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GATE 2018 EE – Question 11

Electrical Machines · Single phase transformer: equivalent circuit, phasor diagram, open circuit and short circuit tests, regulation and efficiency · 1 mark · Multiple choice

A single-phase 100 kVA, 1000 V / 100 V, 50 Hz transformer has a voltage drop of 5% across its series impedance at full load. Of this, 3% is due to resistance. The percentage regulation of the transformer at full load with 0.8 lagging power factor is

  1. 4.8
  2. 6.8
  3. 8.8
  4. 10.8

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Correct answer: (A) 4.8

Explanation

$\%R=3$ and $\%X=\sqrt{25-9}=4$. The regulation is $\%R\cos\phi+\%X\sin\phi=3(0.8)+4(0.6)=4.8\%$.