## Sunday, 15 March 2015

### ELECTRICAL MACHINES – II IMPORTANT QUESTIONS For Exams

ELECTRICAL MACHINES – II  IMPORTANT QUESTIONS For Exams

1. Write the causes of harmonics in the voltage and current waves of electrical
machinery?
2. What are conditions for parallel operation of alternators?
3. List the inherent disadvantages of synchronous motor?
4. When is a synchronous motor said to receive 100% excitation?
5. Under what condition, the slip in an induction motor is
(a) Negative
(b) Greater than one.
6. What are the two fundamental characteristics of a rotating magnetic field?
7. State two advantages of speed control of induction motor by injecting an e.m.f
in the rotor circuit.
8. What is the effect of change in input voltage on starting torque of induction
motor?
9. What are the drawbacks of the presence of backward rotating magnetic field in
a single phase induction motor?
10. What are the demerits of repulsion motor?

PART B — (5 × 16 = 80 Marks)

11. (a) (i) Derive the e.m.f. equation of an alternator. Explain pitch factor and
distribution factor. (12)
(ii) A 3 phase, 6 pole, star-connected alternator revolves at 1000 r.p.m.
The stator has 90 slots and 8 conductors per slot. The flux per pole
is 0.05 wb (sinusoidally distributed). Calculate the voltage
generated by the machine if the winding factor is 0.96. (4)
Or
(b) (i) Elaborate the discussion on capability curve with its boundaries of
synchronous machine. (8)
(ii) Discuss the parallel operation of two alternators with identical
speed/load characteristics. (8)
12. (a) (i) Draw the equivalent circuit and phasor diagram of a synchronous
motor. (8)
(ii) Explain the significance of V and inverted V curves. (8)
Or
(b) (i) Discuss the methods of starting and procedure for starting
synchronous motor. (10)
(ii) A 3000 V, 3 phase synchronous motor running at 1500 r.p.m, has its
excitation kept constant corresponding to no-load terminal voltage
of 3000 V. Determine the power input, power factor and torque
developed for all armature current of 250 A if the synchronous
reactance is 5 per phase and armature resistance is neglected. (6)
13. (a) (i) Discuss the different power stages of an induction motor with
losses. (8)
(ii) An 18.65 KW, 4 pole, 50 Hz, 3 phase induction motor has friction
and windage losses of 2.5% of the output. Full load slip is 4%. Find
for full load
(1) rotor copper loss,
(2) rotor input
(3) shaft torque
(4) the gross electromagnetic torque. (8)
Or
(b) (i) Write a brief note on induction generator. (8)
(ii) Write a brief note on double cage rotor induction motors. (8)
14. (a) (i) Explain the speed control of three phase induction motor by pole
changing. (8)
(ii) Explain the rotor rheostat control of 3 phase slip ring induction
motor. (8)
Or
(b) Discuss the various starting methods of squirrel cage induction motors.
(16)
15. (a) (i) Explain the operation of a single phase induction motor using
double field revolving theory. (8)
(ii) Explain the operation of shaded pole induction motor with neat
diagram. (8)
Or
(b) (i) Explain the principle and operation of AC series motor. (8)
(ii) Explain the principle and operation of reluctance motor and state
its applications. (8)