Quantifying the energy efficiency of motors fed by adjustable frequency inverters

Abstract

With ever-increasing focus on energy efficiency, along with the more widespread application of adjustable frequency inverters, it is important to be able to accurately quantify the energy efficiency of motors fed with adjustable frequency power. Three phase AC induction motors demonstrate high efficiency levels and are able to maintain a high efficiency across a substantial range of loading conditions. When fed by inverter power, there are additional losses which occur in these motors, incrementally degrading the energy efficiency. While prior inverter waveforms (such as 6-step) had substantial harmonic content in the motor current, today's PWM inverters are capable of producing waveforms with very little lower order harmonic content. In fact, the majority of the “non-fundamental” frequency content in modern PWM current waveforms is at the switching frequency — which is not really a “harmonic” for the typical asynchronous carrier modulation schemes. This paper will quantify the impact of inverter waveforms on the component losses and overall losses of motors. The effects of various filters which may be applied between the inverter and motor will also be considered. This paper will also address the challenges associated with making accurate efficiency measurements on inverter-fed motors, especially those with extremely high efficiency. Recommendations will be made to allow informed decisions about the economic impact of energy usage in motors fed by adjustable frequency inverters. Both low voltage and medium voltage cases will be included.