Anna univ EE 256 — ELECTRICAL MACHINES

PART A — (10 ? 2 = 20 marks)
1. State the different parts of a dc generator and explain their functions.
2. Why do we need starters for dc motors?
3. Explain, why the Open Circuit test is generally performed at rated voltage on the LV side of a transformer.
4. Show that the maximum efficiency in a transformer occurs when its variable loss is equal to constant loss.
5. A 4 pole, 3 phase, synchronous motor run from a 50 H supply is mechanically coupled to a 24 pole synchronous generator. At what speed will the set rotate? What will be the frequency of the emf induced in the generator?
6. Why is a synchronous motor not self–starting?
7. If a 3-phase induction motor runs at 960 rpm on a 50 Hz supply, what is the number of poles in the motor?
8. Why can't an induction motor develop torque at synchronous speed?
9. Explain, how the starting winding assist to develop starting torque in single–phase induction motors.
10. How will you tell the direction of rotation of a shaded pole induction motor from its construction (without actually running the motor)?
PART B — (5 ? 16 = 80 marks)
11. Explain the principle of operation of a three–phase induction motor and draw its equivalent circuit. (8 + 8)
12. (a) (i) Draw and explain the load characteristics of a dc shunt generator. (8)
(ii) A 6 pole, wave wound 500 rpm, dc shunt generator has armature and field resistances of 0.5 ohm and 250 ohm respectively. The armature has 250 conductors and the flux per pole is 40 m Wb. If the load resistance is 15 ohm, determine the terminal voltage and load current. (8)
Or
(b) A 250 volts dc shunt motor takes an armature current of 20 amps and runs at 1000 rpm against full load torque. The armature resistance is 0.5 ohm. What resistance must be inserted in series with armature to reduce the speed to 500 rpm at the same load torque? With this resistance in circuit, determine the speed when the load torque reduced to 50%. Assume that the flux remains constant throughout. (16)
13. (a) Explain the constructional details and principle of operation of a single–phase transformer. (16)
Or
(b) A 4 kVA, 400/200 V, 50 Hz single phase transformer gave the following test results :
OC Test (LV side) : 60 Watts, 0.7 A, and 200 V
SC Test (HV side) : 21.6 Watts, 6 A, and 9 V
(i) Determine the equivalent circuit parameters of the transformer referred to LV side. (8)
(ii) Determine its voltage regulation at full load, 0.8 PF leading. (8)
14. (a) Derive the equation for the induced emf of a synchronous generator. (16)
Or
(b) Derive the expression for the torque developed by a synchronous motor and draw its torque angle characteristics. (16)
15. (a) Explain how torque is developed in a single phase induction motor according to the double revolving field theory. (16)
Or
(b) Draw a neat diagram of a universal motor and explain its operation. Also draw its torque–speed characteristics when it is operated with ac and dc sources and explain why they are different. (16)