DC Motor MCQ

31. For a constant supply voltage, what are the effects of inserting a series resistance in the field circuit of a. d.c. shunt motor, on its speed and torque?

  1. Speed will decrease and the torque will decrease
  2. Speed will increase and the torque will increase
  3. Speed will increase and the torque will decrease
  4. Speed will decrease and the torque will increase
Answer. c

32. A 50 kW d.c. shunt motor is loaded to draw rated armature current at any given speed. When driven

(i) at half the rated speed by armature voltage control and

(ii) at 1.5 times the rated speed by field control, what are the approximate output powers delivered by the motor?

  1. 25 kW in (i) and 75 kW in (ii)
  2. 25 kW in (i) and 50 kW in (ii)
  3. 50 kW in (i) and 75 kW in (ii)
  4. 50 kW in (i) and 50 kW in (ii)
Answer. b

33. A 400V D.C. shunt motor takes 5 A at no-load. Ra = 0.5 ohms, Rf = 200 ohms. What is the ratio of speed from full load to no-load, when the D.C. shunt motor takes 50 A on full load?

  1. 0.94
  2. 0.8
  3. 0.6
  4. 0.4
Answer. a

34. What is the increase in the torque expressed as percentage of initial torque, if the current drawn by a d.c. series motor is increased from 10 A to 12 A (Neglect saturation)?

  1. 25%
  2. 21%
  3. 44%
  4. 41%
Answer. c

35. Consider the following statements:

The speed of a d.c. motor can be controlled by the variation of

  1. armature voltage
  2. armature circuit resistance
  3. field current
  4. angle of brush shift

Which of these statements are correct?

  1. i, ii and iii
  2. ii, iii and iv
  3. i, iii and iv
  4. i, ii and iv
Answer. a

36. A d.c. shunt motor is excited from an alternating power frequency voltage source. Its brush-axis is rotated by an angle α from the geometrical neutral axis. The torque developed will be proportional to which one of the following?

  1. cos α
  2. sin α
  3. cos 2α
  4. tan α
Answer. a

37. The speed of a d.c. shunt motor may be varied by varying

  1. Field current
  2. Supply voltage
  3. Armature circuit

Select the correct answer using the codes given below

  1. i, ii and iii
  2. i and ii
  3. i and iii
  4. ii and iii
Answer. a

38. Armature torque of a d.c. motor is a function of which of the following factors?

  1. Speed
  2. Field Flux
  3. Armature Current
  4. Residual Magnetism

Select the correct answer using the codes given below:

  1. ii and iii
  2. i and iv
  3. iii and iv
  4. i and ii
Answer. a

39. The current drawn by a 120 V d.c. motor with back e.m.f. of 110 V and armature resistance of 0.4 ohm is

  1. 4 A
  2. 25 A
  3. 274 A
  4. 300 A
Answer. b

40. The direction of rotation of a d.c. series motor can be reversed

  1. By interchanging supply terminals
  2. By interchanging field terminals
  3. Either by interchanging supply terminals or by interchanging field terminals
  4. By interchanging supply terminals as well as field terminals
Answer. c
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