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