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Electrical DC Motor MCQs

Read Out “Electrical DC Motors” topic to better understand all those MCQs

1) Brushes of DC Motors are made of

  1. carbon
  2. soft carbon
  3. hard carbon
  4. aluminium
ANSWER
EXPLANATION

2) Armature reaction of an unsaturated DC machine is

  1. crossmagnetising
  2. demagnetizing
  3. magnetizing
  4. none of the above
ANSWER
EXPLANATION

3) Eddy current are induced in the pole shoes of a DC machine due to

  1. oscillating magnetic field
  2. pulsating magnetic flux
  3. relative rotation between field and armature
  4. none of the above
ANSWER
EXPLANATION

4) In a DC machine, short circuited field coil will result in

  1. odour of baring insulation
  2. unbalanced magnetic pull producing vibrations
  3. reduction of generated voltage for which excitation has to be increased to maintain the voltage
  4. all of the above
ANSWER
EXPLANATION

5) In a DC machine, fractional pitch winding is used

  1. to increase the generated voltage
  2. to reduce sparking
  3. to save copper because of short end connection
  4. due to both 2 and 3
ANSWER
EXPLANATION

6) A DC motor is used in industrial application because it ………..

  1. is cheap
  2. is simple in construction
  3. provide fine speed control
  4. none of the above
ANSWER
EXPLANATION

7) The field poles and armature of a DC machine are laminated to ……………

  1. reduce the weight of the machine
  2. decrease the speed
  3. reduce eddy currents
  4. reduce armature reaction
ANSWER
EXPLANATION

8) The back emf in a DC motor ……….

  1. opposes the applied voltage
  2. aids the applied voltage
  3. aids the armature current
  4. none of the above
ANSWER
EXPLANATION

9) The value of back e.m.f in a DC motor is maximum at ………

  1. no load
  2. full load
  3. half load
  4. none of the above
ANSWER
EXPLANATION

10) The motor equation is given by

  1. V = Eb – IaRa
  2. V = Eb + IaRa
  3. Eb = IaRa – V
  4. none of the above
ANSWER
EXPLANATION

11) The fully loaded current of a 20 HP 500 V DC motor will be closer to

  1. 100 A
  2. 60 A
  3. 35 A
  4. 15 A
ANSWER
EXPLANATION

12) The speed of a DC motor can be varied by varying

  1. field current
  2. applied voltage
  3. resistance in series with armature
  4. any of the above
ANSWER
EXPLANATION

13) In a DC motors, the conditions for maximum power is

  1. supply voltage = 1/2 × back e.m.f
  2. supply voltage = √2 × back e.m.f
  3. back e.m.f = 2 × supply voltage
  4. back e.m.f = 1/2 × supply voltage
ANSWER
EXPLANATION

14) If conditions for maximum power for a DC motor are established, the efficiency of the motor will be

  1. 100%
  2. 90 to 95%
  3. 81%
  4. less than 50%
ANSWER
EXPLANATION

15) When the speed of a DC motor increases, its armature current ………

  1. increases
  2. decreases
  3. remains constant
  4. none of the above
ANSWER
EXPLANATION

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