Impact of cut edges on magnetization curves and iron losses in e-machines for automotive traction

Abstract

At last year's edition of EVS, we presented an improved model for iron losses prediction in Permanent Magnet Synchronous Machines (PMSM) [1]. The benefit of this model holds in that it fits more closely the real material behavior than the standard Steinmetz or Bertotti approaches [2], by including 1) magnetic material characteristics measured at high frequency and 2) an improved representation of iron losses at the approach to saturation (by introducing a higher order term in J). We are taking this model a step further by considering now another phenomenon impacting iron losses in electrical machines: the decrease of magnetic permeability and the increase of local hysteresis loss at the vicinity of lamination edges due to the cutting process. This paper presents a quantitative analysis of the impact of lamination processing (cutting, punching, etc) for high quality low loss electrical steels used in automotive traction applications. It is important to perform the analysis over a wide frequency range, because of the large speed range of PMSM drives in automotive applications and the presence of higher harmonics (PWM supply). Our approach consists in measuring the material characteristics for sample sets with different ratios of degraded vs. non degraded material and at various frequencies. Starting from that experimental data we propose a method to determine the local magnetization curves, as function of distance from the cut edge. These local material characteristics can then be implemented in a FE model so that the effect of punching on the machine performance can be determined quantitatively: (1) the cutting impact on magnetization modification allows more precise field calculations; (2) a proposition is made on the implementation of the cutting impact on the loss calculations in post processing via an enhanced version of the loss model developed in [1].