D.C. Machines

71. The induced emf in the armature conductors of a dc motor is

1. Sinusoidal
2. Trapezoidal
3. Rectangular
4. Alternating

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72. The locked rotor current in a 3-phase, star connected 15kW, 4-pole, 230V, 50Hz induction motor at rated conditions is 50A. Neglecting losses and magnetizing current, the approximate locked rotor line current drawn when the motor is connected to a 236V, 57Hz supply is

1. 58.5A
2. 45.0A
3. 45.7A
4. 55.6A

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73. the main disadvantage of hopkinson's test for finding efficiency of shunt dc motors is that it

1. Requires full load power
2. Ignores any change in iron loss
3. Needs one motor and one generator
4. Requires two identical shunt machines

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74. The maximum value of torque angle a in a synchronous motor is. degrees electrical

1. 45
2. 90
3. between 45 and 90
4. below 60

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75. The most suitable servomotor for low power applications is

1. a dc series motor.
2. a dc shunt motor.
3. an ac two-phase induction motor.
4. an ac series motor.

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76. The power factor of a squirrel cage induction motor is

1. low at light load only.
2. low at heavy load only.
3. low at light and heavy load both.
4. low at rated load only.

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77. The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz supply. The secondary output voltage will be

1. 3.6 V.
2. 2.5 V.
3. 3.0 V.
4. 6.0 V.

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78. The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for a

1. dc machine.
2. 3 phase induction machine.
3. synchronous machine.
4. single phase induction machine.
5. All are correct

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79. The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04 is _________ Hz

1. 8
2. 4
3. 6
4. 2

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80. The size of a conductor used in power cables depends on the

1. operating voltage.
2. power factor.
3. current to be carried.
4. type of insulation used.

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