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

Leakage reactance
Eddy current loss
Copper loss
Iron losses

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

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

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