Electrical Machines Multiple Choice Questions on “Starting of Synchronous Motors”.
1. Synchronous motors are ___________
A. not self starting
B. self starting
C. single excited
D. none of the mentioned
Answer: A
Clarification: The average torque developed in the synchronous motor is zero.
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
Answer: A
Clarification: 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.
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
Answer: A
Clarification: 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
Answer: B
Clarification: No, the dc motor used as pony motor is of lower rating than the synchronous motor it starts.
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
Answer: D
Clarification: 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.
A. IV
B. I, II, IV
C. II, IV
D. I
Answer: A
Clarification: 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
A. A, B
B. A, C, D
C. B, D
D. A, B, D
Answer: A
Clarification: 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
Answer: A
Clarification: 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