DC Motor MCQ Questions

1. For the same power output rating and full-load speed, the motor that will have poorest starting torque, is

(a) cumulative compound.
(b) differential compound.
(c) series.
(d) shunt.

2. A motor used for punching machine is usually subjected to

(a) full load continuously.
(b) part of full load continuously.
(c) no load.
(d) intermittent load.

3. For continuously running rolling mills with intermittent loading, the most suitable dc drive is

(a) dc series motor.
(h) dc shunt motor.
(c) dc differentially compounded motor.
(d) dc cumulatively compounded motor.

4. The speed of a dc motor depends upon

(a) flux per pole.
(b) applied voltage to armature.
(c) voltage drop in armature circuit resistance.
(d) all of the above.

5. The speed of a dc 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 : Codes :
(a) 1, 2, and 3.
(b) 1 and 2.
(c) 1 and 3.
(d) 2 and 3.

6. Consider the following statements: The speed of a dc motor can be controlled by the variation of

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

Which of the statements given above are correct?
(a) 1, 2, 3.
(b) 2, 3, 4.
(c) 1, 3, 4.
(d) 1, 2, 4.

7. The field flux method of speed control of dc shunt motor is generally used for which one of the following?

(a) Constant hp drive below the base speed.
(b) Constant torque drive above the base speed.
(c) Constant torque drive below the base speed.
(d) Constant hp drive above the base speed.

8. Following controls are considered for dc motors

1. Control flux.
2. Armature resistance control.
3. Supply voltage control.

Which of the above controls play significant role in the speed control of dc motors?
(a) I only.
(b) II and III only.
(c) I and III only.
(d) I and II.

9. The resistance of the field regulator of a dc shunt motor is of the order of

(a) 100 Ω
(b) 10 Ω
(c) 2 Ω
(d) 0.5 Ω

10. A dc shunt motor is fitted with a field regulator for speed control. For a constant load torque the speed will be minimum for a regulator resistance of

(a) infinite.
(b) zero.
(c) none of these.

11. The speed of a dc shunt motor can be raised above its rated value

(a) inserting an additional resistance in the armature circuit.
(b) inserting an additional resistance in the field circuit.
(c) reducing the applied voltage.
(d) using either of the above methods.

12. Which of the following statements associated with field control of do shunt motors is correct?

(a) Speed can be varied below and above the rated speed to any extent.
(b) Speed can be varied above to any extent and below to a certain extent.
(c) Speed can be varied above to the extent of satisfactory commutation and below no variation.
(d) Speed can be varied above to the extent of satisfactory commutation and below to any extent.

13. A dc shunt motor is supplying a load at its rated speed. The speed can be reduced by

(a) armature circuit resistance control.
(b) field resistance control.
(c) Ward Leonard control.
(d) any of the above.

14. The speed of a dc shunt motor supplying a load at its rated speed can be reduced by

(a) connecting an additional resistance in the armature circuit.
(b) connecting an additional resistance in the field circuit.
(c) increasing the applied voltage.
(d) any of the above methods.

15. The most inefficient method for speed control of dc motors is

(a) voltage control.
(b) field control.
(c) armature control.

16. A dc shunt motor is operating at no load. If the armature is shunted by a resistance, its speed will

(a) increase.
(b) decrease.
(c) remain the same.
(d) be unpredictable.

17. The shunted armature control method of speed control for dc shunt motors is

(a) less expensive.
(b) less wasteful.
(c) suitable for rapidly changing loads.
(d) none of the above.

18. The armature voltage control is considered suitable for a dc motor driven at constant

(a) torque.
(b) speed.
(c) power.
(d) current.

19. If the voltage applied across the armature of a dc motor is reduced by 5%, keeping the load current and field current unchanged, the speed of the motor will

(a) increase by about 5%.
(b) reduce by about 5%.
(c) remain unaltered.
(d) unpredictable.

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

(a) Speed will decrease and the torque will decrease.
(b) Speed will increase and the torque will increase.
(c) Speed will increase and the torque will decrease.
(d) Speed will decrease and the torque will increase.

21. A 50 kW dc 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, the respective output powers delivered by the motor are approximately

(a) 25 kW in (i) and 75 kW in (ii).
(b) 25 kW in (i) and 50 kW in (ii).
(c) 50 kW in (i) and 75 kW in (ii).
(d) 50 kW in (i) and 50 kW in (ii).

22. In a Ward-Leonard drive, if the field current of the dc generator is suddenly reduced, the controlled dc motor operates

(a) still in the motoring mode.
(b) in dynamic braking mode.
(c) in counter-current braking mode.
(d) in regenerative braking mode.

23. Ward-Leonard system for controlling the speed of a dc motor is used where

(a) the speed in both the directions is required.
(b) very fine speed control over the whole range from zero to normal speed is required.
(c) the accuracy in speed control is required.
(d) all of the above.

24. Variation in speed by Ward Leonard control of dc motors occurs due to variation in

(a) field excitation.
(b) armature current.
(c) armature voltage.
(d) supply voltage.

25. Ward Leonard control is basically a …………. control method.

(a) armature voltage
(b) field
(c) field divertor
(d) armature resistance

26. Ward-Leonard control provides speed control

(a) in one direction only.
(b) in both directions.
(c) above rated speed.
(d) below rated speed.

27. For very sensitive and wide speed control, the preferable control method is

(a) field control.
(b) armature control.
(c) multiple voltage control.
(d) Ward Leonard system.

28. In Ward-Leonard system, the lower limit of speed is imposed by

(a) armature resistance.
(b) field resistance.
(c) residual magnetism of the generator.
(d) none of the above.

29. In Ward Leonard method of speed control, the direction of rotation of the motor is reversed usually by reversing the connections of the

(a) generator field terminals.
(b) generator armature terminals.
(c) motor armature terminals.
(d) motor field terminals.

30. The main disadvantage(s) of the Ward-Leonard system is/are

(a) high initial cost.
(b) poor overall efficiency.
(c) high maintenance cost.
(d) all of the above.

 

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