Code No: 310202

III/IV B-Tech I-Semester Examinations

November - 2002
Set No: 3

ELECTROMECHANICS - III

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Explain various methods of making the wave form of generated emf in a 3-phase alternator sinusoidal.

b) The phase emf of a 3-phase 50HZ alternator consists of a fundamental, a 20% 3rd harmonic and a 10% 5th harmonic, the maximum amplitude of the fundamental voltage is 1000v. Calculate the rms line voltage windings are in i) star, ii) delta.

2. a) Discuss the factors that govern the change in terminal voltage of an isolated synchronous generator when feeding its own load.

b) A 3-phase Y-connected alternator is delivering 20MW and 8MVAr to an infinite bus at 11 KV. Synchronous impedance=0+j3 ohm. Find load angle and excitation emf of the alternator

3. a) Draw and discuss the general phasor diagram of a cylindrical -rotor alternator showing various emfs and mmfs. Also draw the modifications done for emf and kkf methods.

b) Explain how OC and SC tests are conducted on a synchronous machine.

4. a) Develop phasor diagram for a salient pole alternator supply a leading pf load and explain 2-reaction theory.

b) Explain how an alternator is synchronized to the bus bars.

5. a) Explain the phenomenon of hunting, its causes and method of suppression in a synchronous motor.

b) Explain excitation circles and how it is constructed.

6. a) Describe construction and principle of operation of a shaded-pole motor.

b) Describe the double revolving field theory of a single-phase induction motor.

7. a) Describe briefly the methods of starting a synchronous motor.

b) Which motor has zero instantaneous torque during starting and which motor has zero average torque but non-zero instantaneous torque during starting? Explain your answer

8. Write short notes on: a) Synchronous condenser. b) Synchronous induction motor.