Transformers
Leakage flux in a transformer occurs because

Applied voltage is sinusoidal
Transformer is not an efficient device
Iron core has high permeability
Air is not a good magnetic insulator

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

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

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