61. Synchronous motors are inherently not self starting motors as
- the direction of instantaneous torque on the rotor reverses after each half cycle
- there is no slip
- the stator does not produce revolving magnetic field
- it has no starting winding
62. During short circuit test on a synchronous motor ______ is/are short circuited.
- armature terminals
- field terminals
- all stator phase windings
- one of the stator phase windings
63. The rotor copper losses of a synchronous motor are met by
- armature input
- dc source
- ac supply mains
- none of these
64. The efficiency of a properly designed synchronous machine is of
- 60%
- 80%
- 92%
- 99%
65. The negative phase sequence in 3-phase synchronous motor exists when the motor is
- underloaded
- overloaded
- supplied with unbalanced voltage
- hot
66. In case one phase of a synchronous motor is short circuited, the motor will
- not start
- run at 2/3 of synchronous speed
- run with excessive speed
- take less than the rated load
67. A synchronous motor may fail to pull into synchronism owing to
- excessive load
- low excitation
- high friction
- any of these
68. Which of the following synchronous motors has cost comparable to that of an induction motor?
- high kW output high speed
- high kW output low speed
- low kW output low speed
- low kW output high speed
69. Which of the following is not the advantage of a synchronous motor?
- high operation efficiency
- operation over a wide range of power factor
- operation at constant speed from no load to full load
- its rotor has 2 slip rings
70. Which of the following is not the disadvantage of a synchronous motor?
- its torque is less sensitive to variations in supply voltage
- DC excitation is required
- it has tendency to hunt
- its starting under load is not possible