MCQ on synchronous motor 41-60

41. If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor
(a) become more
(b) become less✔
(c) remain constant
(d) none of the above

42. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will
(a) remain same
(b) go down
(c) improve✔
(d) none of the above

43. While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by
(a) induction motor torque in field winding
(b) induction motor torque in damper winding
(c) eddy current and hysteresis torque in pole
(d) all of the above methods✔

45. Armature of a synchronous machine is
(a) of reducing number of slip rings on the rotor
(b) armature is associated with large power as compared to the field circuits
(c) of difficulty of providing high voltage insulation on rotor
(d) all of the above reasons✔

46. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to
(a) increase in both armature current and power factor angle✔
(b) increase in back e.m.f. but decrease in armature current
(c) increase in both armature current and power factor which is lagging
(d) increase in torque angle but decrease in back e.m.f.

47. When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way
(a) augments it directly
(b) directly opposes it✔
(c) cross-magnetises it
(d) none of the above

48. Stability of a synchronous machine
(a) decreases with increase in its excitation
(b) increases with increase in its excitation✔
(c) remains unaffected with increase in excitation
(d) any of the above

49. The power factor of a synchronous motor is better than that of induction motor because
(a) stator supply is relieved of responsibility of producing magnetic field✔
(b) mechanical load on the motor can be adjusted
(c) synchronous motor runs at synchronous speed
(d) synchronous motor has large air gap

50. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor
(a) will become more
(b) will become less✔
(c) will remain unchanged
(d) none of the above.

51. A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
(a) speed of the motor is reduced
(b) power factor is decreased
(c) Eb (back e.m.f.) becomes less than V (applied voltage)
(d) Er (net resultant voltage) in armature is increased✔

52. If one-phase of a 3-phase synchronous motor is short-circuited, motor
(a) will refuse to start✔
(b) will overheat in spots
(c) will not come upto speed
(d) will fail to pull into step

53. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then
(a) it runs at a slower speed
(b) the motor stops✔
(c) it continues to run at the same speed
(d) it runs at a very high speed

54. In which of the following motors the stator and rotor fields rotate simultaneously ?
(a) D.C. motor
(b) Reluctance motor
(c) Universal motor
(d) Synchronous motor✔
(e) Induction motor

55. The speed of a synchronous motor
(a) increases as the load increases
(b) decreases as the load decreases
(c) always remains constant✔
(d) none of the above

56. A rotory converter can also be run as a
(a) d.c. shunt motor
(b) d.c. series motor
(c) d.c. compound motor
(d) induction motor
(e) synchronous motor
Ans: e

57. The maximum speed variation in a 3-phase synchronous motor is
(a) 10 per cent
(b) 6 per cent
(c) 4 per cent
(d) zero✔

58. Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ?
(a) Phase to phase winding resistance
(b) Stator winding to earthed frame
(c) Rotor winding to earthed shaft
(d) All of the above✔

59. Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?
(a) High field current
(b) Low short circuit ratio
(c) High core losses
(d) Low field current✔

60. Which of the following motors will be used in electric clocks ?
(a) D.C. shunt motor
(b) D.C. series motor
(c) A.C. induction motor
(d) A.C. synchronous motor✔