# Design of Electrical Machines Questions and Answers – Losses and Efficiency

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

1. How many losses are present in induction motors?
a) 4
b) 3
c) 5
d) 2

Explanation: There are 5 losses present in the induction motor. They are i) Stator copper losses, ii) Rotor copper losses, iii) Stator iron losses, iv) Friction and winding losses, v) Additional losses.

2. What is the formula for efficiency at full load?
a) efficiency at full load = output / output + losses
b) efficiency at full load = output / output – losses
c) efficiency at full load = output / output * losses
d) efficiency at full load = output * output + losses

Explanation: First the various losses are calculated for the machine. Then the output obtained is observed and the substitution of the values in the formula gives the efficiency at full load.

3. How many types of additional losses are present?
a) 1
b) 4
c) 2
d) 3

Explanation: The additional losses are divided into 2 types. They are i) Additional copper loss ii) Additional iron losses.

4. What factor does the additional copper losses depend upon?
a) skin effect
b) mmf harmonics
c) machine design
d) mmf harmonics and skin effect

Explanation: With a sinusoidal voltage impressed over the terminal of the motor, the additional copper losses are caused. They are caused due to the higher order mmf harmonics and skin effect.

5. The additional losses owing to the higher order mmf harmonics occur mainly in windings of squirrel cage rotor.
a) true
b) false

Explanation: The additional losses are depending on the higher order mmf harmonics and skin effect. The losses occur mainly in the squirrel cage rotor.
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6. How can the additional losses be decreased in the induction motor?
a) chording the stator winding
b) skewing the rotor
c) having a proper slot combination
d) chording the stator winding, skewing the rotor, having a proper slot combination

Explanation: There are 3 methods to decrease the additional losses in induction motor. They are chording the stator winding, skewing the rotor, having a proper slot combination.

7. What is the use of skin effects in the induction motor?
a) it helps in improving the efficiency
b) it helps in improving the stopping characteristics
c) it helps in improving the starting characteristics
d) it helps in improving the running characteristics

Explanation: The skin effect phenomenon is observed in stator and rotor windings in the induction motor. The effect may be used for improving starting characteristics.

8. What should be the maximum permissible level for frequency in normal operating conditions?
a) < 2 Hz
b) > 3 Hz
c) < 4 Hz
d) > 3 Hz

Explanation: The normal condition operation depends upon the frequency levels in the machine. It should not exceed 3 Hz.

9. How many types are the additional losses in iron classified into?
a) 2
b) 3
c) 4
d) 5

Explanation: The additional iron losses are classified into 2 types. They are i) pulsation losses and ii) surface losses.

10. The pulsation losses are caused by the direct axis pulsation of magnetic flux.
a) true
b) false

Explanation: The pulsation losses are one type of additional iron losses produced. They are produced by the direct axis pulsation of magnetic flux due to the variation of permeance caused by the continuous change in mutual positions of rotor and stator teeth during rotation of rotor.

11. How much does the addition iron losses relate with the supplied power?
a) additional iron losses = 0.5% of supplied power
b) additional iron losses = 0.6% of supplied power
c) additional iron losses = 0.8% of supplied power
d) additional iron losses = 0.9% of supplied power

Explanation: The additional iron losses are a small amount when compared with the supplied power. They are 0.5% of the supplied power.

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