Electrical Machines Questions and Answers_pag45

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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 
View Answer 
Answer: c 
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. Consider the following statements regarding efficiency of electrical machines: 
(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) 
Which of the above statements are correct? 
a) (i),(iii),(vi) 
b) (ii),(iv),(v) 
c) (i),(ii),(iii) 
d) (iv),(v),(vi) 
View Answer 
Answer: b 
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 
View Answer 
Answer: b 
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