Method of reducing the effect of supply unbalance on induction motors

The operation of induction motors under unbalanced supply conditions is well documented. Because of the resulting unbalanced currents, there is an increase in losses and consequent overheating of the windings. IEC Standard 34-1-1996 and NEMA Standard MG-1-1993 stipulate that a motor should be able to withstand continuous operation with 1% negative sequence voltage without derating. Where motors are continuously subjected to unbalance beyond this, the effects become significant and derating the motor becomes essential. Where an unbalance in the supply cannot be easily corrected, the general method adopted is to use a motor of larger rating. Although this solves the problem of the motor getting overheated, the performance parameters of the motor (viz., power factor, efficiency, starting current, etc.) deteriorate. In fact, the negative sequence currents in the supply lines increase, causing additional losses and requiring larger capacity supply cables. Thus, although simpler to implement, use of a larger size motor does not lead to the desirable solution (viz., reduction of unbalance in the motor winding currents). This letter studies the feasibility of modifying the machine in order to minimize the unbalance in the currents of a motor subjected to unbalanced supply voltages.