The impact of the physical, electrical and magnetic properties of ferromagnetic materials on their excess loss

The excess loss in a ferromagnetic material is linked to the eddy currents generated due to the domain wall movements during the subsequent magnetizing and demagnetizing fields in AC applications. Bertotti’s theory describes a dependency of this excess loss to the frequency with a power of 1.5. This implies the excess loss does not increase as fast with the frequency as the induced eddy current loss, but it does bring a substantial loss component in high speed automotive and flight powertrain applications. The excess loss of a series of electrical steels with different thickness, electrical resistivity, texture and grain size, is studied in an Epstein frame set-up. The total measured losses are split between hysteresis loss (corrected for flux redistribution), eddy current loss (corrected for flux redistribution and edge effects due to the limited width of the set-up), resulting in an excess loss matrix per material over a frequency and polarization range, that is matching machine exploitation conditions. The models of Bertotti and Mayergoyz are the basis for describing the excess loss. This is performed in the time domain in order to integrate all waveforms occurring at different depths within the lamination thickness. The model improvement is based on including the maximum permeability as parameter that differentiates the domain wall movement in the magnetization behavior. The paper identifies the number of active magnetic objects throughout the thickness of the lamination. It proves the impact of the flux redistribution on the excess loss depending on the selected lamination material in an electric machine, demonstrating that local magnetic properties are important for achieving excess loss reduction in materials prone to skin effect.