This paper uses a real load test to investigate the effects of an unbalanced voltage supply on an induction motor＇s performance. Based upon various experiments, including (1) cases with the same unbalance voltage factor but different unbalanced voltages, (2) cases with only one unbalanced voltage but different degrees of unbalance, and (3) cases with the same positive-sequence voltage but different negative-sequence voltages, the importance of the positive-sequence voltage in the motor＇s apparent performance and of the negative-sequence voltage in the hidden damage are pointed out. Finally, it is strongly suggested that the related regulations, and a motor＇s derating factor and temperature rise curves should be based on not only a voltage unbalance factor, but also the magnitude of the positive-sequence voltage.