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Electrical Engineering QUESTION #2185
Question 1
A $100\,\text{kVA}$, $11\,\text{kV}/400\,\text{V}$ transformer has iron losses $P_i = 500\,\text{W}$ and full-load copper losses $P_{cu} = 1200\,\text{W}$. The efficiency at half full-load and $0.8$ pf lagging is:
  • $98.0\%$✔️
  • $96.1\%$
  • $97.3\%$
  • $94.5\%$
Correct Answer Explanation
Output at half load $= 0.5\times100\times10^3\times0.8 = 40{,}000\,\text{W}$. Cu loss $= (0.5)^2\times1200 = 300\,\text{W}$. Iron loss $= 500\,\text{W}$ (constant). $\eta = \frac{40000}{40000+800}\times100 = \frac{40000}{40800} \approx 98.0\%$.