d c motors MCQs – Page 2

Practise focused d c motors questions with marked answers for exam preparation and subject revision.

d c motors Mcqs
21. The main disadvantage of the Ward-Leonard control method is______________?
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A. all of the above

B. high maintenance cost

C. low efficiency at Hght loads

D. high initial cost

22. If the field winding resistance of a DC shunt motor is increases, its speed will____________?
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A. increases

B. none of the above

C. the field winding resistance of a DC shunt motor is increases, its speed will

D. remains same

E. decreases

23. Which one of the following is not the function of pole shoes in a D.C. machine ?
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A. To reduce eddy current loss

B. To reduce the reluctance of the magnetic path

C. To spread out flux for better uniformity

D. To support the field coils

24. In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be______________?
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A. around 90%

B. anywhere between 75% and 90%

C. less than 50%

D. 100%

25. Regenerative method of braking is based on that___________?
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A. none of the above

B. back e.m.f. of rotor is more than the applied voltage

C. back e.m.f. is less than the applied voltage

D. back e.m.f. is equal to the applied voltage
26. Hopkinsons test on D.C. machines is conducted at_______________?
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A. no-load

B. full-load

C. part load

D. overload

27. Which of the following test can be performed on DC series motor?
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A. Field test

B. Hopkinson test

C. Swinburne test

D. Brake test

28. Which of the following loss in a D.C. generator is dissipated in the form of heat?
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A. Core loss

B. Mechanical loss

C. All of the above

D. Copper loss

29. In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to
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A. none of the above

B. 10% of the previous value

C. 100% of the previous value

D. 50% of the previous value

E. 25% of the previous value
30. The speed of a D.C. series motor is_____________?
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A. proportional to the armature current

B. proportional to field current

C. inversely proportional to the armature current

D. proportional to the square of the armature current

31. Torque developed by a D.C. motor depends upon_____________?
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A. current flow through the conductors

B. radius of armature

C. magnetic field

D. active length of the conductor

E. all above factors

F. number of conductors

32. The armature torque of the D.C. shunt motor is proportional to____________?
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A. field flux only

B. armature current only

C. both A. and B.

D. none of the above

33. Which D.C. motor is generally preferred for cranes and hoists ?
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A. Shunt motor

B. Series motor

C. Differentially compounded motor

D. Cumulatively compounded motor

34. A dc series motor should not be started on no load because it would___________?
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A. draw more current without producing sufficient torque

B. run at dangerously high speed

C. not develop starting torque

D. fail to start without load

35. In D.C. machines the residual magnetism is of the order of_________________?
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A. 10 to 15 per cent

B. 20 to 25 per cent

C. 50 to 75 per cent

D. 2 to 3 per cent
36. In a DC motor the inter-pole winding is connected in________________?
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A. parallel big field winding

B. parallel with armature winding

C. series with field winding

D. series speed armature winding
37. For which of the following applications a D.C. motor is preferred over an A.C. motor ?
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A. High speed operation

B. Low speed operation

C. Variable speed operation

D. Fixed speed operation

38. If a D.C. motor designed for 40C ambient temperature is to be used for 50C ambient temperature, then the motor_____________?
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A. is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

B. can be used for 50C ambient temperature also

C. of higher H.P. should be selected

D. of lower H.P. should be selected

39. In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be_____________?
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A. more than full-load current

B. less than full-load current

C. almost negligible

D. rated full-load current

40. Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?
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A. Hopkinsons test

B. Brake test

C. Swinburnes test

D. Field test