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

The stamped cut edges of electrical steel NGO laminations in electric motors experience mechanical 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’, which includes the iron losses. More advanced models need accurate data on the cut edge damage width and deteriorated material magnetic behaviour. Currently, significantly varying results have been reported in the literature for cut edge damage, depending on the method used for characterisation, the material and the cutting method. This thesis has focussed on characterising the cut edge plastic damage using different methods and quantitatively relating this to the plastic strain level and hence magnetic property deterioration due to plastic strain. The work then considers the contribution of plastic strain and elastic stress magnetic deterioration on the overall magnetic performance of samples with varying cut edge. The cut edge from a stamped tooth of a segmented stator was characterised for the plastic damage region (width and magnitude) using EBSD kernel average misorientation maps and nano-hardness. The relationship between the characterised cut edge damage and plastic strain level was found using tensile samples tested to varying applied strain levels combined with EBSD and nano-hardness characterisation. Therefore, the plastic strain gradient with respect to distance from the cut edge was determined. It was found that both EBSD and nano-hardness gave similar predictions of plastic strain in terms of damaged area and amplitude, up to 40-50% at the cut edge. The relationship between the magnetic performance and elastic stress, plastic strain, and plastic strain with elastic stress were studied using single sheet tester (SST) measurements. In addition, an SST 3D FEA model was built using COMSOL Multi-2 Physics software with a single layer to represent the cut edge plastic damage on the magnetic performance of NGO electrical steel sheets using a cut edge width defined by the EBSD and nano-indentation test characterisation and plastic strain deteriorated magnetic properties for the cut edge. A two-layer model considering both residual elastic stress and residual elastic stress acting on the plastic deformed region was built. The modelling results were compared to literature SST data for NGO electrical steel samples comprised of multiple strips, and hence number of cut edges. 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 with a first layer width of 180 μm with magnetic properties for an effective plastic strain of 17% under residual elastic stress of -133 MPa and a second layer width of 220 μm with magnetic properties for 0% strain under residual elastic stress of -119 MPa giving good results. The effect of damage with respect to the flux direction was also studied in this thesis, relevant for electric machine laminations where the flux flows parallel and perpendicular to cut edges. An SST 3D FEA model was built using COMSOL Multi-Physics software with SST samples having damaged regions implemented both parallel and perpendicular to the flux path way. A novel large tensile test sample was designed to implement the same volume of damage in different orientations allowing SST samples to be machined and SST tested with damage in both orientations with respect to the SST sample length. Both modelled and measured results showed a difference at the knee point of the BH curve, where perpendicular damage to the flux direction give greater magnetic deterioration