Effect of plastic damage orientation with respect to magnetic flux pathway on magnetic performance for NGO electrical steel

Abstract

Non-grain-oriented (NGO) electrical steel laminations are used in electric motor stators and rotors and are usually produced by stamping the complex shape from steel strip. It is known that the cut-edge damage, an unavoidable result of this process, adversely affects magnetic properties. During motor operation, magnetic flux flows through these regions, with pathways that may align parallel or perpendicular to the damage. This paper investigates the effect of flux orientation relative to cut-edge damage on the magnetic performance using both experimental measurements and 3D finite element modelling (FEM) for M250-35A NGO electrical steel. A novel tensile test specimen was designed to allow single-sheet tester (SST) samples (300 x 30 x 0.35 mm) to be extracted with comparable plastic damage levels (average 6.2 % strain) but varying orientations to the magnetic flux pathway. FEM simulations, developed in COMSOL Multiphysics, of the SST samples used BH curves for damaged and undamaged material to determine the magnetic behaviour for the SST samples with the different damage orientations. Experimental and modelled results showed consistent trends, revealing that both damage orientations deteriorate the magnetic performance compared to undamaged material, with damage perpendicular to the flux pathway giving more severe degradation. This effect was particularly notable at the BH curve knee point, where the perpendicular damage sample displayed a 20 % lower magnetic flux density than the parallel damage sample. These results highlight that motor performance will be affected not only by the extent of cut-edge damage but also by the design of the laminations through the magnetic flux pathways relative to the cut-edge damage, especially in areas operating below magnetic saturation.