Code No: 310205

III B-Tech I-Semester Regular Examinations

November - 2003
Set No: 1

ELECTROMECHANICS - III

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. a) Describe the constructional features of cylindrical rotor synchronous generators and the function of each part

b) Distinguish between integral slot and fractional slot winding and their merits and demerits.

2. What is armature reaction? And what is synchronous reactance? How do you calculate synchrous impedance experimentally?

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

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

Field current (A)        2    4    6     7     8    10     12    14

Terminal voltage (V)  155 287 395  440 475  530   570  592

S.C. Current (A)        11   22   34   40   46   57    69    80

Resistance between any two terminals is 0.3 Ω. Find the regulation at full load, 0.8 P.f Lagging, by MMF method

4. Two identical 2 MVA alternators operate in parallel. The governor of the first machine is such that frequency drops uniformly from 50Hz on no load to 48Hz on full load. The corresponding uniform speed drop of the second machine is 50Hz to 47.5Hz.

a) How will the two machines share a load of 3 MW

b) What is the maximum load at UPF that can be delivered without overloading either machine.

5. A 750KW, 11KV, 3Ph star connected synchronous motor has a synchronous reactance of 35 Ω / ph and negligible resistance. Determine the excitation emf per phase when the motor is operating on full load at 0.8 pf leading. Its efficiency under this condition is 93%.

6. a) What are the advantages of synchronous motor over induction motors?

b) Why at any load, the power factor decreases and the armature current increase if the field current is varied above and below the normal excitation.

7. a) Prove that a single phase motor winding when excited by a single phase supply produces two equal and opposite revolving fields.

b)"The centrifugal switch of a single phase motor failed to open". Explain the after effects in the performance.

8. a) Compare the constructional features of a.c series motor with d.c. Series motor.

b) Why cannot a shaded pole motor be made to rotate in the reverse direction?