Code No: 310202

III/IV B-Tech I-Semester Examinations

November - 2002
Set No: 4

ELECTROMECHANICS - III

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Derive the e.m.f equation of an alternator having a distributed and fractional pitch winding in its armature.

b) An alternator has 72 stator slots, 10 poles and a 3-phase double layer winding with a coil pitch of 6 slots. If the flux wave has a third harmonic whose magnitude is 15% of the fundamental components, find the corresponding value of the third harmonic in the phase voltage wave-form as no-load.

2. a) Explain voltage regulation of alternator.

Field current (A)                 40     50     110      140      180

Line voltage O.C.C. (volts): 5800  7000  12,500  13,750   15,000

Line voltage Z.p.f (volts):     0    1500   8,500   10,500   12,500  

b) A 11 KV, 1000 KVA, 3-phase star connected alternator has a resistance of 2 ohms/phase. The O.C.C and the characteristics with rated current at zero p.f are given in the following table. Find the voltage regulation of the alternator for full load current at 0.8 p.f leading.

3. a) With the help of a phasor diagram, explain the effect of excitation control of an alternator operating with another alternator supply a common load.

b) A 3-phase, 400KVA, 6.6KV, 1500RPM, 50c/s alternator is running in parallel with infinite bus bars. Its synchronous reactance drop is 25%. Calculate the synchronizing poser and torque per unit mechanical angle of phase displacement when the alternator operates at full load 0.8 p.f lagging.

4. a) Briefly explain how a 3-phase synchronous motor works.

b) Given a synchronous motor operating with a constant load an normal excitation, explain the effect on torque angle, armature current and power factor of motor with i) decreased excitation and ii) increased excitation.

5. a) Explain how power circle diagram of a synchronous motor can be constructed.

b) A 3-phase star connected synchronous motor has a synchronous reactance of 4 ohms per phase and is working on 1100 volts bus bars. Calculate the power factor of this machine when taking 90KW from the mains, the excitation being adjusted to a value corresponding to an induced e.m.f 1200 volts. Neglect armature resistance.

6. a) Explain briefly the principle of operation of a single-phase induction motor.

b) State the reasons for the inferior performance of single-phase motors in comparison to three-phase motors.

c) Describe the different methods of starting single-phase induction motors.

7. a) Explain the terms:- i) direct axis-synchronous reactance and ii) quadrature axis synchronous reactance and the methods if determining in a laboratory.

b) Explain the phenomenon of hunting in synchronous machines.

c) Discuss the methods of starting of synchronous motor.

8. Write short notes on :-

a) Short circuit current wave motor.
b) Synchronous induction motor.
c) Repulsion motor.