Question 215Electricity & Electronics - Assistant Engineer

70% to pass

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?

AThe motor tripped-out on under load.
BThe branch circuit breaker tripped open.
CThe source voltage or frequency was too low.
DOne of the lines feeding the motor has an open phase, resulting in single-phasing.
AI Explanation

The correct answer is D) One of the lines feeding the motor has an open phase, resulting in single-phasing. This is the correct answer because when one of the three phases feeding an induction motor is lost, it causes the motor to operate in a single-phase mode. This leads to excessive vibration, overheating, and the inability to restart the motor, as observed in the problem. The other options are incorrect because they do not accurately describe the symptoms and underlying cause of the issue.

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Official Resources
46 CFR Electrical Engineering 46 CFR Machinery & Electrical USCG National Maritime Center
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