating magnetic field is initially established by the prime source of supply V. The main field then causes an emf e to get induced in the 3 phase winding. Hence when the machine operates as a synchronous motor the emf phasor always lags the terminal voltage phasor by the load/torque.

23. What is meant by inverted V curves of synchronous motor?

The variation of magnitude of line current with respect to the field current is called V curve The variation of power factor with respect to the field current is called inverted V curve.

24. What happens when the field current of a synchronous motor is increased beyond the normal value at constant input?

Increase in emf causes the motor to have reactive current in the leading direction. The additional leading reactive current causes the magnitude of line current, accompanied by the decrease in power factor.

25. Distinguish between synchronous phase modifier and synchronous condenser?

A synchronous motor used to change the power factor or power factor in the supply lines is called synchronous phase modifier.

A synchronous motor operated at no load with over excitation condition to draw large leading reactive current and power is called a synchronous condenser.

26. How the synchronous motor can be used as s synchronous condenser?

Synchronous motor is operated on over excitation so as to draw leading reactive current and power from the supply lines. This compensates the lagging current and power requirement of the load making the system power factor to become unity. The motor does the job of capacitors and hence called as synchronous condenser.

Unit –III

THREE PHASE INDUCTION MOTOR

1.)What is the function of slip ring in 3 phase induction motor?

Slip rings are used to connect external stationary circuit to internal rotating circuit.

2.) Under what condition the slip in an induction motor is? a)negative b)greater than 1

a )When rotor is running at a speed above the synchronous speed slip is negative.

b) When motor is rotated in opposite direction to that of rotating field slip is greater than 1

3.) What are the 2 fundamental characteristics of a rotating magnetic field?

a ) The resultant of three alternating fluxes separated from each other by 120 degree has constant amplitude of 1.5 .

b) The resultant always keeps on rotating with a certain speed in space.4.) What is induction generator?

When the slip of the induction motor is negative the induction motor that runs as a generator is called induction generator.

5.) What are the purposes that could be served by external resistors connected in the rotor Circuit of phase wound induction motor?

a) Increasing starting torque.

b) For speed control

c) Limiting starting current.

d)

6.) What are the merits of inner and outer cage of double cage induction motor?

Merits of inner cage,

a)leakage reactance is high. b)resistance is small.

Merits of outer cage,

a)has high starting torque. b)resistance is high.

7) Define Synchronous speed in a 3 phase induction motor?

The speed at which the revolving flux rotates is called synchronous speed Ns and is given by,

Ns =120f / P

Where f - Supply Frequency

P- Number of poles on the stator 8.)

What are the types of induction motor?

a) Split type induction motor.

b) Capacitor start induction motor.

c) Shaded pole induction motor.

d) Deep bar rotor induction motors

9.) What are the losses in induction motor?

a) Constant losses

b) Variable losses.

10) What is crawling in induction motor?

The tendency of the motor to run stably at speeds as low as one seventh of its synchronous speed with a low pitched howling sound is called crawling.

11.) What are the applications of 3 phase I.M motors?

a) driving fans

b) Blowers.

c)l lathes

c) Lifts

12. What are the characteristics of double squirrel cage motor, compared to a squirrel cage motor?

(i) High starting torque

(ii) Excellent running performance

13. State the principle of 3 phase IM?

While starting, rotor conductors are stationary and they cut the revolving magnetic field and so an emf is induced in them by electromagnetic induction. This induced emf produces a current if the circuit is closed. This current opposes the cause by Lenz’s law and hence the rotor starts revolving in the same direction as that of the magnetic field.

14. Name the tests to be conducted for predetermining the performance of 3-phase induction machine?

(a)No load test(b) Blocked rotor test

15. What are the information obtained from no-load test in a 3-phase induction motor?

(i) No –load input current per phase (Io)(ii) No load power factor and hence no load phase angle(iii) Iron and mechanical losses together(iv) Elements of equivalent circuit shunt branch

16. What are the information obtained from blocked rotor test in a 3-phase induction motor?

(i)Blocked rotor input current per phase at normal voltage(ii) Blocked rotor power factor and hence phase angle(iii) Total resistance and leakage reactance per phase of the Motor as referred to the stator

17. What is circle diagram of an induction motor?

When an induction motor operates on constant voltage and constant frequency source, the loci of stator current phasor is found to fall on a circle. This circle diagram is used to predict the performance of the machine at different loading conditions as well as mode of operation.

18. What are the advantages and disadvantages of circle diagram method of predetermining the performance of 3 –phase I.M?

The prediction can be carried out when any of the following information is available, The input line current., the input power factor, The active power input, The reactive power input, The apparent power input, The output power , The slip of operation, The torque developed, The equivalent rotor current per phase, Maximum output power, Maximum torque developed.

The only disadvantage is, being a geometrical solution, errors made during measurements will affect the accuracy of the result.

19. What are the advantages and disadvantages of direct load test for 3 – phase I M?

Advantages: Direct measurement of input and output parameters yield accurate

results Aside from the usual performance other performances like mechanical

vibration, noise etc can be studied. By operating the motor at full load for a continuous period, the final Steady temperature can be measured.

Disadvantages: Testing involves large amount of power and the input energy and the

entire energy delivered is wasted Loading arrangement cannot be provided for motors of large power

rating

20. Why is there not appreciable magnetic losses in the rotor core of Induction motors?Although the rotor core is also subjected to magnetic flux reversals and since the

frequency of flux reversals in the rotor, fr = Sfs, is very small, the iron loss incurred in the rotor core is negligibly small.

21. What care should be taken at the time of construction to reduce eddy current losses in Induction Motor?

Make the resistance of the core body as large as possible. This is achieved by laminating the stator core, stacked and revetted at right angles to the path of eddy current. The laminations are insulated from each other by thin coat of varnish.

22. What are the losses occurring in an I M and on what factors do they depend?

Magnetic losses Wi Electrical losses Wcu Mechanical losses Wm

For I M operating in normal condition (with constant voltage and frequency) magnetic and mechanical losses remain constant whereas electrical losses vary in square proportion to the current.

23. What are the advantages of slip-ring I M over cage I M?

(i) Rotor circuit is accessible for external connection.(ii) By adding external resistance to the rotor circuit the starting current is reduced with the added advantage of improving starting torque.(iv) Additional speed control methods can be employed with the accessibility in the rotor circuit.

24. State the condition at which the torque developed in a 3 phase induction motor is maximum?

The torque developed in a 3 phase induction motor is maximum, when R2=SX2

25. What would happen if a 3 phase induction motor is switched on with one phase disconnected?

The motor is likely to burn.

Unit –IV

STARTING AND SPEED CONTROL OF THREE PHASE INDUCTION MOTOR

1.) How do you reverse the direction of rotation of 3 phase?The direction of 3 phase can be reversed by interchanging any 2 terminals of 3 phase

winding.

2.) What are the different speed control methods of squirrel cage induction motors?

a)supply frequency control

b)supply voltage control. c)controlling

number of stator poles. d)adding

rheostats in stator.

3.) What are the types of starters?

a)stator resistance starter

b)autotransformer starter.

c) star delta starter.

d) rotor resistance starter.

e) direct to line starter.

4.) How the stator poles can be changed?

a)consequent poles method. b)multiple

stator winding method. c)pole

amplitude modulation method.

5.) What is the effect of change in input voltage on starting torque of induction motor?

There is no change in starting torque due to change in input voltage. The motor reacts by drawing more current at lower speeds to keep the same torque.

6.)What are the definitions associated with stepping motor?

a)holding torque.

b)detent torque

c)step angle.

d)critical torque.

e)limiting torque.

f)slewing rate.

7.What is meant by Slip power recovery Scheme?

This slip power can be returned to the supply source and can be used to supply an additional motor which is mechanically coupled to the main motor. This type of drive is known as a slip power recovery system and improves the overall efficiency of the system.

8.What are the various methods of speed control of 3 phase induction motor?

(i) stator voltage control

(ii) stator frequency control

(iii) V/f method

(iv) pole changing method

9. In what respect does a 1-phase Induction motor differ from a 3-phase Induction motor?

Construction wise a plain 1-phase Induction motor is more or less similar to a 3-phase squirrel-cage Induction motor except that its stator is provided with only 1-phase winding.

10. Induction motor can run at synchronous speed ? True or false? Explain .

No, if the speed of induction motor is Ns then the relative speed between the rotating flux and the rotor will be zero and so no torque is produced.

11. An induction motor is generally analogous to ?

It is analogous to a winding rotating transformer with its secondary circuit closed’.

12. Can the starting torque of of a slip ring induction motor being increased?

Yes. It can be increased by adding resistances to the rotor.

13. What happens if the air gap flux density in an induction motor increases?

The increase in air gap flux increases iron loss and hence efficiency decreases.

14. State the advantages of skewing?

It reduces humming and hence quite running of motor is achieved. It reduces magnetic locking of the stator and rotor.15. What are the effects of increasing rotor resistance on starting current and starting torque?

The additional external resistance reduces the rotor current and hence the current drawn from the supply. It improves the starting torque developed by improving the power factor in high proportion to the decrease in rotor current.

16. What is slip of an induction motor?The slip speed expressed as the ratio of synchronous speed is defined as slip.

Percentage slip S=Ns-N/Ns*100

17. How the magnitude of rotor emf is related to the slip in an I M?

Rotor circuit emf per phase E2r=SE2

18. How the frequency of rotor emf is related to the slip in an I M?

Frequency of rotor emf/current fr=Sfs

19. What is the normal value of slip of an I M operating at full load?

3 - 5%

20. Why is not possible for the rotor speed of an I M to be equal to the speed of its rotating magnetic field?

The machine will not be able to develop any mechanical torque to run as a motor.

21. Which winding in a double cage induction motor has high resistance and low inductance?

Outer cage winding

22. What is meant by synchronous watt?

With the power input to the motor Pi, after the losses in the stator winding. Wcu1 and stator core, Wi, are met with, the remaining power is transferred to the rotor by the rotating magnetic field as power input to the rotor Pir,

Pir = Pi –wau1 - Wi

23. What are the losses occurring in an I M and on what factors do they depend?

Magnetic losses Wi Electrical losses Wcu Mechanical losses Wm

For I M operating in normal condition (with constant voltage and frequency) magnetic and mechanical losses remain constant whereas electrical losses vary in square proportion to the current.

24. What care should be taken at the time of construction to reduce eddy current losses in I M?

Make the resistance of the core body as large as possible. This is achieved by laminating the stator core, stacked and rewetted at right angles to the path of eddy current. The laminations are insulated from each other by thin coat of varnish.

25. Why is there not appreciable magnetic losses in the rotor core of Induction motors?

Although the rotor core is also subjected to magnetic flux reversals and since the frequency of flux reversals in the rotor, fr = Sfs, is very small, the iron loss incurred in the rotor core is negligibly small.

Unit –V

SINGLE PHASE INDUCTION MOTORS AND SPECIAL MACHINES

1. In what respect does a 1-phase Induction motor differ from a 3-phase Induction motor?

Construction wise a plain 1-phase Induction motor is more or less similar to a 3-phase squirrel-cage Induction motor except that its stator is provided with only 1-phase winding.

2. What are the inherent characteristics of plain 1-phase Induction motor?

A plain 1-phase Induction motor is not used in practice due to the following inherent characteristics,

A plain 1-phase Induction motor does not have any starting torque However, if the rotor is initially given a starting torque, by

some means, the motor can pick up its speed in a direction at which the initial torque is given and deliver the required output.

3. Name the two different theories with which principle of 1-phase induction motors are explained.

The two different theories are,

Double revolving field theory Cross field theory

4. State double revolving field theory.

Double revolving theory, formulated by Ferrari, states that a single pulsating synchronous speed proportional to the frequency of the pulsating field.

5. Name any four types of 1-phase induction motors.

Based on the method of starting arrangement provided, the 1-phase Induction motors are classified as follows,

(i) Split-phase motor(ii) Capacitor start motor(iii) Capacitor start and run motor(iv) Shaded pole motor(v) Repulsion start Induction run motor

6. Why are centrifugal switches provided on many 1-phase Induction motors?Centrifugal switches are provided on many 1-phase Induction motors to

disconnect the starting / auxiliary winding from the supply when the motor reaches about 70% of its synchronous speed.

7. How is the direction of a capacitor start Induction motor be reversed?

The direction of rotation can be reversed by interchanging the terminals of either the main winding or the starting winding.

8. What type of single phase induction motor would you use for the following applications?

(i) Ceiling fan (ii) Wet grinder

Ceiling fan – capacitor start and run motor Wet grinder – capacitor start motor

9. After servicing a single phase fan it was found to run in reverse direction. What could be the reason?

The connection to the starting/ auxiliary winding would have reversed.

10. What will be the direction of rotation of a shaded pole single phase induction motor?

The motor rotates in the direction specified by the unshaded to shaded region in the pole phase.

11.What is the property of a single phase single winding induction motor?

It has zero starting torque

12. What is the function of capacitor in a single phase induction motor ?

Capacitor is used to improve the power factor of the motor . Due to the capacitor connected in series with the auxiliary winding , the capacitive circuit draws a leading current which increases the split phase angle ά between two currents Im and Ist .

13. What is the use of shading coil in the shaded pole motor ?

In shaded pole motors the necessary phase -splitting is produced by induction. These motors have salient poles on stator and a squirrel cage type rotor . The poles are shaded ie each pole carries a copper band one of its unequally divided part is called shading band. When single phase ac supply is given to the stator winding due to shading provided to the poles a rotating magnetic field is generated .

14 . Why capacitor -start induction motors advantageous?In capacitor start induction motors capacitor is connected in series with the

auxiliary winding . when speed of the motor approaches to 75 to80%of the synchronous speed the starting winding gets disconnected due to the operation of the centrifugal switch . The capacitor remains in the circuit only at start . the starting torque is proportional to phase angle ά and hence such motors produce very high starting torque.

15 . List out 4 applications of shaded pole induction motor ?

Shaded pole motors have very low starting torque , low power factor and low efficiency. The motors are commonly used for small fans , toy motors ,advertising displays , film projectors , record players , gramophones ,hair dryers , photocopying machines etc

16. What are the drawbacks of the presence of the backward rotating field in a single phase induction motor?

Due to cutting of flux , emf gets induced in the rotor which circulates rotor current

.the rotor current produces rotor flux . This flux interacts with forward component φf to produce a torque in one particular direction say anticlockwise direction . While rotor flux interacts with backward component φb to produce a torque in the clockwise direction . So if anti clock wise torque is positive then clockwise torque is negative thus net torque experienced by the rotor is zero at start .

17. Is single phase induction motor self starting ? why?

Due to cutting of flux , emf gets induced in the rotor which circulates rotor current. The rotor current produces rotor flux . this flux interacts with forward component φf to produce a torque in one particular direction say anticlockwise direction . While rotor flux interacts with backward component φb to produce a torque in the clockwise direction . So if anti clock wise torque is positive then clockwise torque is negative thus net torque experienced by the rotor is zero at start. Hence net torque experienced by rotor is zero at start and so single phase induction motor are not self starting.

18. Why is hysteresis motor free from mechanical and magnetic vibrations?

The stator of hysteresis motor carries main and auxiliary windings to produce rotating magnetic field or of shaded pole type also. The rotor is smooth cylindrical type made up of hard magnetic material. The torque in this motor is constant at all speeds it runs at synchronous speed. there is not relative motion between stator and rotor field so the torque due to eddy current vanishes .only hysteresis torque is present which keeps rotor running at synchronous speeds .the high retentivity ensures continuous magnetic locking between stator and rotor . Hence it is free from magnetic vibrations

19. What types of motor is used in computer drives and wet grinders?For computer drives permanent magnet dc motors are used while in wet

grinders universal motor may be used .

20. Give two advantages and two applications of stepper motor.Advantages:

*These motors are compatible with digital equipments and are flexible in operation.*The dynamic response is fast

Applications:

Stepper motors are widely used in computer peripherals such as serial printers tape drives , floppy disk drivers . they are also used in control of machine tools . robotics.

21. List some applications of linear induction motor?

They are used in machine tool industry and in robotics .They are used in trains operated on magnetic levitation, reciprocating compressors can also be driven by linear motors.

22. What are the specific characteristic features of the repulsion motor?

Repulsion motors give excellent performance characteristics. A very high starting torque of about 300 to350% of full load can be obtained with starting currents of about 3 to 4 times the full load current. Thus it has got very good operating characteristics. The speed of the motor changes with load .with compensated type of repulsion motor the motor runs with improved power factor as the quadrature drop in the field winding is neutralized. Also the leakage between armature and field is reduced which gives better regulation.

23. Discuss characteristics of single phase series motor?

* To reduce the eddy current losses , yoke and pole core construction is laminated

*The power factor can be improved by reducing the number of turns. But this reduces the field flux. But this reduction in flux increases the speed and reducing the torque. To keep the torque same it is necessary to increase the armature turns proportionately. This increases the armature inductance.

24. What are the demerits of repulsion motor?

*very expensive

*speed changes with load* On no load speed is very high causing sparking at brushes

*low power factor on no load

25. List four applications of reluctance motors?

This motor is used in signaling devices, control apparatus, automatic regulators,

recording instruments, clocks and all kinds of timing devices , teleprinters ,gramophones

26. What is a universal motor?

There are small capacity series motors which can be operated on dc supply or single phase ac supply of same voltage with similar characteristics called universal motors. The construction of this motor is similar to that of ac series motor.

16 MARKS

EE6504 – ELECTRICAL MACHINES

UNIT I

1. SYNCHRONOUS GENERATOR 9

Constructional details – Types of rotors – emf equation – Synchronous reactance – Armature reaction – Voltage regulation – EMF, MMF, ZPF and A.S.A methods – Synchronizing and parallel operation – Synchronizing torque - Change of excitation and mechanical input – Two reaction theory – Determination of direct and quadrature axis synchronous reactance using slip test – Operating characteristics - Capability curves.

SYNCHRONOUS GENERATOR

Constructional Details

Two types of Alternators

(a) Stationary armature - rotating field

(b) Stationary field – rotating armature

Constructional Details

Advantages of stationary armature - rotating field:

i) The HV ac winding and its insulation not subjected to centrifugal forces.

ii) Easier to collect large currents from a stationary member.

iii) The LV dc excitation easily supplied through slip rings and brushes to the rotor field winding.

Stationary Armature - Rotating Field

Stator:

Laminated core with slots to hold the armature

conductors. Rotor:

i) Salient pole type –

Projecting poles dove tailed on the shaft - Used in low speed alternators driven by water turbines or IC engines

Stationary Armature - Rotating Field

ii) Non Salient pole type

Smooth cylindrical rotor - slots cut to house the field winding - used in high speed alternators driven by steam turbines - smaller diameter and larger axial length compared to salient pole type machines, of the same rating.

EMF Equation

Where,Eph 4.44Kc Kd f Tph

Kc = cos (α/2),

Kd = sin (mβ/2) / m sin (β/2) f

= PNs/120, Hz;

Φ = flux per pole, Wb

Tph = Turns in series per phase

= (No. of slots * No. of cond. per slot) / (2 x 3)

EMF Equation

Short pitching and distribution of the winding:

• time harmonics of induced voltage reduced

• The waveform made more sinusoidal.

• Short pitching also reduces the length of the coil end connections

Armature Reaction

• Effect of the armature flux on the main flux.

• Three phase current in a three - phase winding - a rotating magnetic field produced (MMF = 1.5 Im Tph).

• UPF - cross magnetizing.

• Lag PF - demagnetizing.

• Lead PF - magnetizing.

Armature Leakage Reactance(XL)

• Three major components -Slot leakage reactance, end winding leakage reactance and tooth tip leakage reactance.

• Synchronous reactance/phase

Xs = XL + Xar, where Xar is the fictitious armature reaction reactance.

• Synchronous impedance/phase

Zs = (Ra + jXs ).

Voltage Regulation of Alternators

Eph Vph Ia Zs

E ph

Eph Vph 100Vph

Methods Of Predetermination Of Regulation

• Synchronous impedance method (EMF method)

• Magneto Motive Force method (MMF method)

• Zero Power Factor method (ZPF method)

• American Standards Association method (ASA method)

Synchronous Impedance Method (EMF Method)

• OC and SC tests conducted.

• Zs is calculated.

• Ra measured and Xs obtained.

• For a given armature current and power

factor, Eph determined - regulation is calculated.

Magneto Motive Force Method (MMF Method)

• OC & SC tests conducted.

• field currents If1 (field current required to produce a voltage of(Vph + IaphRacos Φ) on OC) and If2 (field current required to produce the given armature current on SC) are added at an angle of (90± Φ).

• For this total field current, Eph found from OCC and regulation calculated.

Zero Power Factor Method (ZPF Method)

OC test and ZPF test is conducted – characteristics are drawn. This is Potier triangle

method From this triangle the potier reactance (leakage reactance of the alternator), XLph

V cos I R 2 V sin

I X

ph

a a

ph

a

s2

is obtained.

The terminal voltage and the leakage reactance drop added vectorially - load induced EMF found

Zero Power Factor Method (ZPF Method)

• For this load induced emf, the corresponding field current If1 obtained from OCC.

• The field current If2 required to balance armature reaction obtained from potier triangle.

If1 and If2 are added at an angle of (90± Φ). For this total field current, Eph found from OCC – regulation calculated

American Standards Association Method (ASA Method

• The field currents If1 (field current required to produce the rated voltage of Vph from theair gap line).

• If2 (field current required to produce the given armature current on short circuit) added at an angle of (90± Φ).

• Load induced EMF calculated as was done in the ZPF method - Corresponding to this EMF, the additional field current (If3) due to saturation obtained from OCC and air gap

line - If3 added to the resultant of If1 and If2 -For this total field current, Eph found from OCC and regulation calculated

Synchronizing And Parallel Operation Of Alternators

• Necessary conditions for synchronization :

The terminal voltage, frequency and phase sequence of the incoming machine should

be same as those of the bus bars.

• Synchronization can be carried out using either i) Dark lamp method ii) Bright lamp method or iii) Synchroscope.

Synchronizing Power and Torque

• Power developed by an alternator

Pi

Where

E E cos V cos( )Z s

θ is the internal angle of the

machine and δ is the power

angle.

Synchronizing power On no load . Synchronizing torque = PSY / ( 2πNs/60).

Two Axis Theory

Xd , Xq : Direct & Quadrature

axis synchronous

reactances in .

Id, Iq : The current components of

Ia in the d & q axis

Part A

1. Calculate the distribution factor for a 36- slot, 4 -pole, single layer three phase winding.

2. Define voltage regulation of an alternator.

3. What are the two components of field current required for the predetermination of

regulation by MMF method ?

4. What are the conditions to be fulfilled for connecting two alternators in parallel ?

5. Define short circuit ratio of an alternator.

PART B

1.The open circuit and short circuit test readings obtained on a 3 phase,

star connected, 1000 kVA, 2000 V , 50 Hz alternator are :

Field Current, A 10 20 25 30 40 50

OC terminal

voltage , V 800 1500 1760 2000 2350 2600

SC armature

current, A - 200 250 300 - -

The effective armature resistance is 0.2 Ω per phase. Draw the

characteristic curves and estimate the full-load regulation at a power

factor of 0.8 lagging by synchronous impedance method.

2.Derive the EMF equation of an alternator.

3 What do you understand by direct and quadrature axis reactance in a

salient pole alternator? Draw the phasor diagram of this alternator

for a lagging power factor load

4. Derive the expression for power developed in a 3 phase synchronous motor

in terms of E, V, Zs, θ and δ .

5. Sketch an excitation circle and a power circle of a synchronous motor and state

what each one represents

UNIT – II

SYNCHRONOUS MOTOR

Principle of Operation

3Ø supply given to the 3Ø stator winding - Rotating magnetic field produced – rotating at synchronous speed(Ns) – Field winding on the rotor excited with dc – Field poles produced - if the rotor is brought to near synchronous speed, rotor pulled into synchronism - also rotates at Ns due to magnetic locking between the stator and rotor poles.

• Torque equation

Torque developed =

Nm Where,

Pm (mechanical developed) = P1 – 3I2 ra ,W

P1 (Power input to the stator) = 3VI cosΦ ,W

Ns (Synchronous speed) = (120f)/P, rpm

I = Stator current in A, P = No. of Poles,

f = frequency in Hz

V - Curves

The V – curves shows the variation of armature current with field current for different values of constant power input. Curves joining points of equal power factor are called compounding curves.Variation of power factor with field current gives the inverted V – curves.

• Effect of changing the Excitation (Load constant)

• Changing the excitation varies the power factor of the motor

• Normal excitation: E = V (PF Lag)

• Under excitation : E < V (PF lag)

• Over excitation : E > V (PF lead)

• Minimum armature current occurs at UPF

Effect of changing the Load (Excitation Constant)

Change in load changes the torque angle – armature current changes - induced emf does notchange.

Pin = √3 VL IL cosΦ

• Power developed Mechanical

power developed, Pm = If Ra is

neglected, θ , then Pm =

The maximum power developed =

• Circle Diagrams

• Excitation Circles :

The extremity of the armature current phasor varies over a circle when load varies ( for a constant excitation)

Radius of the excitation circle =

V = applied phase voltage,

volts

Zs = synchronous impedance per phase, Ω

• Circle Diagrams

Power Circles :

The extremity of the armature current phasor varies over a circle when excitation varies (load constant).

Radius of the power circle

= Amp

• Starting of synchronous Motors

• Using Pony motors

• Using damper winding

• As a slip ring induction motor

• Using small D.C. machine

• Circle Diagrams

Power Circles

• The extremity of the armature current phasor varies over a circle when excitation varies (load constant).

• Radius of the power circle = Amp

• Starting of synchronous Motors

• Using Pony motors

• Using damper winding

• As a slip ring induction motor

• Using small D.C. machine

PART A

• Why is the field system of an alternator made as a rotor?

• Sketch salient pole and non salient pole rotors.

• What is synchronous reactance?

• Define Pitch factor.

• Calculate the distribution factor for an 18 slots, 4-pole three-phase winding.

UNIT III

STARTING AND SPEED CONTROL OF THREE PHASE INDUCTION MOTOR 7

Need for starting – Types of starters – Rotor resistance, Autotransformer and Star-delta starters – Speed control – Change of voltage, torque, number of poles and slip – Cascaded connection – Slip power recovery scheme.

Three phase Induction

Motor Construction

The stator is similar to that of Synchronous machine and is wound for three phases.

Rotor is of two types (i) wound rotor

(ii) squirrel–cage rotor

The rotor core is laminated with slots punched for accommodating the rotor winding/ rotor bars.

Rotors

Slip ring: The winding is polyphase with coils placed in the slots of rotor core. The number of slots is smaller and fewer turns per phase of heavier conductor are used.

Squirrel-cage: These rotors has solid bars of conducting material placed in rotor slots and shorted through end-rings on each side.

Principle of operation

The stator is fed from a 3-phase supply. The resultant air gap flux/pole is established in the air- gap. The mmf vector Fr with associated flux density vector Br rotates at synchronous speed. The relative speed between Br and rotor causes induction of current in shorted rotor. Due to interaction of Br and rotor current torque is produced and the rotor tend to move in the direction of Br. Thus the motor is self starting.

Slip

The flux density vector Br moves at speed (ns-n) with respect to rotor conductors, this is known as slip speed.

Equivalent circuit

An induction motor can be assumed as a transformer having primary and secondary windings separated by an air gap. The mechanical load may be replaced by a variable resistance RL = r2(1- s)/s where r2 is the rotor resistance and s the slip.

Power input to rotor/phase = (I22

r2)/s Rotor copper loss = I22 r2

Rotor current I2 = s2/(√(r22/s)2 + X22

PART A

6. Draw the phasor diagram of synchronous motor working with lagging power factor.

7. Sketch the V and inverted V- curve of a synchronous motor, clearly indicating the X and Y axes for each curve.

8. State the differences in construction between squirrel cage and slip ring rotors of an induction motor.

9. A 3 phase, 4 pole, 50 Hz induction motor runs at a speed of 1425 rpm. Calculate the slip and frequency of the rotor EMF.

10. Sketch the circle diagram of a 3 phase induction motor.

PART B

1. Using relevant phasor diagrams, describe the effect of changing the excitation,

on the armature current and power factor of a synchronous motor and

thereby obtain ‘V’ and inverted ‘V’ curves.

2. Deduce the Equivalent circuit of 3 phase induction motors

3.The power input to a 3 phase, 500 V, 50 Hz, 6 Pole, induction motor is 40

kW. The motor runs at a speed of 975 rpm. The stator losses amount to 1 kW

and the friction and windage losses amount to 2 kW. Calculate the i) slip ii)

rotor copper loss iii) hp output and iv) efficiency.

4.Draw the Circle diagram for a 5.6 kW, 400 V, 3 phase, star connected 4

pole 50 Hz slip ring induction motor which gave the following test data :

No-load: 400 V, 6 A, 360 W : Blocked rotor : 100 V, 12 A, 720 W

The ratio of stator to rotor turns is 2.6. The stator resistance per phase = 0.67 Ω . The

rotor resistance per phase = 0.185 Ω. Calculate the full load current and maximum

output.

5. With neat diagrams , explain the working of any two types of starters used for

starting 3 phase squirrel cage induction motors.

UNIT IV

STARTING AND SPEED CONTROL OF THREE PHASE INDUCTION MOTOR 7

Need for starting – Types of starters – Rotor resistance, Autotransformer and Star-delta starters – Speed control – Change of voltage, torque, number of poles and slip – Cascaded connection – Slip power recovery scheme.

Need for Starting – Types of Starters

At starting when the rotor is at standstill, the squirrel cage rotor is just like a short circuited secondary. Therefore the current in the rotor circuit will be high and consequently the stator also will draw a high current from the supply lines if full line voltage were applied at start.

Auto –Transformer Starter

A three phase auto transformer can be used to reduce the voltage applied to the stator. The advantage of this method is that the voltage is reduced by transformation and not by dropping the excess in resistor and hence the input current and power from the supply are also reduced compared to stator resistor starting.

Auto –Transformer Starter

The ratio of starting torque (Tst) to full

load torque (Tf):

Ist = starting current and Ist = full load current

X = Transformer tapping as p.u. of rated

voltage Sf = Full load slip

Star-Delta Starter

This method applicable for motors designed to run normally with delta connected stator windings - At starting, the stator windings connected in star - After the motor has reached nearly the steady state speed, the windings are connected in

delta – over load and single phasing protection are provided.

• Star-Delta Starter

• At starting the stator phase voltage reduced by 1/√3 times the voltage.

• Phase current reduced by 1/√3 times the current with the direct online starting.

• Line current reduce by 3 times.

• Rotor Resistance Starter

Applicable to slip ring induction motors - Rated voltage applied to the stator - balanced three phase resistors connected in series with the rotor through slip rings – Resistance kept at maximum at starting – starting current reduced – starting torque increased – after starting resistance can be cut out .

Speed Control of Induction Motors

Synchronous speed of the rotating magnetic field produced by the stator, Ns = 120 f / P

• By changing the frequency.

The available AC voltage (50 Hz) is rectified and then inverted back to AC of variable frequency/ Variable voltage using inverters.

Inverter can be Voltage source or current source inverter.

• Speed Control of Induction Motors

• By changing the number of poles.

The stator winding is designed for operation for two different pole numbers: 4/6,4/8,6/8 etc. This can be applied only to squirrel cage induction motors.

• Stator voltage control.

The stator voltage is varied – slip and operating speed varies.

• Speed Control of Induction Motors

• Rotor resistance control.

This method is applied to slip ring induction motor – rotor is connected to variable resistance through slip rings – resistance varied – slip and hence the operating speed varies – this method results in power loss in the resistor

Speed Control of Induction Motors

• Using cascade connection – Three phase voltage applied to the stator of a slip ring induction motor(P1 – poles ) – slip ring voltage applied to the stator of squirrel cage induction motor (P2 – poles)– two rotors are coupled.

Ns = 120 f / (P1±P2)

Slip Power Recovery Scheme

This scheme applied to slip ring induction motor:- Rated voltage applied to the stator - the rotor voltage is rectified using a diode bridge rectifier – the resulting DC voltage is inverted using line commutated inverter and the AC voltage is fed back to the supply through appropriate transformer – slip power is thus recovered from the motor and the speed reduced

PARTA

11. Calculate the pitch factor for a 36 slot, 4 pole winding having a coil pitch of 6 slots.

12. A 5 MVA, 10 kV, 1500 rpm, 3 phase, 50 Hz, 4 pole, star connected alternator is operating on infinite bus bars. Determine the synchronizing power per mechanical degree of displacement under no-load condition. Xs = 20 %.

13. When is a synchronous motor said to be under excited? What will be the nature of power factor under that condition?

14. What is hunting in a synchronous motor?

15. Under what condition, the slip in an induction machine is i) negative ii) greater than one?

PARTB

1. Describe the following methods of speed control of slip ring induction motors:

i) Using external rotor resistanceii) Slip power recovery scheme

2. Based on double field revolving theory and using the torque – speed

characteristics, explain the operation of single- phase induction motors.

ii) Draw the equivalent circuit of single-phase induction motor.

3. Describe the constructional features and the operation of the following:

i) Stepper motorii) Shaded pole motoriii) Hysteresis motor

4. A 3 phase star connected synchronous motor is rated for 6600 V. For a particular load, the motor takes 50 A line current from the supply. The effective armature resistance is 1.0 Ω per phase and synchronous reactance is 20 Ω per phase. Calculate (a) the power supplied to the motor and (b) induced EMF for a power factor of 0.8 lag.

5. Explain why a synchronous motor is not self starting. Describe the method of starting the synchronous motor using damper winding.

B.E./B.TEChD. EGREE EXAMINATION, NOVEMBER/DECEMBER 2007.Fourth Semester

(Regulation 2004)Electrical and Electronics Engineering EE 1251

- ELECTRICAL MACHINES - II(Common to B.E. (Part-Time) Third Semester Regulation 2005)

Time : Three hours Maximum : 100 marksAnswer ALL questions.

PART A - (10 x2 = 20 marks)

1. Calculate the distribution facior for a 36 slots, 4 poles, single layer three phase winding.2. Write down the causes for reduction in terminal voltage of Alternator from it's no ioad value Eo to V for a lagging power factor.3. Write down the applications of synchronous motor.4. Write down the equation for power developed by a synchronous motor.5. A 3 phase Induction motor is wound for 4 poles and is supplied from 50 Hz system. Calculate rotor frequency when rotor runs at 600 r.p.m'6. Write down the advantages of double cage rotor Induction motor over single squirrel cage motor.7 . Name the type of starters used in 3 phase Induction motors.8. Write down the methods to control the speed of 3 phase lnduction motor from its rotor side.9. Explain how to start the single phase Induction motor?10. Write down any two applications of stepper motor.

PARTB-(5x16=80marks)

11. (a) (i) Explain in detail E.M.F. method of finding voltage regulation of 3 phase Alternator. (8)

(ii) Find the no load phase and line voltage of a star connected 3 phase, 6 pole alternator which runs at 1200 r.p.m, having flux per pole of0.1 Wb sinusoidalally distributed. It's stator has 54 slots having double layer winding. Each coil has 8 turns and the coil is chorded by 1 slot

Or(b) (i) Explain with neat diagrams on alternator.

12. (a) Explain the effect of excitation on synchronous motor and thereby(8)the armature reaction and it's effects (8)

(ii) Explain synchronizing of 3 phase alternator by dark lamp method. (8)armature current and power factor of obtain 'V and inverted 'V curves.

13. (a) (i) Explain slip-Torque characteristics of 3 phase Induction motor with necessary equations. (8)(ii) A 100 kW(output), 3300 V, 3 phase,s tar connectedin duction motor has a synchronous speed of 500 r.p.m. The full load slip is 1.8 Va and full load power factor 0.85. Stator copper loss = 2440 W. Iron loss = 3500 V/. Rotational losses = 7200 W. Calculate (1) the rotor copper loss (2) the line current (3) the full load efficiency.

OrExplain the principle of operation of synchronous motor and starting methods. (16)

14. (a) (i) Explain the necessity of starters in induction motor @) Explain with neat sketches the different methods of speed control in 3 phase induction motor.

Or( (b) Explain with neat diagram the Static Scherbius drive system of slip power recovery scheme. (16)

15. (a) Explain why the single phase induction motor is double field revolving theorY.

Or(b) (i) Explain the operation of Repulsion motor(ii) Explain the operation of Stepper motor' not self starting, n'ith