Transformers
The maximum efficiency of a distribution transformer is

at 80% full load
at 50% full load
at no load
at full load

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

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

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