Synchronous Motor Multiple Choice Questions And Answers In English -03

1. When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor
(A) remains same as the original value
(B) decreases to half the original value
(C) tends to becomes zero
(D) increases to two times the original value
Ans: D

2. Synchronsizingpower of a synchronous machine is
(A) direcly proportional to the synchronous reactance
(6) inversely proportional to the synchronous reactance
(A) equal to the synchronous reactance
(D) none of the above
Ans: B

3. The standard full-load power factor ratings for synchronous motors are
(A) zero or 0.8 leading
(B) unity or 0.8 lagging
(C) unity or 0.8 leading
(D) unity or zero
Ans: C

4. Slip rings are usually made of
(A) carbon or graphite
(B) brass or steel
(C) silver or gold
(D) copper or aluminium
Ans: B

5. An over excited synchronous motor is used for
(A) fluctuating loads
(B) variable speed loads
(C) low torque loads
(D) power factor corrections
Ans: D

6. When the voltage applied to a synchronous motor is increased, which of the following will reduce ?
(A) Stator flux
(B) Pull in torque
(C) Both (A) and (B)
(D) None of the above
Ans: D

7. To limit the operating temperature an electrical machine should have proper
(A) voltage rating
(B) current rating
(C) power factor
(D) speed
Ans: B

8. A synchronous machine with large air gap has
(A) a higher value of stability limit
(6) a small value of inherent regulation
(C) a higher synchronizing power which makes the machine less sensitive to load variations
(D) all of the above
Ans: D

9. In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately
(A) one-third
(B) twice
(C) thrice
(D) six times
(e) nine times
Ans: C

10. Which of the following methods is used to start a synchronous motor ?
(A) Damper winding
(B) Star-delta starter
(C) Damper winding in conjunction with star-delta starter
(D) Resistance starter in the armature circuit
Ans: C

11. The angle between the rotating stator flux and rotor poles is called _____ angle.
(A) torque
(B) obtuse
(C) synchronizing
(D) power factor
Ans: A

12. Slip-rings in a synchronous motor carry
(A) direct current
(B) alternating current
(C) no current
(D) all of the above
Ans: A

13. The armature current of the synchronous motor has higher values for
(A) high excitation only
(B) low excitation only
(C) both (A) and (B)
(D) none of the above
Ans: C

14. In which of the following motors the stator and rotor magnetic field rotate at the same speed ?
(A) Universal motor
(B) Synchronous motor
(C) Induction motor
(D) Reluctance motor
Ans: B

15. Synchronous motors are
(A) not-self starting
(B) self-starting
(C) essentially self-starting
(D) none of the above
Ans: A

16. The efficiency of a properly designed synchronous motor will usually fall in range
(A) 60 to 70%
(6) 75 to 80%
(C) 85 to 95%
(D) 99 to 99.5%
Ans: C

17. The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of
(A) zero degree
(B) two degrees
(C) five degrees
(D) ten degrees
Ans: C

18. While starting a synchronous motor by induction motor action, field winding is usually
(A) connected to D.C. supply
(B) short-circuited by low resistance
(C) kept open-circuited
(D) none of the above
Ans: B

19. Power factor of a synchronous motor is unity when
(A) the armature current is maximum
(B) the armature current is minimum
(C) the armature current is zero
(D) none of the above
Ans: B

20. Change of D.C. excitation of a synchronous motor changes
(A) applied voltage of the motor
(B) motor speed
(C) power factor of power drawn by the motor
(D) any of the above
Ans: C