# Synchronous Machines - Electrical Engineering (MCQ) questions and answers

1)   The open circuit voltage and short circuit current of a 3 phase, star connected alternator are 1050 V and 250 A respectively. Its field current is 12 A. Then the synchronous impedance of alternator will be

a. 1.34 ohm
b. 1.69 ohm
c. 2.42 ohm
d. 2.85 ohm
 Answer  Explanation ANSWER: 2.42 ohm Explanation: No explanation is available for this question!

2)   A 6 kVA, 220 v 3 phase, star connected synchronous generator has a winding resistance of 0.15 ohm per phase and synchronous reactance of 6.1 ohm per phase. The voltage regulation of alternator at a load power factor of 0.8 lagging will be

a. 58.10 %
b. 68.43 %
c. 69.02 %
d. 72.81 %
 Answer  Explanation ANSWER: 58.10 % Explanation: No explanation is available for this question!

3)   Generally, in alternator divided winding rotor is preferred to a conventional winding rotor, because it gives

a. Better damping
b. Less harmonics
c. More efficiency
d. None of these
 Answer  Explanation ANSWER: Better damping Explanation: No explanation is available for this question!

4)   The given figure shows the load characteristics of an alternator. Which among the following is correct? a. A – Leading p.f., B – unity p.f., C – lagging p.f.
b. A – Lagging p.f., B – unity p.f., C – leading p.f.
c. A – Leading p.f., B – lagging p.f., C – unity p.f.
d. A – Unity p.f., B – leading p.f., C – lagging p.f.
 Answer  Explanation ANSWER: A – Leading p.f., B – unity p.f., C – lagging p.f. Explanation: No explanation is available for this question!

5)   For a two pole, 50 Hz, 3 phase synchronous motor the speed of rotating magnetic field is

a. 1000 rpm
b. 2000 rpm
c. 3000 rpm
d. 3500 rpm
 Answer  Explanation ANSWER: 3000 rpm Explanation: No explanation is available for this question!

6)   Which type of rotor is generally used for high speed alternators or turbo alternators?

a. Salient pole type
b. Projected pole type
c. Smooth cylindrical type
d. Both (a) & (b)
 Answer  Explanation ANSWER: Smooth cylindrical type Explanation: No explanation is available for this question!

7)   If the field of the synchronous motor is overexcited then its power factor will be

a. Lagging
c. Unity
d. Zero

8)   To have effective synchronization without any interruption, the necessary conditions which must be satisfied are

a. The terminal voltage and frequency of the incoming machine must be same as that of bus bar voltage and frequency
b. With respect to the external load, the phase of alternator voltage must be identical with that of the bus bar voltage
c. Both (a) and (b)
d. None of these
 Answer  Explanation ANSWER: Both (a) and (b) Explanation: No explanation is available for this question!

9)   Back emf in case of synchronous motor depends on the

a. Excitation given to the field winding
b. Speed
c. Both (a) and (b)
d. None of the above
 Answer  Explanation ANSWER: Excitation given to the field winding Explanation: No explanation is available for this question!

10)   The reluctance offered to the mmf wave is lowest when

a. It is aligned with the field pole axis
b. It is oriented at 90 degree to the field pole axis
c. Both (a) and (b)
d. None of these
 Answer  Explanation ANSWER: It is aligned with the field pole axis Explanation: No explanation is available for this question!

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