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GATE 2021 IN – Question 37

Electrical Circuits and Machines · Transformers and electrical machines · 2 marks · Multiple choice

A single-phase transformer has maximum efficiency 98%, core loss 80 W, equivalent winding resistance 0.5 $\Omega$ referred to the primary, and rated primary current 25 A. Maximum efficiency occurs at what percentage of rated current?

  1. 35.7%
  2. 50.6%
  3. 80.5%
  4. 100%

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Correct answer: (B) 50.6%

Explanation

Maximum efficiency occurs when copper loss equals the constant core loss: I²·0.5=80 W.

Thus I=√160=12.649 A. Relative to the rated 25 A, this is 100·12.649/25=**50.6%, B**. The stated 98% efficiency is not needed for this load condition.