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GATE 2019 EE – Question 55

Electrical Machines · Single phase transformer: equivalent circuit, phasor diagram, open circuit and short circuit tests, regulation and efficiency · 2 marks · Numerical answer

A single-phase transformer of rating 25 kVA, supplies a 12 kW load at power factor of 0.6 lagging. The additional load at unity power factor in kW (round off to two decimal places) that may be added before this transformer exceeds its rated kVA is ________.

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Correct answer: 7.16 to 7.24

Explanation

The present load is $S=\frac{12}{0.6}=20$ kVA with $Q=\sqrt{20^2-12^2}=16$ kvar. Adding $P$ kW at unity power factor, $\sqrt{(12+P)^2+16^2}=25$ gives $12+P=19.2$, so $P=7.2$ kW.