Concept for more correct magnetic power loss measurements considering path length dynamics

Abstract

In principle, single sheet testers (SSTs) or Epstein testers (ETs) determine iron losses p through the time integral over the field H and the time derivative of the induction B. Both quantities should be determined in equivalent ways, i.e. considering an identical quasi-homogeneously magnetized sample section. Partly, this condition is fulfilled by H-coil methods. However, the so far used very small coils do not meet the demand of equivalence. At least outside of Japan, most labs prefer apparatuses where H is assumed to be proportional to the magnetization current i. But exact proportionality does not exist due to the periphery – the yoke of SST and the corners of ET, respectively. Usually, the periphery is considered through a constant effective magnetic path length l_M based on the peak induction B_p . But in fact, l_M varies during the period, i.e. it is a function of B(t) and thus also of time. Our novel "PLC method" is characterized by path length consideration. We consider dynamics of length through a "path length function" Λ (B) which is determined for each grade of material in a calibration process by means of a large, rigid H-coil. Considering Λ [B(t)] by software, routine measurements on similar materials can be based on the "corrected" current i(t)/Λ (t). First results are presented for HGO SiFe investigated in an ET. Here, Λ proves to be above 1 for low B and below 1 for very high B. As a final target, application of the method for both ETs and SSTs should yield almost identical loss values that are close to "true" ones.