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Multiple Choice Question (MCQ) of D.C. Machines page-7

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61. If back emf of d.c. motor is doubled while its speed is also doubled, then torque developed by the machine will

a) Remain same
b) Become four times
c) Double
d) Becomes half

Ans: (a)

62. If field of d.c. shunt motor is opened, then

a) Current in the armature will increase
b) It will run at its normal speed
c) Speed of motor will be reduced
d) Speed of motor will be high

Ans: (d)

63. Which of the following motors has high starting torque?

a) D.C. shunt motor
b) Induction motor
c) D.C. series motor
d) A.C. series motor

Ans: (c)

64. What is the standard direction of a motor?

a) Clockwise
b) Anti-clockwise
c) None of these
d) Either (a) or (b)

Ans: (b)

65. Left hand rule is applicable to

a) Motor
b) Generator
c) Transformer
d) Mercury are rectifier

Ans: (a)

66. A dc shunt motor is driving a mechanical load at rated voltage and rated excitation. If the load torque becomes double, then the speed of the motor

a) Increases slightly
b) Decreases slightly
c) Becomes double
d) Becomes half

Ans: (b)

67. A dc series motor is running with a diverter connected across its field winding. If the diverter resistance is increased, then the speed of the motor

a) Decreases/increase
b) Increase
c) Remains unchanged
d) Becomes very high

Ans: (a)

68. A dc series motor is running at rated speed. If a resistance is placed in series, the speed of the motor

a) Increases
b) Decreases
c) Remains unchanged
d) Increases very much

Ans: (b)

69. In a dc shunt motor, field excitation is kept at maximum value during starting to

a) Increase acceleration time
b) Reduce armature heating
c) Prevent voltage dip in the supply mains
d) Decrease starting torque

Ans: (b)

70. If the field circuit of a dc shunt motor running at rated speed gets open circuited, then immediately thereafter the speed of the motor would tent to

a) Decrease
b) Increase
c) Remain unchanged
d) Increase excessively

Ans: (b)

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