GATE EE 2014 Set 2 — Question 23
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Electrical Machines → Three-Phase Induction Machines → Induction Generator
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Question
A three-phase, 4-pole, self excited induction generator is feeding power to a load at a frequency . If the load is partially removed, the frequency becomes . If the speed of the generator is maintained at 1500 rpm in both the cases, then
Correct answer
(C) f₁, f₂ < 50 Hz and f₂ f₁
Solution
For an induction machine, the synchronous speed is given by . For a 4-pole machine at , .In generator mode, the rotor speed must be greater than the synchronous speed (). Given that the generator speed is maintained at , we must have .
.
Thus, both and must be less than .When the load is partially removed, the electrical power output decreases. For an induction generator, this corresponds to a decrease in the magnitude of the negative slip .
. Since , is negative.
As decreases, must increase towards . Since , an increase in implies an increase in frequency . Therefore, .Combining these, and .
.
Thus, both and must be less than .When the load is partially removed, the electrical power output decreases. For an induction generator, this corresponds to a decrease in the magnitude of the negative slip .
. Since , is negative.
As decreases, must increase towards . Since , an increase in implies an increase in frequency . Therefore, .Combining these, and .
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