DC Machines - Electrical Engineering (MCQ) questions and answers

1)   A 120 V shunt generator running at 850 rpm has its armature and shunt field resistance of 0.15 ohm and 50 ohm respectively. It supplies 200 lamps each rated at 60 W, 100 V. The friction and windage  and core loss of the machine is 400 W. its armature copper loss on full load and shunt field loss is

a. 2156.7 W, 200 W
b. 2232.6 W, 200 W
c. 2156.7 W, 240 W
d. 2232.6 W, 240 W
Answer  Explanation 

ANSWER: 2232.6 W, 200 W

Explanation:
No explanation is available for this question!


2)   If A is the number of parallel paths and P is the number of poles, then the number of parallel path in lap winding and in wave winding is

a. A = P, A = 2
b. A = 2P, A = P
c. A = 2, A = P
d. A = P, A = 2P
Answer  Explanation 

ANSWER: A = P, A = 2

Explanation:
No explanation is available for this question!


3)   If % ηm is the mechanical efficiency of a DC machine and % ηe is the electrical efficiency. Then the overall efficiency of DC machine is given by

a. % η m + % η e
b. % η m X % η e
c. (% η m + % η e) / % η m
d. (% η m + % η e) / % η e
Answer  Explanation 

ANSWER: % η m X % η e

Explanation:
No explanation is available for this question!


4)   In a self excited generator, if the series field opposes the shunt field, then the generator is

a. Cumulatively compounded
b. Differentially compounded
c. Series generator
d. None of these
Answer  Explanation 

ANSWER: Differentially compounded

Explanation:
No explanation is available for this question!


5)   If the armature current of dc series motor has become twice then the torque will become

a. Twice of the former
b. Four times of the former
c. One fourth of the former
d. Remains same
Answer  Explanation 

ANSWER: Four times of the former

Explanation:
No explanation is available for this question!


6)   Laminations or stampings are used to reduce the eddy current losses. The direction of laminations should be such that

a. They are perpendicular to the paths of eddy current and flux both
b. They are parallel to the paths of eddy current and flux both
c. They are perpendicular to the paths of eddy current and parallel to the flux
d. They are parallel to the path of eddy current and perpendicular to the flux
Answer  Explanation 

ANSWER: They are perpendicular to the paths of eddy current and parallel to the flux

Explanation:
No explanation is available for this question!


7)   With the increase in temperature, the speed of series and shunt motor will

a. Increase, decrease
b. Decrease, increase
c. Increase, increase
d. Decrease, decrease
Answer  Explanation 

ANSWER: Decrease, increase

Explanation:
No explanation is available for this question!


8)   If Eb is the back emf of d.c. motor and V is the terminal voltage, then the condition for maximum power is

a. Eb = V
b. Eb = 2V
c. Eb = ( V / 2 )
d. Eb = V ∧ 2
Answer  Explanation 

ANSWER: Eb = ( V / 2 )

Explanation:
No explanation is available for this question!


9)   For low reluctance path for the flux in armature, the permeability of the material should be

a. High
b. Low
c. Both (a) and (b)
d. None of these
Answer  Explanation 

ANSWER: Low

Explanation:
No explanation is available for this question!


10)   The armature reaction in d.c. machine causes distortion in the main field flux. This effect of armature reaction can be reduced by

a. Increasing the length of air gap
b. Decreasing the length of air gap
c. Increasing the number of poles
d. Decreasing the number of poles
Answer  Explanation 

ANSWER: Increasing the length of air gap

Explanation:
No explanation is available for this question!


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