Determination of AC losses in segmented rare earth permanent magnets

Abstract

Segmenting rare earth permanent magnets into thin slices is an effective means for avoiding eddy current losses in oscillating magnetic fields. However, the large surface areas created by segmentation can give rise to severe hysteresis losses. The surface hysteresis may lead to increasing rather than decreasing AC losses for narrower segmentation. Increasing losses were observed in NdFeB samples for segment thicknesses below 4 mm, in an AC field of ±80 kA/m at 1000 Hz. In contrast, Sm2Co17 magnets showed no significant hysteresis losses. The strong hysteresis losses in NdFeB were observed for AC fields oscillating around H = 0. DC bias fields of >100 kA/m reduced the hysteresis losses by more than one order of magnitude. Determining the hysteresis losses from quasi-static minor loop measurements gave similar results as found in the AC measurements. An evaluation of the hysteresis losses is thus possible by hysteresisgraphs, which are more widely available.