Electrical Machines Questions and Answers – Starting of Synchronous Motors

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Starting of Synchronous Motors”.

1. Synchronous motors are ___________
a) not self starting
b) self starting
c) single excited
d) none of the mentioned
View Answer

Answer: a
Explanation: The average torque developed in the synchronous motor is zero.
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2. Which method is/are used to perform magnetic locking?

I. Auxiliary motor
II. Dc motor
III. induction motor

a) I, II, III
b) II, III
c) II, I
d) II
View Answer

Answer: a
Explanation: A sub motor is needed to make the synchronous machine run till the speed of synchronous speed and to achieve that we need additional motor.
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3. Auxiliary method of starting is not possib1e due to the fact that ____________
a) it can not be started under load
b) it can be only started under load
c) it is less efficient
d) all of the mentioned
View Answer

Answer: a
Explanation: Auxiliary motors usually reduces the performance indices of the synchronous machine when started under load.

4. Rating of the dc motor to be used in the starting of SM, should be higher than the synchronous motor.
a) True
b) False
View Answer

Answer: b
Explanation: No, the dc motor used as pony motor is of lower rating than the synchronous motor it starts.
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5. A synchronous machine with its field winding on stator and polyphase armature winding on rotor. At steady state, which of the following is not true for its air gap field?
a) stationary w.r.t. stator
b) rotating at double the speed Ns w.r.t. rotor
c) rotating in direction opposite to rotor
d) rotating at Ns w.r.t. rotor
View Answer

Answer: d
Explanation: The air gap will not be at synchronous speed w.r.t. rotor.

6. A synchronous machine with its field winding on rotor and polyphase armature winding on stator. At steady state running condition, its air gap field is?

I. stationary w.r.t. stator
II. rotating at double the Ns w.r.t. rotor
III. rotating at -Ns w.r.t. rotor
IV. stationary w.r.t. rotor.
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a) IV
b) I, II, IV
c) II, IV
d) I
View Answer

Answer: a
Explanation: The excitation is given on rotor so the air gap field will be stationary w.r.t. rotor.

7. Consider a 3-phase cylindrical-rotor alternator.

A. E.m.f. generated by armature reaction lags armature current by 90°.
B. Air gap voltage leads the field flux by 90°
C. Air gap voltage lags the field flux by 90°    
D. Armature reaction due to intermediate lagging p.f is partly cross magnetizing & partly magnetizing
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a) A, B
b) A, C, D
c) B, D
d) A, B, D
View Answer

Answer: a
Explanation: Having cross magnetizing armature reaction in the machine means partial magnetization and demagnetization, adding another clause is useless.

8. These days alternators are designed to have larger air gaps for __________
a) stable parallel operation
b) higher stability limit
c) sinusoidal mmf distribution
d) all of the mentioned
View Answer

Answer: a
Explanation: Large air gap ensures more SCR value and better preformances.

9. Consider the following statements and mark appropriately.

 I. In a cylindrical-rotor synchronous machine, armature flux is in phase with armature current.
 II. In salient pole machine, armature flux lags the armature current.

a) I is true
b) II is true
c) I and II are true
d) None of them are true
View Answer

Answer: c
Explanation: In CRSM, there is no saliency and the armature flux follows armature current in phase.

10. In a 3-phase cylindrical-rotor alternator, synchronous reactance is sum of mutual and leakage reactance.
a) True
b) False
View Answer

Answer: b
Explanation: Xs = magnetizing reactance + leakage reactance.

11. A 3-phase, 400V, synchronous motor is providing load at 0.8 p.f. lagging. If the field current of the motor is continuously increased, then the _____________
a) power factor increases and then decreases
b) power factor is not affected
c) power factor decreases upto a certain value of field current and then increases
d) power facor increases simply
View Answer

Answer: a
Explanation: The magnetization current is increased as we increase the excitation, so the power factor will increase upto the saturation and then again it will decrease.

12. A synchronous machine is operating at constant load and at unity power factor. If its excitation is increased and then it operates as ___________
a) motor at leading p.f
b) motor at lagging p.f
c) absorber of reactive power
d) generator at leading p.f
View Answer

Answer: a
Explanation: It will operate at leading p.f. if it was in motoring mode.

13. A synchronous machine is operating at constant load and at unity power factor. If its excitation is increased, then it operates as ___________
a) generator at lagging p.f
b) motor at lagging p.f
c) absorber of reactive power
d) generator at leading p.f
View Answer

Answer: a
Explanation: It will operate at lagging p.f. if it was working as generator.

14. The reactive power output of a synchronous generator is limited by ___________
a) armature current and field current
b) field current and load angle
c) load angle and prime mover input
d) armature current and prime mover input
View Answer

Answer: a
Explanation: Q=f(Ia,If).

15. Power factor of a synchronous motor varies when the ___________
a) applied voltage is varied
b) load is changed
c) supply frequency and field excitation is changed
d) all of the mentioned
View Answer

Answer: a
Explanation: p.f. = f(V,E,f,Field current).

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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