Transformers
Cores of large transformers are built upto nearly circular cross section in order to reduce

Copper loss
Leakage reactance
Iron losses
Eddy current loss

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Transformers
The efficiency of a transformer, under heavy loads, is comparatively low because

iron loss is increased considerably
voltage drop both in primary and secondary becomes large
secondary output is much less as compared to primary input
copper loss becomes high in proportion to the output

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