Transformers
The hum in a transformer is mainly attributed to

load changes
mechanical vibrations
oil in the transformer
magnetostriction

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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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Transformers
No-load current of a transformer has

has high magnitude and high power factor
has small magnitude and high power factor
has small magnitude and low power factor
has high magnitude and low power factor

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