DC Motor MCQ Question Answer

1. In a dc motor; if armature current is increased, keeping field current constant, the torque developed will

(a) remain unchanged.
(b) decrease in inverse proportion.
(c) increase proportionately.
(d) increase in proportion to the square of the armature current.

2. If the supply voltage to a dc shunt motor is increased by 15%, which of the following will reduce ?

(a) Full-load current.
(b) Full-load speed.
(c) Starting torque.
(d) None of the above.

3. A dc shunt motor is running on no load at 600 rpm. If the motor is run, on no load, continuously for 12 hours, the speed of the motor after 12 hour run will

(a) be 600 rpm.
(b) exceed 600 rpm.
(c) fall below 600 rpm.
(d) be 450 rpm.

4. A dc, shunt motor is driving a constant torque load with normal excitation. If the field current is halved then the motor will run at

(a) rated speed.
(b) half of rated speed.
(c) double of rated speed.
(d) slightly less than double the rated speed.

5. A dc shunt motor is driving a mechanical load at rated voltage with normal excitation. If the load torque is increased to double of the normal one, the speed of the motor will

(a) become half.
(b) become double.
(c) increase slightly.
(d) fall slightly.

6. DC shunt motors are used for driving

(a) machine tools.
(c) centrifugal pumps.
(e) blowers and fans.
(b) lathes.
(d) line shafts.
(f) all of these.

7. When a dc cumulative compound wound motor is loaded

(a) torque increases and speed decreases.
(b) torque decreases and speed increases.
(c) both speed and torque increase.
(d) both speed and torque decrease.

8. A cumulative compound long shunt motor is driving a load at rated torque and rated speed. If the series field is shunted by a resistance equal to the resistance of the series field, keeping the torque constant.

(a) the armature current increases.
(b) the motor speed increases.
(c) the armature current decreases.
(d) the motor speed decreases.
(e) both (a) and (b).

9. A belt-driven cumulative compounded dc generator is delivering power to the dc mains. If the belt snaps then the machine will run as a

(a) cumulative compounded motor in the same direction.
(b) differentially compounded motor in the same direction.
(c) cumulative compounded motor in the opposite direction.
(d) differentially compounded motor in the opposite direction.

10. Flywheel is employed with a dc compound motor to reduce peak demand. The compound motor should be

(a) level compound.
(b) differential compound.
(c) cumulative compound.
(d) none of these.

11. In a differential compounded dc motor, if the shunt field gets suddenly opened the motor will

(a) first stop and then run in opposite direction as a series motor.
(b) run as a series motor, but at slower speed in the same direction.
(c) run as a series motor in the same direction at high speed.
(d) not work and come to stand still.

12. A differentially compounded dc motor is running at rated speed. If the series field winding of the motor gets short circuited, the speed of the motor will

(a) remain unaltered.
(b) increase.
(c) decrease.
(d) be zero.

13. When the direction of power flow in a differentially compounded motor reverses, it will operate as a

(a) cumulatively compounded generator.
(b) differentially compounded generator.
(d) shunt motor.
(c) series motor.

14. A differentially compounded dc motor with interpoles and with brushes on the neutral axis is to be driven as a generator in the same direction with the same polarity of the terminal voltage. It will then

(a) be a cumulatively compounded generator but the interpole coil connections are to be reversed.
(b) be a cumulatively compounded generator without reversing the interpole coil connections.
(c) be a differentially compounded generator without reversing the interpole coil connections.
(d) be a differentially compounded generator but the interpole coil connections are to be reversed.

15. The dc motor most suitable for applications requiring high starting torque is

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

16. The dc motor that draws almost same power at different loads is

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

17. In a dc series motor, the torque developed is

(a) inversely proportional to armature current.
(b) directly proportional to armature current.
(c) proportional to the square of armature current.
(d) proportional to the square root of the armature current.

18. Torque developed by the armature of a dc motor is proportional to which one of the following?

(a) (Back emf) x (Armature current).
(b) (Magnetic flux per pole) x (Armature current).
(c) (Back emf) x (Magnetic flux per pole).
(d) (Back emf) / (Magnetic flux per pole).

19. A series motor drawing an armature current of I_a is operating under saturated magnetic conditions. The torque developed in the motor is proportional to

(a) 1/I_a
(b) 1/I²_a
(c) I²_a
(d) I_a

20. Which of the following dc motors will have least percentage increase of input current for the same percentage increase in torque?

(a) Cumulative compound.
(b) Differential compound.
(c) Shunt.
(d) Series.

21. Which of the following dc motors, on removal of load, will run at the maximum speed?

(a) Series.
(b) Shunt.
(c) Cumulative compound.
(d) Differential compound.

22. Which of the following dc motors has got maximum self relieving property?

(a) Shunt.
(b) Series.
(c) Cumulative compound.
(d) Differential compound.

23. The dc motor, that has poorest speed regulation, is

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

24. Which motor should not be used for centrifugal pumps?

(a) Shunt.
(b) Series.
(c) Cumulatively compound.
(d) Differentially compound.

25. The ratio of the starting torque to full-load torque is minimum in case of

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

26. Which of the following dc motors has almost constant speed over their full-load range?

(a) Shunt.
(b) Series.
(c) Cumulative compound.
(d) None of these.

27. Which of the following dc motors will have highest percentage increase of input current, for the same percentage increase in torque?

(a) Series.
(b) Shunt.
(c) Cumulative compound.
(d) All equal.

28. Which of the following dc motors has least drop in speed from no-load to rated full load?

(a) Series motors with interpoles.
(b) Series motors without interpoles.
(c) Shunt motors with interpoles.
(d) Cumulative compound motors with interpoles.

29. A dc motor is to drive a load which is almost nil for certain part of the load cycle and peak value for short duration. The most suitable dc motor is

(a) Series.
(b) Shunt.
(c) Differential compound.
(d) Cumulative compound.

30. On switching a dc motor to dc supply mains, it is found to rotate in the direction opposite to that for which it is designed. The dc motor is

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

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