Design and analysis of asymmetrical rotor for induction motors

Increase of stray losses in the skewed rotor is mainly due to the cross-currents flowing from the conductor into the laminations. These losses are strongly related to the contact resistance between the bar and the laminations which depends on several factors including the casting process. A substantial reduction of cross-current losses can be achieved by changing or removing skew and by employing various slot combinations. Skew removal will cause some parasitic effects such as increase in the noise level and undesirable dips in the starting torque characteristics. To minimize these effects the rotor design needs to be modified and this could be achieved by introducing modulated slots. Two such types of modulated rotor slot designs are proposed in this paper. As a first step in the performance analysis, magnetic forces, which are the major sources of noise and vibration excitations, is considered. In order to study the newly proposed modulated slot designs, a spectral method is employed and analytically calculated electromagnetic forces are validated against FEM results. The analyses shows that the proposed new designs have better force characteristics compared to the conventional design, which illustrate the fact that better performance characteristics can be achieved by removing skew and introducing modulated slots.