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The efficiency of a transformer, under heavy loads, is comparatively low because?
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The efficiency of a transformer, under heavy loads, is comparatively low because copper loss becomes high in proportion to the output.
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An ideal transformer will have maximum efficiency at a load such that copper loss equal to iron loss.

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The efficiency of transformer is Greater than electrical Machine because there is no rotational losses in transformer and there is no air gap losses in transformer so losses is less in transformer compare to electrical machine so efficiency is higher in case of transformer.

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Transformer efficiency is maximum when copper losses are equal to iron losses.

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