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Multiple Choice Question (MCQ) of D.C. Machines page-3

21. Speed of a motor may be varied by varying

a) Field current
b) Applied voltage
c) Resistance in series with armature
d) Any of these

Ans: (d)

22. In a dc motor, maximum power is developed when supply voltage is equal to

a) 1/2 back emf
b) √2 back emf
c) 2 back emf
d) 1/√2 back emf

Ans: (c)

23. The speed of a series wound dc motor

a) Can be controlled by shunt field regulator
b) Can not be controlled by diverter
c) Increases as flux decreases
d) Increases as armature circuit resistance increases

Ans: (c)

24. The speed of a dc motor is

a) Always constant
b) Directly proportional to back emf
c) Directly proportional to flux
d) Inversely proportional to the product of back emf and flux.

Ans: (b)

25. The highest speed attained by a dc shunt motor at rated flux is

a) Infinity
b) Higher than no load speed
c) Equal to the no load speed
d) Lower than the no load speed

Ans: (c)

26. If the applied voltage to a dc shunt motor is halved and the load torque doubled, the armature current will be

a) Unaltered
b) Zero
c) Doubled
d) Halved

Ans: (c)

27. A dc motor is preferred to an ac motor for the application in

a) Low speed operation
b) High speed operation
c) Variable speed variation
d) Fixed speed operation

Ans: (c)

28. The starting resistance of a dc motor is usually

a) Infinitely large
b) Large
c) About 100Ω
d) Small

Ans: (d)

29. If 220 V dc series motor is connected to a 220 V ac supply, then at will

a) Not run
b) Burn out
c) Run smoothly
d) Run with less efficiency and high sparking

Ans: (d)

30. Which of the following dc motor has approximately constant speed?

a) Series motor
b) Shunt motor
c) Cumulatively compound motor
d) Differentially compound motor

Ans: (b)

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