Extraction and elimination of induction machines inherent asymmetry by lamination material anisotropy

The lamination material used in inverter-fed ac machines shows some non ideal effects like magnetic saturation, hysteresis losses, eddy currents, and anisotropy. This presentation focuses on the effect of magnetic anisotropy which can show up in inverter fed operation and may deteriorate the performance if not considered. The manufacturing process with rolling and punching as well as the demand to reduce the sheet thickness in order to keep losses low support the emergence of an anisotropic distribution of the crystalline energy. Though the anisotropy is smaller than in grain oriented lamination material used in transformers it is still detectable as shown by measurements made on specimen discs. In the behavior of inverter fed ac machines this effect is seen in the transient current slope as shown by measurements performed on an especially manufactured induction machine. In the fundamental wave behavior anisotropy can hardly be detected. As modern drive control methods are mostly based on fundamental wave models it has not been considered up to now. However, in the past years first methods have been proposed to exploit also the high frequency and transient electrical behavior of induction machines, especially in the fields of mechanically sensorless control as well as condition monitoring. The influence of the anisotropy effect on the transient current slope is shown by measurement results obtained from rotor laminations manually stacked in order to enlarge as well as to diminish the anisotropy. The separation of the resulting signal from possible rotor eccentricity is done using additional measurement coils in the machine.