# 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
d) both of the winding get overloaded

Explanation: In the short shunt configuration, the winding interchanging does not change the configuration of the set up so none of the winding get affected.

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

Explanation: It is must that both the machines are identical as required by the calculations.

3. Retardation test on DC shunt motor is used for finding _______
a) stray losses
b) copper losses
c) friction losses
d) iron losses

Explanation: Retardation test is used for finding the stray losses.

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

Explanation: We need the angular acceleration and the moment of inertia of the machine.

5. ______ test is used for determining the efficiency of a traction motor.
a) Field
b) Retardation
c) Hopkinson
d) Swineburne’s

Explanation: Field test is used for finding efficiency of a traction motor.
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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

Explanation: When both field current and armature current are reversed then the direction will not change.

7. Mark the possible causes of overheating of commutator in a DC machine.
b) Restricted ventilation
c) Shorted winding
d) Any of the mentioned

Explanation: All the factors can cause the overheating of commutator.

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

Explanation: Ea/Eb = 1.4/1.3
Ea = 1.077 Eb > Eb
So, the machine will act as motor.

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

Explanation: Ea/Eb = 1.4/1.3
Ea = 1.077 Eb > Eb
So, the machine will act as generator.

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

Explanation: Regenerative phenomena utilizes the energy of the running machine to tap the dynamically produced energy and use it to charge the battery.

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

Explanation: For a generator the output can be measured and so this expression is used.

12. The efficiency in the swineburne’s test can be found.
a) True
b) False

Explanation: Because the constant losses are known.

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)

Explanation: There is decrease in flux due to positive temperature coefficient of resistance in shunt machine.

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

Explanation: Because the constant losses are known in advance of the test is conducted.

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

Explanation: The machines have to be mechanically coupled together.

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