Electrical Machines Questions and Answers_pag338

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10. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced 
by reactance method of starting with a fraction of ‘x’, then the 
starting torque reduces by 
a) x 
b) x^2 
c) 1/x 
d) no change in the starting current 
View Answer 
Answer: b 
Explanation: Tst/Tfl = (x^2)(Ist/Ifl)^2*slip. 
 
11. Series reactor method of starting has main disadvantage of 
a) lowering the starting power 
b) increasing the starting current 
c) increasing starting torque 
d) all of the mentioned 
View Answer 
Answer: a 
Explanation: Main disadvantage of using reactors is that ot reduces the starting power of the 
machine and so the efficiency. 
12. Series reactor method of starting has main disadvantage of lowering the starting power. This 
can be overcome by 
a) using series resistance 
b) using parallel reactor 
c) using parallel resistance 
d) using series reactor 
View Answer 
Answer: a 
Explanation: Adding series resistance instead od reactors will help improving the starting power. 
13. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced 
by auto transformer method of starting with a tapping ratio of ‘x’, then the 
starting current reduces by 
a) x 
b) x^2 
c) 1/x 
d) no change in the starting current 
View Answer 
Answer: b 
Explanation: The starting current reduces by a fraction of x^2. 
14. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced 
by auto transformer method of starting with a tapping ratio of ‘x’, then the 
starting torque will be 
a) x *Tfl