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Voltage unbalance between the three phases is harmful to motors because it

  • A.produces a large negative sequence current and extra heating
  • B.reduces the synchronous speed of the motor concerned
  • C.increases the motor's power factor
  • D.reduces the starting torque only

Answer: A

A small percentage of unbalance produces a much larger percentage of negative sequence current, because the negative sequence impedance is low, and that current heats the rotor without producing useful torque. One per cent of unbalance can cost several per cent of derating.

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