Quantitative characterisation and modelling of the effect of cut edge damage on the magnetic properties in NGO electrical steel

Abstract

The cut edges of electrical steel NGO laminations in electric motors experience mechanically induced plastic deformation and residual elastic stress, which deteriorate the magnetic performance of the material. This deterioration is accounted for empirically in electric machine modelling tools by introducing a ‘build factor’ on the iron losses. Currently, a wide range of cut edge deteriorated widths have been reported in the literature, depending on the method used for characterisation, the material and the cutting method. In this paper, cut edge plastic damage characterisation was carried out using EBSD kernel average misorientation (KAM) maps and nano-indentation to quantify the magnitude and width of the plastic damage at the cut edge for a stamped tooth of a segmented stator and a guillotined single sheet tester (SST) sample. The relationship between the EBSD KAM and nano-hardness values and plastic strain was found using tensile samples tested to varying applied strain levels. Therefore, the plastic strain gradient with respect to distance from the cut edge was determined. It was found that both EBSD and nano-indentation gave similar predictions of effective plastic strain values and damage width: up to 1.45–1.50 at stamped cut edge over 180 µm width and up to 1.35–1.4 at guillotine cut edge over 250 µm width. The relationships between the magnetic performance and elastic stress, plastic strain, and plastic strain with elastic stress were determined using single sheet tester (SST) measurements. In addition, SST 3D FEA models of the cut edge sample were built using COMSOL Multi-Physics software considering a single layer for plastic damage only and two layers for plastic damage + elastic stress and elastic stress. The modelling results for the guillotine cut edge SST samples were compared to measured data. It was found that to obtain accurate cut edge magnetic property deterioration for NGO electrical steel laminations both the plastic strain and residual elastic stress cut edge effects need to be included. In the cut edge model, a first layer width of 250 µm with magnetic properties for an effective plastic strain of 1.17 under residual compressive elastic stress of −133 MPa and a second layer width of 220 µm with magnetic properties for residual compressive elastic stress of −119 MPa give excellent results for hysteretic BH curves and specific iron loss evaluation.