Stray load losses in inverter-fed induction machines – calculation and measurement

Both the calculation and measurement of stray load losses in induction machines pose a challenging task. These power losses are typically measured as an unassignable portion of the total power losses, which can also be considered as remainder losses. Thus, breaking them down according to their physical causes is not possible, making it difficult to correlate with calculated power loss components. For the calculation of stray load losses, conventional software use an empirically determined "penalty curve" according to IEC 60034-2-1. However, this method significantly overestimates the magnitude of these power losses, particularly at operating points with high stator frequencies in the field weakening region. The largest proportion of stray load losses can be attributed to stray load iron losses in the stator and rotor, as well as stray load ohmic losses in the rotor bars, which are caused by spatial harmonic fields in the machine. Depending on the specific machine and its operating range, eddy current losses in the housing or cooler must also be considered, as well as any changes in the geometrical air-gap length due to temperature differences within the machine. In this presentation, the aforementioned power loss components are first determined, summed, and compared with the measured remainder losses using options integrated into the software ASYN: the previous analytical "penalty curve" and a time-stepping FEA. Different machines are used as examples. Furthermore, an extended ASYN methodology that allows for the time-efficient determination of stray load iron losses using magnetostatic FEA is presented.