Derating factors for direct online induction machines when supplied with voltage harmonics: A critical view

Induction machines usually have a reduced efficiency when supplied with non linear currents. Most of the induction machines are not used at nominal loading, but at a 60% loading rate. In the paper it will be shown that derating factors for induction motors fed with nonlinear currents, like the NEMA MG1 or the IEC 60034-17, are correct for nominal loading. At lower loading these derating factors can underestimate the losses. The measurements shown in this paper indicate that for low loading percentages, the phase angles of the harmonics have a significant influence on the peak induction and therefore the saturation level of the induction machine. For low loading rates the constant losses, including the magnetization losses, are the dominant losses. For a correct estimation of the losses for motors working at low loading conditions, the phase of the harmonics has to be taken in account.