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Multiple Choice Question (MCQ) of D.C. Machines page-10

91. If the load on a dc shunt motor is increased, its speed decreases primarily due to

a) Increase in its flux
b) Decrease in back emf
c) Increase in armature current
d) Decrease in brush drop

Ans: (b)

92. If the load current and flux of a dc motor are held constant and voltage applied across are held constant and voltage applied across its armature is increased by 10 per cent, its speed will

a) Decrease by about 10 per cent
b) Remain unchanged
c) Increase by about 10 per cent
d) Increase by 20 per cent

Ans: (c)

93. If the field circuit of a loaded shunt motor s suddenly opened

a) It slow down
b) It would draw extremely high armature current
c) Its speed becomes dangerously high
d) Torque developed by the motor would be reduced to zero

Ans: (c)

94. If the pole flux of a dc motor approaches zero, its speed will

a) Approach zero
b) Approach infinity
c) Not change
d) Approach a stable value between zero and infinity

Ans: (b)

95. The speed of a dc shunt motor is required be more than full load speed. This may be achieved by

a) Increasing the armature current
b) Decreasing the armature current
c) Increasing the excitation current
d) Reducing the field current

Ans: (d)

96. If the speed of a dc shunt motor increases, the back emf

a) Increases
b) Decreases
c) Remains unchanged
d) Fist increases and then decreases

Ans: (a)

97.For a dc motor operating under condition of maximum transfer of power, the efficiency of the motor is

a) 100%
b) About 90%
c) 75%
d) Less than 50%

Ans: (d)

98. DC motors are considered most suitable for the applications in

a) Fans
b) Water pumps
c) Traction
d) flour mills

Ans: (c)

99. Which of the following dc motors will have least percentage increase in input current for a given percentage increase in torque?

a) Series motor
b) Shunt motor
c) Separately excited
d) Cumulatively compound motor

Ans: (a)

100. Which dc motor will have highest percentage increase in input current for a given percentage increase in torque?

a) Series motor
b) Shunt motor
c) Cumulatively compound motor
d) Separately excited motor

Ans: (b)

101. In a dc series motor, the shaft torque is less than the armature torque due to

a) Eddy current losses
b) Stray losses
c) Hysteresis losses
d) Copper losses

Ans: (b)

102. If the load current and flux of dc motor are kept construe while the voltage applied across the armature is increased by 4 percent, the aped of the motor will

a) Remain unchanged
b) Decrease by 4%
c) Increase by 4%
d) Increase by 16%

Ans: (c)

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