DC Motor Questions and Answers

1. In the block diagram of a separately excited dc motor, how does the armature induced emf appear as?

(a) Positive feedback.
(b) Negative feedback.
(c) Disturbance input.
(d) Output.

2. The current drawn by a 220 V dc motor of armature resistance 0.5 Ω and back emf 200 V is

(a) 40 A
(b) 44 A
(c) 400 A
(d) 440 A

3. In a dc motor, energy conversion would not have been possible but for

(a) input energy from the supply.
(b) production of back emf in the armature.
(c) use of commutator.
(d) rotation of the machine.

4. With the increase in speed of a dc motor

(a) both back emf as well as line current increase.
(b) both back emf as well as line current fall.
(c) back emf increases but line current falls.
(d) back emf falls and line current increases.

5. If the back emf in a dc motor vanishes suddenly, the motor will

(a) start hunting
(b) burn.
(c) run at very high speed.
(d) run at very slow speed.

6. The output power of any electrical motor is taken from the

(a) armature.
(b) field.
(c) coupling mounted on the shaft.
(d) motor frame.

7. The armature shaft must be able to withstand

(a) any unbalanced magnetic pull on the armature core.
(b) twisting strains due to transmission of torque.
(c) bending moment due to weight of the armature and commutator.
(d) all of the above.

8. The output indicated on the name plate of any motor is always the

(a) gross power.
(b) power drawn in kVA.
(c) power drawn in kW.
(d) output power at the shaft.

9. A thicker wire is used in dc series motor field winding than that in a dc shunt motor

(a) to create more flux.
(b) to reduce resistance.
(c) to carry large load current.
(d) both (b) and (c).

10. The dc compound motors are generally

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

11. In a dc compound motor the field regulator is provided to

(a) control the flux.
(b) limit the armature current.
(c) demagnetize field partially.
(d) none of the above.

12. The direction of rotation of a de shunt motor can be reversed by interchanging

(a) the supply terminals.
(b) the field terminals only.
(c) the armature terminals only.
(d) either field or armature terminals.

13. When the supply terminals of a dc shunt motor are interchanged

(a) the motor will stop.
(b) the motor will run at its normal speed in the same direction as before.
(c) the direction of rotation will reverse.
(d) the motor will run much faster in the same direction.

14. The direction of rotation of a dc series motor can be reversed

(a) by interchanging supply terminals.
(b) by interchanging field terminals.
(c) either by interchanging supply terminals or by interchanging field terminals.
(d) by interchanging supply terminals as well as field terminals.

15. The direction of rotation of a dc compound motor can be reversed by interchanging ……… connections.

(a) armature
(b) series field
(c) shunt field
(d) armature and series field both.

16. The speed of a dc motor is

(a) directly proportional to back emf and flux.
(b) directly proportional to its back emf and inversely proportional to flux.
(c) inversely proportional to both back emf and flux.
(d) directly proportional to flux and inversely proportional to back emf.

17. If the load current and flux of a dc motor are held constant and voltage applied across it armature is increased by 5%, the speed of the motor will

(a) increase by 5%.
(b) reduce by 5%.
(c) remain un-alternated.
(d) depends on other factors.

18. If the flux of a dc motor approaches zero, its speed will

(a) approach infinity.
(b) approach zero.
(c) remain unchanged.
(d) between zero and infinity .

19. A series motor is working drawing a load current of 1 A from the lines. If now the load is reduced such that the current drawn is halved, the speed of the machine (neglecting the saturation and armature resistance) would be

(a) unchanged.
(b) reduced by 50%.
(c) reduced by 100%.
(d) increased by 100%.

20. A dc shunt motor runs at rated speed. If its field circuit gets open-circuited, then soon after this the motor speed would tend to

(a) decrease.
(b) remain unchanged.
(c) increase.
(d) fluctuate around its previous speed.

21. The speed of a dc shunt motor from no load to full load

(a) falls slightly.
(b) improves slightly.
(c) remains unchanged.
(d) falls rapidly.

22. A dc motor having full-load speed of 750 rpm and speed regulation of 10% will have no-load speed of

(a) 825 rpm.
(b) 675 rpm.
(c) 800 rpm.
(d) 700 rpm.

23. When is the mechanical power developed by a dc motor maximum?

(a) Back emf is equal to applied voltage.
(b) Back emf is equal to zero.
(c) Back emf is equal to half the applied voltage.
(d) None of the above.

24. If the applied voltage to a dc machine is 230V, then the back emf, for maximum power developed is

(a) 115 V
(b) 200 V
(c) 230 V
(d) 460V

25. The efficiency of a dc motor when developing maximum power will be about

(a) 100%
(b) 50%
(c) less than 50%
(d) more than 50%

26. A dc motor develops the maximum mechanical power when the armature current is such that

(a) the back emf is equal to 3/4th of the applied emf.
(b) the back emf is equal to one-half of the applied emf.
(c) the back emf is equal to 1/4th of the applied emf.
(d) the back emf is equal to 2/3rd of the applied emf.

27. The dc motor, which can provide zero speed regulation at full load without any controller is

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

28. A dc shunt motor having unsaturated magnetic circuit runs at 1,000 rpm with rated voltage. If the applied voltage is half of the rated voltage, the motor will run at

(a) 2000 rpm.
(c) 750 rpm.
(b) 1,000 rpm.
(d) 500 rpm.

29. Torque developed in a dc motor depends upon

(a) magnetic field.
(b) current flowing through the armature conductors.
(c) active length of the conductors.
(d) number of armature conductors.
(e) radius of armature .
(f) all of these factors.

30. The armature torque in a dc motor is a function of

1. Field flux
2. Armature current
3. Speed
4. Damping

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

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