DC Motors MCQs
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Which of the following statements is incorrect? If a starter is not used with large dc motor, it will draw a starting current which
- A. is many times its full-load current
- B. will produce excessive line voltage drop
- C. will damage the commutator
- D. will produce very low starting torque
- Correct Answer: Option D
The main function of a commutator in a dc motor is to
- A. prevent sparking
- B. convert ac to dc
- C. reduce friction
- D. convert dc to ac
- Correct Answer: Option D
A 230 V dc series motor is connected to 230 V ac. It will
- A. run slowly
- B. not run at all
- C. run with less efficiency
- D. run faster than its rated speed
- Correct Answer: Option C
If the flux of a dc motor approaches zero, its speed will approach
- A. infinity
- B. zero
- C. a stable value nearer to the rated speed
- D. unity
- Correct Answer: Option A
It is preferable to start a dc series motor with some mechanical load on it because
- A. it will not run at no load
- B. it acts as a starter to the motor
- C. it may develop excessive speed and damage itself
- D. none of these
- Correct Answer: Option C
The rated speed of a given dc shunt motor is 1050 rpm. To run this machine at 1200 rpm, which of the following speed control scheme will be used?
- A. armature current resistance control
- B. Field resistance control
- C. Ward-Leonard control
- D. none of these
- Correct Answer: Option B
Which of the following methods is most economical for finding the no-load losses of a large dc shunt motor?
- A. Hopkinson's test
- B. Swinburne's test
- C. Retardation test
- D. none of these
- Correct Answer: Option B
Interpoles are meant for
- A. increasing the speed of the motor
- B. decreasing the counter emf
- C. reducing sparking at the commutator
- D. converting armature current to dc
- Correct Answer: Option C
Why is the air gap between stator and armature of an electric motor kept as small as possible?
- A. to get stronger magnetic field
- B. to make rotation easier
- C. to reach a higher speed of rotation
- D. to improve the air circulation
- Correct Answer: Option A
Which of the following dc motors is the typical field of application mentioned?
- A. shunt motor : electric trains
- B. series motor : machine tools
- C. series motor : belt drive
- D. compound motor : fly wheel drive
- Correct Answer: Option D