Electrical Machines Questions and Answers – Efficiency and Testing of DC Machines

This set of Electrical Machines Questions and Answers for Freshers focuses on “Efficiency and Testing of DC Machines”.

1. A dc motor is connected in the short-shunt configuration, but the series and the shunt windings get interchanged by mistake and the motor is connected to the rated voltage?
a) neither of the windings get short-circuit
b) series winding gets overloaded
c) shunt winding gets overloaded
d) both of the winding get overloaded
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2. What is the main concern before doing hopkinson’s test for finding efficiency?
a) needs one motor and one generator is that it
b) ignores iron and stray losses
c) needs one motor
d) requires identical shunt machines
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3. Retardation test on DC shunt motor is used for finding _______
a) stray losses
b) copper losses
c) friction losses
d) iron losses
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4. The quantities needed to complete retardation test are/is _________
a) dw/dt and moment of inertia
b) dw/dt
c) current
d) any of the mentioned
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5. ______ test is used for determining the efficiency of a traction motor.
a) Field
b) Retardation
c) Hopkinson
d) Swineburne’s
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6. If the field current and armature current are reversed, then the __________
a) direction of rotation remains same
b) direction of rotation reverses
c) stops
d) none of the mentioned
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7. Mark the possible causes of overheating of commutator in a DC machine.
a) Overload
b) Restricted ventilation
c) Shorted winding
d) Any of the mentioned
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8. In the hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has field current of 1.3A. Which machine acts as motor?
a) B
b) A
c) A, B
d) None of the mentioned
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9. In the Hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has field current of 1.3A. Which machine acts as generator?
a) B
b) A
c) A,B
d) None of the mentioned
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10. Hopkinson’s test is also called regenerative test due to ____________
a) energy of one machine is used to drive other
b) feedback
c) losses are least
d) extra motor is used
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11. Which is the most appropriate relation to find efficiency of the generator?
a) output/(output + losses)
b) (output – losses)/input
c) (output – losses)/(output + losses)
d) any of the mentioned
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12. The efficiency in the swineburne’s test can be found.
a) True
b) False
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13. The possible assumption is/are made while doing swineburne’s test.

(i) mechanical loss constant
(ii) Armature reaction neglected
(iii) Increases in flux

a) (i) and (ii)
b) (iii) and (i)
c) (i), (ii), and (iii)
d) (i)
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14. Swineburne’s test is applicable to those machines in which flux is practically _______
a) constant
b) linear
c) non-linear
d) none of the mentioned
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15. Similarity between Hopkinonson’s and field’s test is that _______
a) both need similar mechanically coupled motor
b) both need similar electrically coupled motor
c) regenerative power
d) negligible power
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Sanfoundry Global Education & Learning Series – Electrical Machines.

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