Abstract
Heating and losses of induction motor (IM) grow with the supply voltage unbalance and harmonic distortion. To avoid thermal overload, the IEEE 141-1993, the IEEE 3004.8, NEMA MG1-2014, and the IEC 60034-26 standards establish derating factors for IMs operating under those conditions. In this article, 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.
I. INTRODUCTION
ELECTRIC motors, and in particular induction motors (IMs), represent the most important load in the electric power system. Its participation in the demand for electric energy is close to half of the energy consumed globally [1], [2]. For this reason, even today, there are proposals to increase the electric motors efficiency to yield large energy savings [3].
To promote the development of more efficient IMs, the IEC 60034-30 standard from 2008 specified three efficiency classes. Compliance with these classes guarantees a minimum efficiency level for every type of IM and nominal power. These classes are: standard efficiency or IE1, high efficiency or IE2, and premium efficiency or IE3. A later revision of this standard includes two new efficiency classes, the super-premium or IE4 and ultra-premium or IE5 [4]. VI.
CONCLUSION
In this article, we study the effect of voltage unbalance and harmonic distortion on the losses of IMs in different efficiency classes.
We first studied the nameplate data of 548 IMs to show that higher efficiency class IMs are correlated to higher starting currents in p.u. Then, we drew attention to the work presented in [24], where higher starting current are associated with lower negative-sequence impedances and to higher losses for unbalanced and distorted voltage conditions.
