How does an AC induction motor work? Here’s a little animation to summarize things and hopefully make it all clear:

Why is the three-phase current of the motor unbalanced?
Three-phase voltage unbalance If the three-phase voltage is unbalanced, there will be reverse-sequence current and reverse-sequence magnetic field in the motor, resulting in a large reverse-sequence torque, resulting in an unbalanced distribution of the three-phase current of the motor and
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