Code No: 310205

III B-Tech I-Semester Supplementary Examinations

June - 2003
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. Distinguish between integral slot and fractional slot winding slot winding and their merits and demerits.

2. The field form of a star connected 3-phase machine is given by 100 sinθ + 30sin 3θ + 10 sin5θ, where θ is measured from the interpolar axis. If the stator has 9slots per pole and coils are full pitched, determine the wave form of phase voltage in terms of V_max, the amplitude of the fundamental phase voltage. Determine also the r.m.s value of phase and line voltages what are the phase and line voltages if the flux is sine distributed with the same total flux per pole?

3. a) What happens to the value of synchronous reactance if air gap is increased

b) What is the excitation voltage in case of synchronous machines?

c) A 30KVA, 440V, 50Hz, 3-phase, Star-connected alternator gave the following data:

4. Explain the Phasor diagram of salient-pole synchronous machine

a) At Lagging P.f
b) At Leading P.f

5. a) How do you calculate the time constant in case of an alternator

b) Show that an alternator running in parallel a constant voltage and frequency bus bars has a natural time period of oscillation. Deduce a formulae for the time of one complete oscillation and calculate the value for a 500KVA, 3-phase, 10000V machine running at 1500rpm and constant 50Hz bus bars.

6. a) Define and explain different torques of a synchronous motor?

b) A 400V, 50Hz, 6-pole, 3-Φ Y connected synchronous motor has a synchronous reactance of 4Ω/phase and a resistance of 0.5Ω/phase. On full-load, the excitation is adjusted so that machine takes an armature current of 60 Amperes at 0.866p.f. Leading. Keeping the excitation unchanged, find the maximum power output. Excitation, friction, windage and iron losses total 2KW.

7. a) Why are synchronous condensers also called as synchronous phase modifiers? Where are these used?

b) A substation operating at its full load of 1,000KVA supplies a load of power factor 0.71 lagging. Calculate the permissible additional load at this power factor and the rating of synchronous condensers to raise the substation power factor to 0.87 lagging.

8. a) Show that in a shaded pole motor, the flux in the shaded part of pole segment lags behind the flux of the unshaded pole segment both in space and time, resulting a rotational torque in the motor.

b) A capacitor connected in series with the starting winding of a resistance start split phase induction motor. Explain the changes in the performance characteristics