Synchronous Motor Multiple Choice Questions And Answers In English -02

1. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected
(A) the motor stops
(B) it runs as a reluctance motor at the same speed
(C) it runs as a reluctance motor at a lower speed
(D) none of the above
Ans: A

2. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as
(A) V
(B) V312
(C) V2
(D) 1/V
Ans: A

3. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in
(A) back e.m.f.
(B) armature current
(C) power factor
(D) torque angle
Ans: B

4. A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because
(A) synchronous motor has no slip
(B) stator supply is not required to produce magnetic field
(C) mechanical load on the rotor remains constant
(D) synchronous motor has large airgap
Ans: B

5. A synchronous motor working at leading power factor can be used as
(A) voltage booster
(B) phase advancer
(C) noise generator
(D) mechanical synchronizer
Ans: B

6. The power developed by a synchronous motor will be maximum when the load angle is
(A) zero
(B) 45°
(C) 90°
(D) 120°
Ans: C

7. Mostly, synchronous motors are of
(A) alternator type machines
(6) induction type machines
(C) salient pole type machines
(D) smooth cylindrical type machines
Ans: C

8. The synchronous motor is not inherently self-starting because
(A) the force required to accelerate the rotor to the synchronous speed in an instant is absent
(B) the starting device to accelerate the rotor to near synchronous speed is absent
(C) a rotating magnetic field does not have enough poles
(D) the rotating magnetic field is produced by only 50 Hz frequency currents
Ans: A

9. As the load is applied to a synchronous motor, the motor takes more armature current because
(A) the increased load has to take more current
(B) the rotor by shifting its phase backward causes motor to take more current
(C) the back e.m.f. decreases causing an increase in motor current
(D) the rotor strengthens the rotating field casuing more motor current
Ans: B

10. The working of a synchronous motor is similar to
(A) gear train arrangement
(B) transmission of mechancial power by shaft
(C) distribution transformer
(D) turbine
(e) none of the above
Ans: B

11. The minimum armature current of the synchronous motor corresponds to operation at
(A) zero power factor leading
(B) unity power factor
(C) 0.707 power factor lagging
(D) 0.707 power factor leading
Ans: B

12. In a synchronous motor, the magnitude of stator back e.m.f. £& depends on
(A) d.c. excitation only
(B) speed of the motor
(C) load on the motor
(D) both the speed and rotor flux
Ans: A

13. If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is
(A) 2
(B) 3
(C) 4
(D) 6
Ans: B

14. The shaft of synchronous motor is made of
(A) mild steel
(B) chrome steel
(C) alnico
(D) stainless steel
Ans: A

15. When the field of a synchronous motor is under-excited, the power factor will be
(A) leading
(B) lagging
(C) unity
(D) zero
Ans: B

16. The speed regulation of a synchronous motor is always
(A) 1%
(B) 0.5%
(C) positive
(D) zero
Ans: D

17. The percentage slip in case of a synchronous motor is
(A) 1%
(B) 100%
(C) 0.5%
(D) zero
Ans: D

18. When load on an over-excited or under excited synchronous*motor is increased, rate of change of its armature current as compared with that of power factor is
(A) more
(B) less
(C) equal
(D) twice
Ans: B

19. The rotor copper losses, in a synchronous motor, are met by
(A) d.c. source
(B) armature input
(C) motor input
(D) supply lines
Ans: A

20. The maximum power developed in a synchronous motor occurs at a coupling angle of
(A) 30°
(B) 60°
(C) 90°
(D) 180°
Ans: C