University Of Pune Question Paper

T.E. (Electrical) (Sem. – II) Examination, 2010

ELECTRICAL MACHINES – III

(2003 Course)

Time : 3 Hours Max. Marks : 100

Instructions:1) Answer 3 questions from Section I and 3 questions from

Section II.

2) Answers to the two Sections should be written in separate books.

3) Neat diagrams must be drawn wherever necessary.

4) Use of logarithmic tables, slide rule, Mollier charts, electronic

pocket calculator and steam tables is allowed.

SECTION – I

1. a) Describe the advantages of providing field winding on rotor and the armature

winding on stator in case of large synchronous machines. 4

b) Derive an expression for the emf generated in the armature winding of a

synchronous machine. 6

c) A 3-phase, 10 KVA, 400 V, 50 Hz, star connected alternator supplies the rated

load of 0.8 p.f. lagging. If the armature resistance is 0.5 Ω per ph. and

synchronous reactance is 10 Ω per phase, find :

i) Voltage regulation

ii) Power angle. 6

OR

2. a) What do you mean by distribution factor of an alternator ? Derive expression

for distribution factor. 8

b) Explain 2PF method for finding voltage regulation of a 3-phase synchronous

generator. 8

3. a) Explain the slip test to be conducted on an alternator to determine Xd and Xq. 8

b) A 3-phase alternator has Xd = 0.85 pu and Xq = 0.55 pu. Draw the phasor

diagram for the alternator when operating on full load at 0.8 p.f. lagging and

determine therefrom i) the load angle ii) the no load p.u. voltage iii) p.u. voltage

regulation. Neglect armature resistance. 8

OR

4. a) Derive the expression for synchronising torque of an alternator. 6

b) What are the conditions for proper parallel operation of 3 - phase alternators ? 4

c) Two alternators working in parallel supply a lighting load of 3000 KW and

motor load amounting to 5000 KW at a p.f. of 0.71 lagging. One machine is

loaded upto 5000 KW at 0.8 p.f. lagging. What is the load and p.f. of the

other machine. 6

5. a) Write short note on synchronous induction motor. 6

b) Explain the effect of excitation on power factor on synchronous motor at

constant load. 4

c) A 3980 - V, 50 Hz, 4 - pole star connected synchronous motor generates back

emf of 1790 V per phase. The resistance and synchronous reactance per

phase are 2.2 Ω and 22 Ω respectively. The torque angle 30°C. Calculate the

i) resultant armature voltage/phase ii) armature current / phase iii) power factor

of the motor iv) gross torque developed by the motor. 8

OR

6. a) Describe the hunting phenomenon in synchronous machines. What are the

causes of hunting ? What is the roll of damper winding ? 6

b) With neat sketches explain operation of a synchronous induction motor. How

its performance differ from a synchronous motor ? 6

c) Explain the effect of variation of load on a three phase synchronous motor. 6

SECTION – II

7. a) Explain the constructional features, principle of operation, working and

applications of single phase hysteresis motors. 8

b) For linear induction motor, derive an expression for linear force in terms of

specific loadings and linear dimensions. 8

OR

8. a) Explain the torque - pulse rate characteristics of a stepping motor. 8

b) What are linear electrical machines ? Give the principle of operation of LIM. 8

9. a) What are synchronous harmonic and induction harmonic torques that may

appear in an induction motor ? 8

b) Explain the methods used in synchronous machines to reduce harmonics. 8

OR

10. a) What are the causes of space harmonics in the air gap flux wave of 3 - phase

induction motor ? 8

b) Explain how the space harmonics in the air gap flux wave introduce harmonic

asynchronous torques and give rise to crawling. 8

11. a) What is Kron's primitive machine ? How are the various windings of a machine

represented by the primitive machine ? 8

b) Obtain identical transformation for currents and voltages from rotating balanced

3 - phase (a,b,c) winding to rotating balanced 2 - phase ( ) ,βα winding. Show

that power invariance is maintained under this transformations. 10

OR

12. a) Explain the various limitations of the generalized theory of electrical machines. 8

b) Write voltage equations for the mathematical model of a 3 - phase induction

machine and hence obtain an expression for steady state torque when balanced

3 - phase supply is connected to the stator. 10

––––––––––––––––

T.E. (Electrical) (Sem. – II) Examination, 2010

ELECTRICAL MACHINES – III

(2003 Course)

Time : 3 Hours Max. Marks : 100

Instructions:1) Answer 3 questions from Section I and 3 questions from

Section II.

2) Answers to the two Sections should be written in separate books.

3) Neat diagrams must be drawn wherever necessary.

4) Use of logarithmic tables, slide rule, Mollier charts, electronic

pocket calculator and steam tables is allowed.

SECTION – I

1. a) Describe the advantages of providing field winding on rotor and the armature

winding on stator in case of large synchronous machines. 4

b) Derive an expression for the emf generated in the armature winding of a

synchronous machine. 6

c) A 3-phase, 10 KVA, 400 V, 50 Hz, star connected alternator supplies the rated

load of 0.8 p.f. lagging. If the armature resistance is 0.5 Ω per ph. and

synchronous reactance is 10 Ω per phase, find :

i) Voltage regulation

ii) Power angle. 6

OR

2. a) What do you mean by distribution factor of an alternator ? Derive expression

for distribution factor. 8

b) Explain 2PF method for finding voltage regulation of a 3-phase synchronous

generator. 8

3. a) Explain the slip test to be conducted on an alternator to determine Xd and Xq. 8

b) A 3-phase alternator has Xd = 0.85 pu and Xq = 0.55 pu. Draw the phasor

diagram for the alternator when operating on full load at 0.8 p.f. lagging and

determine therefrom i) the load angle ii) the no load p.u. voltage iii) p.u. voltage

regulation. Neglect armature resistance. 8

OR

4. a) Derive the expression for synchronising torque of an alternator. 6

b) What are the conditions for proper parallel operation of 3 - phase alternators ? 4

c) Two alternators working in parallel supply a lighting load of 3000 KW and

motor load amounting to 5000 KW at a p.f. of 0.71 lagging. One machine is

loaded upto 5000 KW at 0.8 p.f. lagging. What is the load and p.f. of the

other machine. 6

5. a) Write short note on synchronous induction motor. 6

b) Explain the effect of excitation on power factor on synchronous motor at

constant load. 4

c) A 3980 - V, 50 Hz, 4 - pole star connected synchronous motor generates back

emf of 1790 V per phase. The resistance and synchronous reactance per

phase are 2.2 Ω and 22 Ω respectively. The torque angle 30°C. Calculate the

i) resultant armature voltage/phase ii) armature current / phase iii) power factor

of the motor iv) gross torque developed by the motor. 8

OR

6. a) Describe the hunting phenomenon in synchronous machines. What are the

causes of hunting ? What is the roll of damper winding ? 6

b) With neat sketches explain operation of a synchronous induction motor. How

its performance differ from a synchronous motor ? 6

c) Explain the effect of variation of load on a three phase synchronous motor. 6

SECTION – II

7. a) Explain the constructional features, principle of operation, working and

applications of single phase hysteresis motors. 8

b) For linear induction motor, derive an expression for linear force in terms of

specific loadings and linear dimensions. 8

OR

8. a) Explain the torque - pulse rate characteristics of a stepping motor. 8

b) What are linear electrical machines ? Give the principle of operation of LIM. 8

9. a) What are synchronous harmonic and induction harmonic torques that may

appear in an induction motor ? 8

b) Explain the methods used in synchronous machines to reduce harmonics. 8

OR

10. a) What are the causes of space harmonics in the air gap flux wave of 3 - phase

induction motor ? 8

b) Explain how the space harmonics in the air gap flux wave introduce harmonic

asynchronous torques and give rise to crawling. 8

11. a) What is Kron's primitive machine ? How are the various windings of a machine

represented by the primitive machine ? 8

b) Obtain identical transformation for currents and voltages from rotating balanced

3 - phase (a,b,c) winding to rotating balanced 2 - phase ( ) ,βα winding. Show

that power invariance is maintained under this transformations. 10

OR

12. a) Explain the various limitations of the generalized theory of electrical machines. 8

b) Write voltage equations for the mathematical model of a 3 - phase induction

machine and hence obtain an expression for steady state torque when balanced

3 - phase supply is connected to the stator. 10

––––––––––––––––

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