# Electrical Machines Questions and Answers – Losses and Efficiency – 2

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Losses and Efficiency – 2”.

1. A consideration of the power losses in electrical machines is essential for which of the following reasons?
a) Operating cost
b) Temperature rise
c) Voltage drops
d) All of the mentioned

Explanation: A machine with lower efficiency has more losses, and therefore increased operating cost. Also, losses cause heating of the machine and therefore, its temperature rise. Similarly, voltage drop IR is associated with ohmic loss.

2. To determine the efficiency of the machine, direct load test is not advantageous because of which of the following reasons?
a) Cost of providing large inputs
b) Difficulty of dissipating the large outputs
c) Both cost of providing large inputs and difficulty in dissipating the large outputs
d) None of the mentioned

Explanation: Also, a small error in the measurement of either output or input causes the same amount of error in the computed efficiency.

3. Which of the following statements are correct regarding brush contact losses?
a) In DC machine: proportional to armature current
b) In synchronous machine: neglected
c) In induction machine: neglected
d) Any of the mentioned

Explanation: There is brush contact loss at the contacts between the brushes and commutator (in DC machine) or between the brushes and slip rings (synchronous and induction machines). However, in practical, the brush contact loss is neglected for synchronous and induction machines.

4. In rotating electrical machines, when the armature rotates, there are continuous magnetic reversals and power required for their reversals is called _____________
a) Eddy current loss
b) Hysteresis loss
c) Resistance or ohmic losses
d) Mechanical loss

Explanation: Hysteresis loss is directly proportional to the number of magnetic reversals per second or the speed.

5. The usual lamination thickness selected to minimize the eddy current loss in rotor is _____________
a) 0.1 mm to 0.2 mm
b) 0.3 mm to 0.4 mm
c) 0.4 mm to 0.5 mm
d) 0.9 mm to 0.10 mm

Explanation: If lamination thickness is made less than 0.4 mm, the reduction in eddy current losses is achieved, but at the cost of additional labor charges in assembling the rotor.
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6. In an induction motor, which of the following is correct?
a) Stator core loss < rotor core loss
b) Stator core loss = rotor core loss
c) Stator core loss > rotor core loss
d) Any of the mentioned

Explanation: The rotor core loss is almost negligible because of reduced frequency of the flux reversals (equal to slip frequency, sf) in the rotor, and Pe∝f2 and Ph∝f. (Pe = eddy current losses, Ph = hysteresis losses).

7. The pole shoes in DC and synchronous machines are laminated to reduce _____________
a) Resistance losses
b) Pulsation or pole-face losses
c) Mechanical losses
d) None of the mentioned

Explanation: The pulsations in flux density wave arising from slot openings cause losses in the field iron, particularly in the pole faces. This loss occurring due to relative motion, between field poles and slotted armature, is referred to as pulsation loss.

8. What percentage of the rated output for DC machine and synchronous machine is taken as stray load losses respectively?
a) 1% and 0.5%
b) 0.5% and 1%
c) 3% and 0.1%
d) 0.1% and 5%

Explanation: Stray load losses cannot be determined accurately. In DC machine, by convention, it is taken as 1% of the rated output for rating above 150 KW. For synchronous and induction machine, it is taken as 0.5% of their rated output.

9. Which of the given losses are directly proportional to square of speed?
a) Windage loss
b) Eddy current loss
c) Both Windage and eddy current loss
d) Hysteresis loss and brush loss

Explanation: The windage loss includes the power required to circulate air through the machine and ventilating ducts, and is approximately proportional to the square of speed. Also, eddy current losses are proportional to square of speed.

10. Which of the following statements regarding efficiency of electrical machines are true?

```(i) efficiency should be calculated by measuring output and input
(ii) efficiency is maximum when constant losses = variable losses
(iii) electrical machines are designed to have maximum efficiency at full load
(iv) electrical machines are designed to have maximum efficiency at near about full load
(v) efficiency should be calculated by measuring their losses
(vi) efficiency is maximum when constant losses = x times of (variable losses)
```

a) (i), (iii), (vi)
b) (ii), (iv), (v)
c) (i), (ii), (iii)
d) (iv), (v), (vi)

Explanation: The machine efficiency rises with load. but at a particular load, efficiency is maximum and beyond this load, efficiency diminishes. Also, for both motors and generators, machine efficiency is maximum when variable loss = constant loss.

11. The electromechanical energy conversion devices used in power systems are never operated to deliver maximum power output because at maximum power output _____________
a) Efficiency is less than 50%
b) Temperature of the power devices is much more than the specified allowable temperature rise
c) Half of the power input appears as losses
d) Any of the mentioned

Explanation: In practice, devices are operated at a load somewhat less than rated load, at which the efficiency is maximum.

12. For the same rating machines, which of the following statement is correct regarding the efficiency (η)?
a) η of low speed machine > η of high speed machine
b) η of low speed machine < η of high speed machine
c) η of low speed machine = η of high speed machine
d) Any of the mentioned

Explanation: The amount of conductor and iron materials required for a machine of given rating is inversely proportional to its speed. Also, more iron and conductor would entail more losses.

13. No load rotational losses in electrical machine consists of _____________
a) Friction and windage losses
b) Stator core, friction and windage losses
c) Rotor core, friction and windage losses
d) No load core, friction and windage losses

Explanation: The sum of friction and windage loss under no load is called no load rotational losses.

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