Transformers
In case of 3-phase induction motors, plugging means

pulling the motor directly on line without a starter
starting the motor on load which is more than the rated load
locking of rotor due to harmonics
interchanging two supply phases for quick stopping

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

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