DC Motor Objective Type Question Answer

1. Which of the following losses occurring in a dc machine has the largest percentage ?

(a) Iron loss.
(b) Windage loss.
(c) Copper loss.
(d) Iron loss.

2. Which of the following losses occurring in a dc machine varies significantly with the load ?

(a) Armature copper loss.
(b) Shunt field copper loss.
(c) Magnetic or iron losses.
(d) Mechanical losses.

3. The armature core of a dc machine gets overheated due to

(a) copper loss occurring in armature winding only.
(b) eddy current loss only.
(c) hysteresis loss only.
(d) all of the above.

4. Efficiency of a dc shunt machine can be computed by knowing

(a) output and input.
(b) rating of machine and account of constant losses and armature resistance.
(c) either (a) or (b).

5. The overall efficiency of a well designed dc machine may be expected to be around

(a) 99%
(b) 92%
(c) 75%
(d) 50%

6. Which of the following motors is expected to have maximum full-load efficiency ?

(a) 1 kW
(b) 5 kW
(c) 30 kW
(d) 100 kW

7. The electrical efficiency of a dc machine is given as the ratio of

(a) VI_L and E_gI_a in case of generator.
(b) E_bl_a and VI_L in case of motor.
(c) output/input.
(d) output/output + total loss.
(e) both (a) and (b).

8. Efficiency of a dc machine is maximum when the variable losses are equal to

(a) constant losses.
(b) square of the constant losses.
(c) square root of the constant losses.
(d) none of the above.

9. The efficiency of a dc machine will be ……….. when the variable losses are equal to constant losses.

(a) 100%
(b) maximum
(c) minimum
(d) 50%

10. DC machines are generally designed for maximum efficiency around

(a) full load.
(b) 75%
(c) 50%
(d) 25%

11. In direct method of determination of efficiency of a dc machine

(a) copper losses, magnetic losses and mechanical losses are measured.
(b) the machine is actually loaded and input power and output power are measured.
(c) either (a) or (b).
(d) none of these.

12. A brake test is usually restricted to …….. dc motors.

(a) small kW output rating
(b) variable speed
(c) high speed
(d) constant speed
(e) open frame

13. Which of the following test(s) need(s) two machines ?

(a) Brake.
(b) Swinburne’s.
(c) Retardation.
(d) Hopkinson’ s.
(e) Field.
(f) both (d) and (e).

14. The efficiency of a dc machine can be determined without directly loading it with the following information(s).

(a) Machine rating.
(b) Armature and field resistances.
(c) No-load input of the machine.
(d) All of the above information.

15. Indirect method of testing is preferred because

(a) it enables the determination of losses without actually loading the machine. (b) power requirement is very small and so large machines can be tested economically and conveniently.
(c) it does not require any loading arrangement.
(d) the efficiency can be determined at any load.
(e) all of the above.

16. The efficiency of a dc series machine cannot be determined by Swinburne’s test as

(a) a series motor has very high starting torque.
(b) a series motor draws excessive current from the supply mains on no load.
(c) a series motor runs at dangerously high speed on no load.
(d) all of the above.

17. In Swinburne’s test of a dc machine

(a) no-load losses are calculated and copper losses are measured.
(b) no-load losses are measured and the copper losses are calculated.
(c) both the no-load losses and the copper losses are calculated.
(d) both the no-load losses and the copper losses are measured.

18. The test that can be used for determination of no-load losses in a large dc shunt machine is

(a) brake test.
(b) field test.
(c) Hopkinson’s test.
(d) Swinburne’s test.

19. In Swinburne’s test of determination of efficiency of a dc shunt machine the no load input power supplies

(a) armature and shunt field copper losses.
(b) iron losses.
(c) friction and windage losses.
(d) all of the above.

20. Performance of large dc machines regarding commutation and temperature rise etc. at full load can be checked by .……. test.

(a) brake
(b) Swinburne’s
(c) Hopkinson’s
(d) running down

21. In the Kapp’s modification of Hopkinson’s test for dc machines

(a) the power losses in the two machines are supplied electrically.
(b) the power losses in the two machines are supplied mechanically.
(c) the two machines are not coupled mechanically.
(d) none of the above.

22. The main disadvantage of Hopkinson’s test is that

(a) it needs large power.
(b) it needs two identical machines.
(c) no account is taken for change in iron losses due to flux distortion on full load conditions.
(d) all of the above.

23. In Hopkinson’s efficiency test for dc machines

(a) both the machines are mechanically coupled.
(b) both the machines are operated at full load.
(c) one machine acts as a generator and another as a motor.
(d) the armature current of the machine operating as a motor is more than that of the machine operating as a generator.
(e) the field current of the machine operating as a generator is more than that of the machine operating as a motor.
(f) all of the above.

24. Two dc shunt machines 200 kW each are tested by Hopkinson’s test. The power input would be of the order of

(a) 200 kW
(b) 100 kW
(c) 35 kW
(d) 10 kW

25. The retardation test for dc shunt machines is employed for determination of

(a) stray losses.
(b) windage and friction losses.
(c) magnetic or core losses.
(d) copper losses.

26. In a field’s test for dc series machines, one of the two machines under test operates as a motor and drives the other which operates as a ………… generator.

(a) series
(b) shunt
(c) separately excited
(d) cumulatively compounded
(e) differentially compounded

27. In a field’s test for dc series machines, the series fields of two machines are connected in series so as to make ………. losses of both machines equal.

(a) copper
(b) iron
(c) friction and windage
(d) total

28. In the field’s test for dc series machines, the difficulty of obtaining readings at relatively light loads is overcome by connecting

(a) the generator series field in series with the motor armature.
(b) the motor series field in series with the generator armature.
(c) the generator armature in series with the motor.
(d) the motor armature in series with the generator.

29. In Hopkinson’s and field’s tests, the main common thing is that

(a) both tests are regenerative.
(b) both tests use two identical mechanically coupled dc machines.
(c) the power drawn in both the tests is negligible to meet the internal losses only. (d) all of the above.

30. Which of the following tests can be conducted on other than shunt machines ?

(a) Swinburne’s test.
(b) Retardation test.
(c) Field’s test.
(d) Back-to-back test.

31. In case of a cross-field machine, the amplification ratio may be as high as

(a) 10,000:1
(b) 2,000:1
(c) 500:1
(d) 100:1

32. The application(s) of cross-field machines is/are

(a) in automobiles and train services.
(b) in control systems.
(c) both (a) and (b).
(d) none of these.

33. In cross-field generators

(a) the reversal of speed does not reverse the polarity of the output.
(b) compensating winding is provided in order to increase the amplification factor.
(c) the number of brush studs is double of that of poles.
(d) all of the above.

34. The number of interpoles in a 2-pole cross-field generator will be

(a) 2
(b) 4
(c) 8
(d) none of these

35. In Rosenberg generators

(a) use of armature reaction is made.
(b) the field coils are shunt connected.
(c) the direction of load current is independent of direction of rotation and remains almost constant between wider limits of speed.
(d) all of the above.

36. Which of the following statement(s) associated with a Rosenberg generator is/are incorrect ?

(a) It is an armature excited high speed machine.
(b) It provides constant current over wider limits of speed.
(c) It is a constant voltage generator at low speeds.
(d) It is essentially a 2-pole machine with two sets of brushes arranged on the axis at right angles to each other.
(e) Its voltage polarity is independent of direction of rotation.

37. In an amplidyne, compensating winding is provided in order to

(a) neutralize the armature reaction setup by the load current.
(b) regulate the load voltage.
(c) reduce sparking at the commutator.
(d) none of the above.

38. An amplidyne is provided with split poles so as to

(a) increase amplification factor.
(b) provide space for interpoles.
(c) increase efficiency.
(d) dampout mechanical oscillations.

39. Which of the following statements associated with Amplidyne and Metadyne are correct ?

(a) These are cross-field machines.
(b) Amplidyne is fully compensated machine and Metadyne is under compensated machine.
(c) Commutating poles are also provided in these machines.
(d) Compensating winding is connected in series with the load output brush terminals.
(e) All of the above.

40. The inductor of a three-wire generator should be iron-cored so as to

(a) increase the current through it.
(b) reduce the voltage ripples in the output circuit.
(c) reduce the current through it.
(d) improve the voltage regulation of the machine.

41. Which of the following statements associated with a 3-wire generator are correct?

(a) It is an ordinary dc generator with a special provision for connecting the neutral wire to the armature winding.
(b) It is somewhat more expensive than the standard one.
(c) For operating 3-wire compound generators in parallel it is necessary that two equalizer connections are used to connect all series fields in parallel.
(d) There are two series fields on each 3-wire compound generator.
(e) All of the above.

42. The proper alignment of a machine is essential otherwise

(a) excessive wear and tear will take place on the bearings.
(b) the load current will become excessively high.
(c) the speed of the motor may become abnormal.
(d) the motor may not operate at all.

43. Before connecting the motor to the supply mains it is necessary to ensure that

(a) there is a proper insulation between field winding, armature winding and earth.
(b) protective devices have been adjusted at their proper settings.
(c) all the necessary connections show continuity.
(d) supply voltage is appropriate.
(e) all of the above.

44. For preventive maintenance of a dc machine it is to be ensured that

(a) insulation resistances of the field and armature windings are proper.
(b) commutator and brushes are in good condition.
(c) bearings are properly greased.
(d) motor and its control gear arc free from oil and grease.
(e) all of the above.

45. Which of the following statement(s) associated with installation and preventive maintenance of dc machines is incorrect ?

(a) Accumulation of carbon dust on the commutator surface may result in flash-over.
(b) The commutator should be lubricated in order to reduce brush friction.
(c) Accumulation of dust on the machine may result in overheating of the machine.
(d) Sufficient space is necessary all around the motor for movement of operational and maintenance personals.
(e) Resistance of earth continuity conductor is to be checked periodically in order to ensure the effectiveness of the earthing system.

46. Which of the following statements associated with the installation and preventive maintenance of dc machines is correct ?

(a) The commutator should be regularly lubricated in order to reduce brush friction.
(b) Accumulation of dust on the machine may cause electrical failure of the machine.
(c) Periodic greasing increases the life of the bearings and also provides trouble free service.
(d) None of the above.

47. Growler is used to detect

(a) condition of insulation used.
(b) working condition of bearing.
(c) working condition of bearing.
(d) none of the above.

48. The sparking at brushes may occur due to

(a) trouble in brushes.
(b) trouble in commutator.
(c) trouble in armature.
(d) excessive load.
(e) any of the above causes.

 

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