Magneto-optical and field-metric evaluation of the punching effect on magnetic properties of electrical steels with varying alloying content and grain size

The punching of electrical steel sheets to manufacture iron cores for electrical machines can influence drastically the final magnetic properties of stator and rotor cores. In this paper, the influence of punching on the magnetic properties is studied on 5 mm wide ring core samples. Six different fully processed non-oriented electrical steels with varying alloy content and varying grain size were investigated. Both global (integral) as well as local magnetic characterisation is carried out. The global magnetic properties are obtained by field-metric magnetic measurements. Punching leads to an increase in the global ring core losses: for the specific case of 5mm wide punched rings, this increase is in the range of 10% to 30% (for losses at 1 T and 400 Hz) depending on the investigated material (this range corresponds to a loss increase of 2.5 to 5.5 W/kg). The local magnetic degradation due to punching becomes more important when approaching the cutting edge. For local magnetic characterisation the magneto-optical Kerr-effect is used. The method allows the observation of the magnetic domain behaviour near the cut edge. Moreover this magneto-optical method allows for a quantitative analysis of the effect of punching on the local magnetic contrast as a function of distance from the cut edge. In addition also micro hardness measurements are performed near the cut edges to determine the mechanically affected zone. Furthermore the influence of punching on the investigated local and global magnetic properties is discussed as a function of the differences in alloying content and grain size of the investigated materials. For the high alloyed electrical steels, the material with medium grain size deteriorates less due to punching than the material with high grain size. This can be observed both from the dc magnetization curves which exhibit higher permeability and from the iron losses at medium frequency (400Hz) after punching. At 50Hz the losses remain lowest for the high grain sized material, even after punching.