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Losses and efficiency

Real transformers have losses. Iron loss comes from magnetizing the core and is almost constant. Copper loss comes from the resistance of the windings and grows with the square of the current. The efficiency is typically above 95 %.

\eta = \frac{P_2}{P_2 + P_{Fe} + P_{Cu}}

efficiency

P_{Cu} = R\,I^2

copper loss

Symbols

$\eta$	efficiency
$P_{Fe}$	iron loss	W
$P_{Cu}$	copper loss	W

Example

Delivered 10 kW, iron loss 150 W and copper loss 250 W:

$\eta = 10\,000/10\,400 \approx 96\,\%$ .

Half load gives a quarter of the copper loss, but the same iron loss.

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Part of Electric Power and Machines: The transformer.