DC Motor MCQ Question and Answer

1. If the handle of a three point starter goes back to OFF position, it may be due to

(a) zero or very low supply voltage.
(b) overload.
(c) high field circuit resistance.
(d) all of the above.

2. A 3 point starter is considered suitable for ………… dc motors.

(a) series
(b) shunt
(c) compound
(d) shunt as well as compound
(e) series as well as compound

3. If the no-voltage release in a dc motor starter fails to operate on failure of supply, on resumption of supply the motor will

(a) not start automatically.
(b) start automatically without trouble.
(c) develop very low starting torque.
(d) get damaged due to flow of excessive current in the armature.

4. The dc motor starter handle should be moved

(a) very slowly in steps.
(b) smoothly in steps.
(c) quickly.
(d) with a jerk to final position.

5. DC motors are stopped by opening the line switches and not by forcing the handle of the motor starter back to the OFF position. This is due to occurrence of

(a) heavy sparking at the first stud of the starting resistance steps.
(b) heavy sparking at the brushes.
(c) both (a) and (b).
(d) none of these.

6. A 4-point starter is used to start and control the speed of a

(a) dc shunt motor with armature resistance control.
(b) dc shunt motor with field weakening control.
(c) dc series motor.
(d) dc compound motor.

6. In a dc compound motor, 4-point starter is employed

(a) to reduce the field current.
(b) to increase the field current.
(c) not to affect the current flowing through ‘Hold On’ coil even when the field current changes.
(d) none of the above.

8. In a dc compound motor, 4-point starter provides

(a) overload protection.
(b) under-voltage protection.
(c) the facility for wide range of speed control by field regulator.
(d) all of the above.

9. A large dc shunt motor can be started preferably using a

(a) compensator.
(b) strong field.
(c) weak field.
(d) none of these.

10. The simplest form of a motor controller is

(a) relay.
(b) toggle switch.
(c) drum switch.
(d) magnetic switch.

11. Field copper losses are constant in ……….. dc motors.

(a) series
(b) shunt
(c) compound
(d) none

12. The variable loss in a dc shunt machine is

(a) iron loss.
(b) shunt field loss.
(c) armature copper loss.
(d) friction and windage loss.

13. Two dc series motors are mechanically coupled. One is operating as a generator and the other as a motor. Which of the following statements are correct ?

(a) The friction losses of the two machines will be the same when both are running at the same speed.
(b) The iron losses of the two machines will be identical when their excitations are identical.
(c) The mechanical losses of the two machines will be identical for identical speeds and excitation of the two machines.
(d) All of the above.

14. Consider the following statements: In a dc machine, iron loss occurs in

1. armature core.
2. yoke.
3. pole cores.
4. pole shoes.

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

15. Two series motors are mechanically coupled. One machine is running as a motor and the other as a generator. The iron and friction losses of the machines will be identical when

(a) their speeds are identical.
(b) their speeds and excitations are identical.
(c) their speeds are equal but back emfs are half the supply voltages.
(d) their ratings and armature sizes are identical.

16. In a dc machine mechanical losses occur due to

(a) air resistance of rotation to armature and fan.
(b) brush friction.
(c) bearing friction.
(d) all of the above.

17. In a dc machine, mechanical losses are primarily function of

(a) voltage.
(b) current.
(c) speed.
(d) none of these.

18. The loss in a dc machine, which has the least proportion is

(a) mechanical loss.
(b) iron loss.
(c) armature copper loss.
(d) field copper loss.

19. The hysteresis loss in a dc machine varies with the frequency of magnetic reversals as

(a) f²
(b) f
(c) 1/f
(d) 1/f²

20. Hysteresis loss in a dc machine is independent of

(a) volume and grade of iron.
(b) frequency of magnetic reversals.
(c) rate of flow of air.
(d) maximum value of flux density.

21. In a dc shunt machine, hysteresis loss

(a) is independent of lamination thickness.
(b) varies as the supply frequency.
(c) varies as 1.6 power flux density.
(d) varies as the volume of the iron core.
(e) all of the above.

22. The eddy current loss in a dc machine varies with the maximum flux density as

(a) (B_max)²
(b) B_max
(c) (B_max)^1.6
(d) (B_max)^3/2

23. In a dc machine, which loss increases rapidly with the increase in frequency of magnetic reversals ?

(a) Copper loss.
(b) Hysteresis loss.
(c) Eddy current loss.
(d) Mechanical loss.

24. The eddy current loss in a dc machine varies with the thickness of laminations as

(a) t²
(b) t
(c) 1/t
(d) 1/t²

25. The core (or iron) losses, in a dc machine occur due to

(a) eddy current loss only.
(b) hysteresis loss only.
(c) both hysteresis and eddy current losses.
(d) air friction only.

26. Core loss in a dc machine occurs in

(a) armature only.
(b) pole shoe only.
(c) yoke only.
(d) in all the above three.

27. In a dc machine iron losses are independent of variations in

(a) voltage.
(b) speed.
(c) load.
(d) both
(a) and (b).

28. In a dc machine, iron losses result in

(a) excessive heating of core.
(b) loss of efficiency.
(c) rise in temperature of ventilating air.
(d) all of the above.

29. In a dc machine, ohmic or copper losses occur in the

(a) armature winding only.
(b) field winding only.
(c) brush contact only.
(d) interpole winding only.
(e) all of the above.

30. In a dc machine, armature copper loss varies as

(a) load current.
(b) (load current)².
(c) 1/load current.
(d) 1/(load current)².

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