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Sunday, 31 January 2016

D. C. Motors - Multiple Choice Questions

D. C. Motors - Multiple Choice Questions Points : direct current motor mcqs, d. c. motors multiple choice questions and answers, electrical objective type question answers, mcqs 1. When the speed of a d.c. motor increases, its armature current …………..
(i) increases
(ii) decreases
(iii) remains constantly
(iv) none of the above

2. The back e.m.f. in a d.c. motor ……………
(i) opposes the applied voltage
(ii) aids the applied voltage
(iii) aids the armature current
(iv) none of the above

3. A series motor will over speed when ……….
(i) the load is increased
(ii) the field is opened
(iii) the armature circuit is opened
(iv) load is removed

4. A d.c .motor is used to ……….
(i) generate power
(ii) change mechanical energy to electrical energy
(iii) change electrical energy to mechanical energy
(iv) increase energy put into it

5. The amount of back e.m.f. of a shunt motor will increase when …………..
(i) the load is increased
(ii) the field is weakened
(iii) the field is strengthened
(iv) none of the above

6. The field poles and armature of a d.c. machine are laminated to …………..
(i) reduce the weight of the machine
(ii) decrease the speed
(iii) reduce eddy currents
(iv) reduce armature reaction

7. Armature reaction in a d.c. motor is increased………..
(i) when the armature current increases
(ii) when the armature current decreases
(iii) when the field current increases
(iv) by interpoles

8. A d.c. motor is still used in industrial applications because it ……….
(i) is cheap
(ii) is simple in construction
(iii) provides fine speed control
(iv) none of the above

9. Carbon brushes are preferable to copper brushes because …………
(i) they have longer life
(ii) they reduce armature reaction
(iii) they have lower resistance
(iv) they reduce sparking

10. ………… motor is variables speed motor.
(i) series
(ii) shunt
(iii) cumulatively compounded
(iv) differentially compounded

11. The speed of a d.c. motor is ………………
(i) directly proportional to flux per pole
(ii) inversely proportional to flux per pole
(iii) inversely proportional to applied voltage
(iv) none of the above

12. The deciding factor in the selection of a d.c. motor for a particular application is its ……….. characteristic.
(i) speed-torque
(ii) speed-armature current
(iii) torque-armature current
(iv) none of the above

13. In very large d.c. motors with severe heavy duty, armature reaction effects are corrected by …………..
(i) using interpoles only
(ii) using compensatory windings in addition to interpotes
(iii) shifting the brush position
(iv) none of the above

14. The torque developed by a d.c. motor is directly proportional to ……….
(i) flux per pole x armature current
(ii) armature resistance x applied voltage
(iii) armature resistance x armature current
(iv) none of the above

15 ......... motor has the best speed regulation.
(i) series
(ii) cumulatively compounded
(iii) shunt
(iv) differentially compounded

16. After a shunt motor is up to speed, the speed maybe increased considerably by ……………..
(i) increasing field circuit resistance
(ii) decreasing field circuit resistance
(iii) increasing armature circuit resistance
(iv) reducing the load

17. With respect to the direction of rotation, interpoles on a d.c., motor must have the same polarity as the main poles ……………
(i) ahead of them
(ii) behind them
(iii) none of the above

18. The most commonly used method of speed control of a d.c. motor is by varying …………….
(i) voltage applied to the motor
(ii) field strength
(iii) effective number of conductors in series
(iv) armature circuit resistance

19. A cumulatively compounded motor does not run at dangerous speed at light Loads because of the presence of ………….. (i) shunt winding
(ii) series winding
(iii) interpoles
(iv) compensating windings

20. The speed of a …………… motor is practically constant.
(i) cumulatively compounded
(ii) series
(iii) differentially compounded
(iv) shunt

21. The demand for a large increase in torque of a d.c. series motor is met by a …………..
(i) large decrease in current
(ii) large decrease in speed
(iii) large increase in speed
(iv) small decrease in speed

22. In a d.c..moor, the brushes are shifted from the mechanical neutral plane in a direction opposite to the rotation to
(i) decrease speed
(ii) increase speed
(iii) reduce sparking
(iv) produce flat characteristics

23. The running speed of a d.c. series motor is basically determined by ……………
(i) field excitation
(ii) load
(iii) armature resistance
(iv) none of the above

24. The demand for a large increase in torque of a d.c. shunt motor is met by a ………….
(i) large decrease in speed
(ii) large increase in speed
(iii) large increase in current
(iv) small increase in current

25. As the load increases, a ………….. motor will speed up.
(i) series
(ii) shunt
(iii) cumulatively compounded
(iv) differentially compounded

26. …………… motor is most suitable for punch presses.
(i) shunt
(ii) series
(iii) differentially compounded
(iv) cumulatively compounded

27. D. C. shunt motor are used in those applications where ……………. is required.
(i) high starting torque
(ii) practically constant speed
(iii) high no-load speed
(iv) variable speed

28. For the same rating ………….. motor has the highest starting torque.
(i) shunt
(ii) cumulatively compounded
(iii) differentially compounded
(iv) series

29. When load is removed, the motor that will run at the highest speed is the ………….. motor.
(i) shunt
(ii) series
(iii) cumulatively compounded
(iv) differentially compounded

30. D. C. series motors are used in those applications where ………….. is required.
(i) high starting torque
(ii) constant speed
(iii) Low no-load speed
(iv) none of the above

31. Cumulatively compounded motors are used where …………. we require
(i) variable speed
(ii) poor speed regulation
(iii) sudden heavy Loads for short duration
(iv) none of the above

32. ………….. motors never use belt-connected Loads.
(i) series
(ii) shunt
(iii) cumulatively compounded
(iv) differentially compounded

33. The iron losses in a d.c. motor depend upon ………………
(i) flux only
(ii) speed only
(iii) both flux and speed
(iv) none of the above

34. For the same rating, ………….. motor has the least starting torque.
(i) cumulatively compounded
(ii) series
(iii) shunt

35. The most suitable motor for elevators is the ……………. motor.
(i) series
(ii) shunt
(iii) differentially compounded
(iv) cumulatively compounded

36. For 20% increase in current, the motor that will give the greatest increase in torque is …………. motor.
(i) shunt
(ii) series
(iii) cumulatively compounded
(iv) differentially compounded

37. The friction and windage losses in a d.c. motor depends upon ……………
(i) speed
(ii) flux
(iii) armature current
(iv) field and armature resistance

38. Excessive sparking at the brushes may be caused due to ……………..
(i) dirt on the commutator
(ii) loose coupling
(iii) misalignment of machine
(iv) worn bearings

39. Hot bearings of a d.c. motor may be caused by ……………..
(i) poor ventilation
(ii) incorrect voltage
(iii) loose coupling
(iv) lack of or dirty lubricant

40. A d.c. series motor is most suitable for …………..
(i) cranes
(ii) pump
(iii) lathes
(iv) punch presses

41. The ……….. motor is used to start heavy loads.
(i) series
(ii) shunt
(iii) differentially compounded
(iv) none of the above

42. Intermittent sparking at the brushes of a d.c. motor may be caused due to …………….
(i) an open armature coil
(ii) intermittent load
(iii) loose coupling
(iv) incorrect voltage

43. In a vacuum cleaner, we generally use ………….. motor.
(i) shunt
(ii) cumulatively compounded
(iii) series
(iv) differentially compounded

44. The greatest percentage of power loss in a d.c. motor is due to ……………..
(i) windage loss
(ii) copper loss
(iii) core loss
(iv) friction loss

45. Excessive motor vibration is caused by ………………..
(i) too much brush tension
(ii) worn bearings
(iii) open armature coil
(iv) bent shaft

46. Overheating of a d.c. motor is often due to ………………
(i) insufficient end play
(ii) Loose parts
(iii) overloads
(iv) rough commutator

47. A d.c. motor runs at 1725 r.p.m. at full-load and 1775 r.p.m. at no load. The speed regulation is ………..
(i) 47%
(ii) 2.9%
(iii) 76%
(iv) 15%

48. The resistance of shunt field winding as compared to series field winding is
(i) less
(ii) more
(iii) the same
(iv) none of the above

49. The commutating winding is basically used in a d.c. machine
(i) to reduce armature reaction
(ii) to improve commutation
(iii) to control the speed of machine
(iv) none of the above

50. The commutating-pole winding is connected in
(i) series with the armature
(ii) parallel with the armature
(iii) series-parallel with the armature
(iv) none of the above

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