Motor Amperage Imbalance

Question: Motor Amperage Imbalance
There is much information on the effects of voltage imbalance on 3 phase electric motors but I can not find specifics on the affects of amperage imbalance. We have three motors with amperage imbalances in the range of 20 and 30 percent on one leg of the 3 phase motors. The other two phases are very close. The imbalance is not on the same phase and switching the leads tot he pumps only moves the imbalance with it. These are two speed single winding motors. What affects does this have on the efficiency and longevity of the motors?

Answer:
A current imbalance of 20-30% is rather significant. The effect of the higher current is higher temperatures in that phase. The higher temperatures will adversely affect the insulation. The insulation will deteriorate and cause coil-to-coil shorts to develop resulting in catastrophic failure of the motor. A current imbalance is normally caused by a high resistance somewhere in the circuit downstream from where the current readings were taken. I would recommend opening the motor junction box and record the currents in all three phases. Assuming that the currents are balanced at the motor junction box, start looking for the cause of the high current between the motor and the Motor Control Center (MCC). If the currents are not balanced at the motor junction box the problem is in the motor and the motor needs to be replaced. Efficiency with an imbalance like this would not be what would be expected form a balanced current situation.

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