Phase shift method for radial magnetic force analysis in induction motors with non-skewed asymmetrical rotor slots

Abstract

Skewing which is commonly used in smaller sized induction motors for the reduction of magnetic noise causes higher cross currents in the rotor bars. This leads to a substantial increase of stray losses in the skewed rotor. Reduction of these losses can be achieved by eliminating skew, but at the cost of increased noise level and torque ripple in the motor. A design change is proposed and studied in this paper to minimize the increased noise level by introducing asymmetrical rotor slots. A simpler method of analysis using numerical calculations for the radial air gap forces is proposed and the vibration behavior of the motor is explained. The method is based on phase shift analysis of radial force waves in the air gap of the machine computed at certain equidistant points in the air gap with respect to the rotation of the rotor. Phase information obtained from the Fourier analysis is analyzed to identify and separate the order of the radial force waves i.e., the various modes of vibration. The final results showed improved force spectrum for the newly proposed asymmetrical rotor designs from which better performance of the newly proposed rotors is predicted. This method of analysis is compared with the previously obtained results from the noise measured on a standard motor and the results show a reasonable reliability of the proposed method to predict the noise level in the machine.