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A dc generator delivers 20 kW with total losses of 2 kW. Its efficiency is

  • A.91 per cent
  • B.90 per cent
  • C.100 per cent
  • D.10 per cent

Answer: A

Correct answer: 91 per cent

Efficiency is output over input, and the input is the output plus the losses:

\[ \eta = \frac{20}{20 + 2} = \frac{20}{22} \approx 0.909 = 91\% \]

Dividing the output by itself plus losses is the generator convention; for a motor the input is measured and the output is input less losses. The 90 per cent answer comes from dividing the losses by the output rather than by the input.

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