Magnetic losses for non-sinusoidal waveforms found in AC motors

Abstract

Both high-power and low-power ac motors are in common use in a wide range of applications. However, it is currently very difficult to predict the efficiency of ac motors because the magnetic losses are difficult to calculate under the non-sinusoidal voltage waveforms that are applied to the ac motors by the inverters. The manufacturers of the magnetic materials usually supply loss data for sinusoidal waveforms only. As a result, a power de-rating factor has to be applied to the design in order to avoid thermal problems. In this letter, a model is provided for calculating magnetic losses under non-sinusoidal voltage waveforms. It is shown that the magnetic losses for non-sinusoidal voltage waveforms can far exceed the magnetic losses for sinusoidal voltage waveforms if either the eddy current loss or excess eddy current loss dominates the total magnetic loss, even when the peak flux density and the frequency are kept the same for both waveforms. The model is applied to experimental data on two different materials and to two common voltage inverter waveforms. Very good agreement with experimental data is found.