Synchronous motor mcq part-3

1. Synchronous motors are
(a) not-self starting
(b) self-starting
(c) essentially self-starting
(d) none of the above

Ans: a

2. 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

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. Slip rings are usually made of
(a) carbon or graphite
(b) brass or steel
(c) silver or gold
(d) copper or aluminium

Ans: b

7. 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

8. 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

11. 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

12. To limit the operating temperature an electrical machine should have proper
(a) voltage rating
(b) current rating
(c) power factor
(d) speed

Ans: b

13. Slip-rings in a synchronous motor carry
(a) direct current
(b) alternating current
(c) no current
(d) all of the above

Ans: a

14. 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

15. 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

16. 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

17. The angle between the rotating stator flux and rotor poles is called angle.
(a) torque
(b) obtuse
(c) synchronizing
(d) power factor

Ans: a

18. 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

19. When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop
(a) inductor motor torque
(b) induction generator torque
(c) synchronous motor torque
(d) d.c. motor toque
(e) none of the above

Ans: b

20. An important advantage of a synchronous motor over wound round induction motor is that
(a) its power factor may be varied at will
(b) its speed is independent of supply frequency
(c) its speed may be controlled more easily
(d) none of the above

Ans: a