Derating of Induction Motors due to Power Quality Issues Considering the Motor Efficiency Class

Heating and losses of Induction Motor (IM) grow with the supply voltage unbalance and harmonic distortion. To avoid thermal overload, the NEMA MG1-2014 and the IEC 60034-26 standards establish derating factors for IMs operating under those conditions. In this paper, we show that while the derating factors proposed by these standards adequately protect standard-efficiency IMs, they are only marginally adequate to protect modern higher-efficiency IMs. To this end, we compare the derating factors provided by the standards with the derating factors required to maintain the losses at rated values in a standard-efficiency IM, in a premium-efficiency IM and in a super-premium-efficiency IM. To extrapolate the results from these IM, we compared the nameplate data of 548 IMs of different efficiency classes and found that higher efficiency classes correlate to higher IM starting currents and lower impedances to the negative sequence and harmonic voltages. These lower impedances in turn may lead to higher losses for unbalanced and harmonic voltages conditions.