A lightly loaded three-phase squirrel-cage induction motor, while running, begins to vibrate excessively but continues to run. After shutting down the motor, the motor will not restart, but makes growling noises, overheats, and smokes. What caused this?

Correct Answer: Option D — One of the lines feeding the motor has an open phase, resulting in single-phasing.

Loss of one supply phase, known as single-phasing, unbalances the motor magnetic fields, causing severe vibration while running. Once stopped, the remaining single-phase circuit lacks starting torque, resulting in high currents, growling noises, and thermal overload.