D.C. Motors

61. Sparking, is discouraged in a D.C. motor because

1. it increases the input power con-sumption
2. commutator gets damaged
3. both (a) and (b)
4. none of the above

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62. Speed control by Ward Leonard method gives uniform speed variation

1. in one direction
2. in both directions
3. below normal speed only
4. above normal speed only.

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63. Starters are used with D.C. motors because

1. these motors have high starting torque
2. these motors are not self-starting
3. back e.m.f. of these motors is zero initially
4. to restrict armature current as there is no back e.m.f. while starting

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64. The armature torque of the D.C. shunt motor is proportional to

1. field flux only
2. armature current only
3. both (a) and (b)
4. none of the above

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65. The armature voltage control of D.C. motor provides

1. constant torque drive
2. constant voltage drive
3. constant current drive
4. none of the above

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66. The condition for maximum efficiency for a D.C. generator is

1. eddy current losses = stray losses
2. hysteresis losses = eddy current losses
3. copper losses = 0
4. variable losses = constant losses

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67. The condition for maximum power in case of D.C. motor is

1. back e.m.f. = 2 x supply voltage
2. back e.m.f. = | x supply voltage
3. supply voltage = | x back e.m.f.
4. supply voltage = back e.m.f.

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68. The current drawn by the armature of D.C. motor is directly proportional to

1. the torque required
2. the speed of the motor
3. the voltage across the terminals
4. none of the above

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69. The direction of rotation of a D.C. series motor can be changed by

1. interchanging supply terminals
2. interchanging field terminals
3. either of (a) and (b) above
4. None of the above

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70. The hysteresis loss in a D.C. machine least depends on

1. Frequency of magnetic reversals
2. Maximum value of flux density
3. Volume and grade of iron
4. Rate of flow of ventilating air

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