Synchronous Motor MCQ

46. An ideal synchronous motor has no starting torque because the

1. Rotor is made up of salient poles.
2. Relative velocity between the stator and rotor mmf is zero.
3. Relative velocity between stator and rotor mmf is not zero.
4. Rotor winding is highly inductive.

47. A large size synchronous generator is protected against overloads by

1. overcurrent relay
2. mho relay
3. temperature-sensitive relay
4. Buchholz relay

48. A salient-pole synchronous motor is operating at 1/4 full load. If its field current is suddenly switched off, it would?

1. run at super-synchronous speed
2. stop running
3. run at sub-synchronous speed
4. synchronous speed

49. A synchronous phase modifier as compared to a synchronous motor used for mechanical load has

1. larger shaft and higher speed
2. smaller shaft and higher speed
3. larger shaft and smaller speed
4. smaller shaft and smaller speed

50. The percentage slip in case of synchronous motor is

1. 1%
2. 100%
3. 5%
4. zero

51. While starting synchronous motor its field winding should be

1. kept open
2. connected to a dc source
3. connected to ac source
4. kept short-circuited

52. For a 3-phase, 4-pole, 50 Hz synchronous motor the frequency, no. of poles, and the load torque are all halved. The motor speed will be

1. 75 rpm
2. 375 rpm
3. 3000 rpm
4. 1500 rpm

53. The negative phase sequence in a 3-phase synchronous motor exists when the motor is:

1. underload
2. overloaded
3. supplied with unbalanced voltage
4. hot

54. In synchronous motor, minimum armature current occurs at

1. zero power factor
2. leading power factor
3. lagging power factor
4. unity power factor

55. Which one of the following is correct?

1. Synchronous motor is supplied with dc voltage in the armature winding
2. Synchronous motor is supplied with ac voltage in the field winding
3. Synchronous motor is supplied with rectified voltage in the armature winding
4. Synchronous motor is supplied with dc voltage in the field winding

56. The pitch factor for a full pitched winding of a synchronous machine is

1. 0.5
2. 0.9
3. 0
4. 1

57. A 3-phase synchronous motor is started by utilizing the torque developed in

1. the high-speed steam-turbine.
2. the damper winding on the rotor.
3. the damper winding on the stator
4. the low-speed water-turbine.

58. A synchronous motor working at leading power factor can be used as

1. phase advancer
2. noise generator
3. mechanical synchronizer
4. voltage booster

59. A 10 pole 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor then the number of poles in a synchronous motor are?

1. 48 poles
2. 12 poles
3. 24 poles
4. None of the options

60. A synchronous machine is called a doubly excited machine because

1. it can be overexcited
2. it has two sets of rotor poles
3. both its rotor and stator are excited
4. it needs twice the normal exciting current