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Multiple Choice Question (MCQ) of D.C. Machines page-5

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41. In a dc motor, the flux leakage coefficient is typically

a) 0.5 to 0.7
b) 0.8 to 1.0
c) 1.1 to 1.3
d) 1.4 to 1.6

Ans: (c)

42. The direction of rotation of a dc motor can be reversed by reversing the connection to

a) Armature
b) Series field
c) Shunt field
d) Any of the above

Ans: (a)

43. The current flowing in conductors of a dc motor is

a) Ac
b) Dc
c) Ac as well as dc
d) Transient

Ans: (a)

44. The armature reaction in a dc motor is attributed to

a) The effect of magnetic field set up by field current
b) The effect of magnetic field set up by armature current
c) The effect of magnetic field set up by back emf
d) Copper losses in the armature

Ans: (b)

45. When an electric traian is moving down a hill the dc motor acts as

a) Dc series motor
b) Dc shunt motor
c) Dc series generator
d) Dc shunt generator

Ans: (c)

46. If the armature current of a dc motor is increased keeping the field flux constant, then the developed torque

a) Increases proportionately
b) Decreases in reverse proportion
c) Remains constant
d) Increases proportional to the square root of current

Ans: (a)

47. The armature current drawn by a dc motor is proportional to

a) The speed of the motor
b) Flux required in the moor
c) Torque required
d) Voltage applied to the motor

Ans: (c)

48. In a dc motor if the brushes are given a backward shift, then

a) Commutation improves and speed decreases
b) Commutation is unaffected and speed increases
c) Commutation improves and speed increases
d) Commutation worsens and speed decreases

Ans: (c)

49. A dc shunt motor is driving a constant torque load with rated excitation. If the field current is reduced to half, then the speed of the motor will become

a) Half
b) Slightly more than half
c) Double
d) Slightly less than double

Ans: (d)

50. The back emf Vb of a dc motor

a) Opposes the applied voltage
b) Assists the applied voltage
c) Does not influence the applied voltage
d) None of these

Ans: (a)

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