D.C. Motors

71. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator?

1. hysteresis loss
2. field copper loss
3. armature copper loss
4. eddy current loss

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72. The main disadvantage of the Ward-Leonard control method is

1. high initial cost
2. high maintenance cost
3. low efficiency at Hght loads
4. all of the above

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73. The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is

1. 4
2. 2
3. 1
4. 0.5

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74. The plugging gives the

1. zero torque braking
2. smallest torque braking
3. highest torque braking
4. none of the above

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75. The power mentioned on the name plate of an electric motor indicates

1. the power drawn in kW
2. the power drawn in kVA
3. the gross power
4. the output power available at the shaft

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76. The purpose of retardation test on D.C. shunt machines is to find out

1. stray losses
2. eddy current losses
3. field copper losses
4. windage losses

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77. The ratio of starting torque to full-load torque is least in case of

1. series motors
2. shunt motors
3. compound motors
4. none of the above

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78. The speed of a D.C. motor can be varied by varying

1. field current
2. applied voltage
3. resistance in series with armature
4. any of the above

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79. The speed of a D.C. series motor is

1. proportional to the armature current
2. proportional to the square of the armature current
3. proportional to field current
4. inversely proportional to the armature current

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80. The speed of a D.C. shunt motor can be increased by

1. increasing the resistance in armature circuit
2. increasing the resistance in field circuit
3. reducing the resistance in the field circuit
4. reducing the resistance in the armature circuit

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