The influence of residual stress on flux-barriers of non-oriented electrical steel

In rotating electrical machines, cutouts in the rotor laminations control the magnetic flux density distribution in the d- and q-axis of the magnetic core. Guiding the magnetic flux by cutouts leads to very narrow bridges in the electrical steel. However, at the same time, this can limit the maximum speed of a rotor e.g. due to the material's maximum allowed mechanical stress. Therefore, the centrifugal forces confine the achievable power density. The aim of this study is the examination of the effect of flux-barriers fabricated by mechanical embossing on the magnetic properties of non-oriented electrical steel. The embossing process causes a static residual stress distribution and thereby a reduction of the permeability resulting from Villari's effect. The non-oriented electrical steel samples are embossed with varying distances between the barriers and measured at different angles to the barrier edges on a single-sheet tester. A comparison with respect to their directional and embossing geometry-dependent effectiveness as magnetic flux barriers is carried out. A correlation of the results with measurements of the local flux density, obtained by neutron-grating interferometry, is performed in order to enable a consideration of the mechanical embossing on the local magnetic flux density distribution.