Most Synchronous Motors

61. Power factor of a synchronous motor is unity when

1. the armature current is maximum
2. the armature current is minimum
3. the armature current is zero
4. none of the above

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62. riunting in a synchronous motor takes place

1. when supply voltage fluctuates
2. when load varies
3. when power factor is unity
4. motor is under loaded

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63. Slip rings are usually made of

1. carbon or graphite
2. brass or steel
3. silver or gold
4. copper or aluminium

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64. Slip-rings in a synchronous motor carry

1. direct current
2. alternating current
3. no current
4. all of the above

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65. Stability of a synchronous machine

1. decreases with increase in its excitation
2. increases with increase in its excitation
3. remains unaffected with increase in excitation
4. any of the above

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66. Synchronous motor always runs at

1. the synchronous speed
2. less than synchronous speed
3. more than synchronous speed
4. none of the above

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67. Synchronous motors are

1. not-self starting
2. self-starting
3. essentially self-starting
4. none of the above

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68. Synchronous motors are generally not self-starting because

1. the direction of rotation is not fixed
2. the direction of instantaneous torque reverses after half cycle
3. startes cannot be used on these machines
4. starting winding is not provided on the machines

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69. Synchronsizingpower of a synchronous machine is

1. direcly proportional to the synchronous reactance
2. inversely proportional to the synchronous reactance
3. equal to the synchronous reactance
4. none of the above

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70. The angle between the rotating stator flux and rotor poles is called _________ angle.

1. torque
2. obtuse
3. synchronizing
4. power factor

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