Multiple choice questions on dc motor # 4

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objectives of electrical engineering on dc motors 4


1) In a dc motor if the brushes are given a back shift, then
A) commutation is improved
B) commutation is worsened
C) speed increases
D) speed decreases
E) speed remains unaffected
F) both (A) and (C)

Ans:F

2) In comparison to shunt and compound motors, a dc series motor develops high torque at start. It is due to its comparatively
A) few series turns
B) stronger series field
C) larger armature current
D) smaller armature resistance

Ans:B

3) In a dc motor, the shaft torque is less than armature torque. This is due to
A) eddy current loss
B) hysteresis loss
C) stray loss
D) all of the above

Ans:C

4) For smaller values of armature current Ia, the torque developed by a dc series motor is proportional to
A) Ia
B) Ia^2
C) √Ia
D) 1/Ia

Ans:B

5) For higher values of armature current Ia, the torque developed by a dc series motor is proportional to
A) Ia
B) Ia^2
C) √Ia
D) 1/Ia

Ans:A

6) The dc motor, which can provide zero speed regulation at full load without any controller is
A) series
B) shunt
C) cumulatively compound
D) differential compound

Ans:B

7) A series motor drawing an armature current of Ia is operating under saturated magnetic conditions. The torque developed in the motor is proportional to
A) 1/Ia
B) 1/Ia^2
C) Ia^2
D) Ia

Ans:D

8) If N is the speed of a dc series motor and Ia is the armature current then
A) N∝(1/Ia)
B) N∝Ia
C) N∝Ia^2
D) N∝(1/Ia^2)

Ans:A

9) For higher values of armature current, the speed of a dc series motor is
A) proportional to armature current
B) proportional to the square of armature current
C) inversely proportional to armature current
D) constant

Ans:D

10) A 240 V dc series motor takes 40 A when giving its rated output at 1500 rpm. Its resistance is 0.3 Ω. The value of resistance which must be added to obtain rated torque at 1000 rpm is
A) 6 Ω
B) 5.7 Ω
C) 2.2 Ω
D) 1.9 Ω

Ans:D

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