Transformers
A tap changer is used on a transformer for

Adjustments in secondary voltage
None of the listed here
Adjustment in both primary and secondary
Adjustment in primary voltage

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

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

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