Dependence of coercivity, relative permeability and magnetic losses on oscillating magnetic field frequencies and maximum polarisation for NGO and HGO steels

Abstract

This study investigated two types of electrical steels: high permeability grain oriented (HGO) 3.25 wt%Si and 0.25 mm thick and two non-grain oriented (NGO) grades 2 wt%Si and 0.50 mm and 0.54 mm thick The dependence of the frequency of the external magnetic field H and maximum polarisation Jmax on the different magnetic losses (hysteresis, classical and excess), coercivity, relative permeability were investigated over a wide frequency range of 3-2000 Hz (10 and 100 Hz increments for low and high frequencies, respectively), and Jmax range of 0.1–1.5 T (0.1 T increment). With this large data set it was observed that the relative permeability as a function of Jmax exhibited a maximum at approximately 1.1 T (HGO) and 0.8 T (NGOs), and the coercivity was dependent on the frequency up to 2000 Hz. Further, the remanence increased up to frequencies of 250 Hz and then became constant. All these distinct dependencies with f and Jmax show the key role of grain size, morphology and crystallographic and magnetic texture on magnetic properties. Furthermore, a scenario showing the component losses that dominated under certain frequency and Jmax ranges was established; we found that the hysteresis loss was dominant for the NGO at frequencies up to 100 Hz and Jmax up to 1 T, whereas the excess loss was the smallest of all in the entire range studied. Above this 1 T field, the classical loss was the most expressive. Regarding HGO, another scenario was observed; the excess loss was the highest of all up to 500 Hz with Jmax up to 0.4 T, and the classical loss dominated above 400 Hz. This clearly shows that different scales in the magnetisation process relate different physical mechanisms (the wall pinning domain and magnetic domain), which regulate the loss components. Thus, our contribution shows the processes and/or parameters that must be considered to tune a certain magnetisation scale to obtain the best material efficiency and thus the best possible performance of a device.