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MCQ of D.C motor page-17

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121. A brake test is usually restricted to ……. dc motors.

A) small kW output rating
B) variable speed
C) high speed
D) constant speed.

Ans : (A)

122. The core ( or iron) losses, in a dc machine, occur due to

A) air friction only
B) both hysteresis and eddy current losses
C) hysteresis loss only
D) eddy current loss only.

Ans : (B)

123. The efficiency of a dc shunt generator is maximum when the

A) armature copper losses equal constant losses
B) stray losses equal constant losses
C) field copper losses are equal to armature copper losses
D) magnetic losses are equal to mechanical losses.

Ans : (A)

124. The main disadvantage (s) of the Ward-Leonard system is/are

A) high initial cost
B) poor overall efficiency
C) high maintenance cost
D) all of the above.

Ans : (D)

125. The field diverter resistance for a dc series motor is of the order of

A) 0.1 Ω
B) 10 Ω
C) 50 Ω
D) 100 Ω.

Ans : (A)

126. If the handle of a three point starter goes back to OFF position, it may be due to

A) zero or very low supply voltage
B) over-load
C) high field circuit resistance
D) all of the above.

Ans : (D)

127. A dc motor is running with a certain load. The effect of adding an external resistance in the shunt field circuit is

A) to step the motor
B) to reduce the armature current of motor
C) to reduce the motor speed
D) to increase the motor speed.

Ans : (D)

128. The most in efficient method for speed control of dc motors is

A) voltage control
B) field control
C) armature control
D) none.

Ans : (C)

129. Core loss in a dc machine occurs in

A) armature only
B) pole shoe only
C) yoke only
D) in all the above three.

Ans : (A)

130. Which of the following losses occurring in a dc machine varies significantly with the load ?

A) mechanical losses
B) magnetic or iron losses
C) shunt field copper loss
D) armature copper loss.

Ans : (D)

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