DC Motor Objective Questions Answers

1. In an electric motor, the electromagnetic torque developed is

(a) in opposite direction to both load torque and frictional torque.
(b) in the same direction as the load torque but in opposite direction to frictional torque.
(c) in the same direction as the load torque and frictional torque.
(d) in the same direction as the frictional torque but in the opposite direction to the load torque.

2. The toque-speed characteristic of a dc shunt motor is

(a) a rectangular hyperbola.
(b) a drooping straight line.
(c) a parabola.
(d) none of the above.

3. The horse power obtained from the motor shaft is called the

(a) IHP.
(b) BHP.
(c) useful output.
(d) none of these.

4. In a dc motor, the shaft torque is less than armature torque. This is due to

(a) eddy current loss.
(b) hysteresis loss.
(c) stray loss.
(d) all of the above.

5. The mechanical output available at the dc motor shaft is equal to

(a) electrical power input to motor.
(b) electrical power input less iron and friction losses.
(c) electrical power input less copper losses.
(d) electrical power input less total losses (copper loss, iron loss, friction and windage losses).

6. In a dc motor if the brushes are given a backward shift, then

(a) commutation is improved.
(b) commutation is worsened.
(c) speed increases.
(d) speed decreases.
(e) speed remains unaffected.
(f)both (a) and (c).

7. T and P are the developed electromagnetic torque and the shaft power of a dc series motor. If losses are neglected, then for constant terminal voltage

(a) T α P
(b) T α 1/P
(c) T α √P
(d) T α P²

8. Which of the following statements associated with a dc series motor is correct?

(a) It has poor torque.
(b) It gives almost constant speed.
(c) Its field winding consists of few turns of thick wire.
(d) It may run away if its field gets opened.
(e) It can be started on no load.

9. In comparison to shunt and compound motors, a dc series motor develops high torque at start. It is due to its comparatively

(a) few series turns.
(b) stronger series field.
(c) larger armature current.
(d) smaller armature resistance.

10. A flywheel is normally fitted to

(a) dc series motor driving a constant torque load.
(b) separately excited dc motor driving pulsed torque load.
(c) cumulatively compound motor driving torque load.
(d) differentially compound motor driving pulsed torque load.

11. An unsaturated dc shunt motor drives a constant torque load at a speed at 1000 rpm taking an armature current of 20 A when 230 V is applied to the motor. If the applied voltage is reduced to 115 V, the motor runs at

(a) 500 rpm taking an armature current of 20 A.
(b) 1000 rpm taking an armature current of 20 A.
(c) 500 rpm taking an armature current of 40 A.
(d) 1000 rpm taking an armature current of 40 A.

12. An electric motor with “constant output power” will have a torque-speed characteristic in the form of a

(a) straight line through the origin.
(b) straight-line parallel to speed axis.
(c) circle about the origin.
(d) rectangular hyperbola.

13. T is the load torque of a dc series motor having linear magnetization and negligible armature resistance. Speed of the motor is

(a) inversely proportional to √T.
(b) directly proportional to √T.
(c) inversely proportional to T².
(d) directly proportional to T².

14. An electric train employing a dc series motor is running at a fixed speed, when a sudden slight drop in the mains voltage occurs. This would result in

(a) drop in speed and rise in current.
(b) rise in speed and drop in current.
(c) rise in speed and rise in current.
(d) drop in speed with current unaltered.

15. In a dc series motor, if armature current is reduced to 70% of its original value, the torque of the motor will be reduced by

(a) 49%
(b) 51%
(c) 50%
(d) 70%

16. When the torque of a dc series motor is tripled, the power approximately increases by

(a) 33%
(b) 50%
(c) 75%
(d) 150%

17. When an electric train is moving down a hill, the dc motor will operate as a dc

(a) series motor.
(b) series generator.
(c) shunt motor.
(d) shunt generator.

18. A dc series motor must not be started on no load because it would

(a) not develop starting torque.
(b) fail to start without load.
(c) run at dangerously high speed.
(d) draw more current without producing sufficient torque.

19. In case of a dc series motor it is possible to have finite no-load speed if a resistance is connected across its

(a) field terminals.
(b) armature terminals.
(c) field and armature together.
(d) it is always very high at no-load.

20. It is advisable to start a dc series motor with some load to

(a) limit the flux.
(b) limit the current.
(c) limit the speed.
(d) avoid sparking.
(e) avoid demagnetization of field system.

21. The speed of a 4-pole dc series motor at no load will be

(a) zero.
(b) 3000 rpm.
(c) infinite.
(d) 1500 rpm.
(e) depending upon the applied voltage.

22. A dc series motor is best suited for driving

(a) line shafting, blowers and fans.
(b) machine tools.
(c) shear and punches.
(d) cranes and hoists.
(e) none of these.

23. DC series motor is best suited for electric traction as

(a) it develops high starting torque.
(b) it runs at very high speed at light load.
(r) it is cheaper in cost.
(d) it consumes less power.

24. A dc series motor is running at rated speed without any additional resistance in series. If an additional resistance is placed in series, the speed of motor

(a) increases.
(b) decreases.
(c) remains unchanged.
(d) oscillates around the rated speed.

25. A dc series motor is used for an overhauling load. It can work stably if

(a) the armature is shunted by a resistor.
(b) the field winding is reversed.
(c) a resistor is put in series with the machine.
(d) a divertor is put across the field.

26. A dc shunt motor is driving a constant torque load. On inserting an additional resistance in the armature circuit, the speed of the motor will

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

27. A dc shunt motor may have rising speed-torque characteristic due to

(a) very high armature circuit resistance.
(b) very high field circuit resistance.
(c) very high demagnetizing armature reaction.
(d) very low demagnetizing armature reaction.

28. Consider the following statements: If the terminal voltage of a dc shunt motor is halved with the load torque varying as the square of speed, then

1. speed is halved.
2. speed remains unaltered.
3. armature current is doubled.
4. armature current remains unaltered.

Of these statements
(a) 1 and 3 are correct.
(b) 1 and 4 are correct.
(c) 2 and 3 are correct.
(d) 2 and 4 are correct.

29. If the speed of a dc motor increases with load torque, then it is a

(a) series motor.
(b) permanent magnet motor.
(c) differentially compounded motor.
(d) cumulatively compounded motor.

30. A separately excited dc generator is feeding a dc shunt motor. If the load torque on the motor is halved approximately

(a) armature current of both motor and generator are halved.
(b) armature current of motor is halved and that of the generator, is unaltered.
(c) armature current of generator is halved and that of motor is unaltered.
(d) armature current of both motor and generator are unaltered.

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