Influence of machine symmetry on reduction of cogging torque in permanent-magnet brushless motors

Abstract

We study two methods to reduce cogging torque, both based on the analysis of the rotational symmetry in a brushless permanent-magnet motor. The first one seeks to lower the symmetry in the flux of a conventional motor with asymmetrical magnet distribution. The second one uses auxiliary slots to increase the frequency of the cogging. Analytical calculations have been made to predict the harmonic spectra of the models. Numerical calculations by the finite-element method were carried out. The results support the analytical conclusions. The frequency of the torque pulsation was increased, as predicted. We obtain a peak-to-peak cogging torque reduction of about 85% for each of the two methods. Dynamical calculations show no changes in the mean torque produced. The asymmetrical motor could introduce some ripple due to the EMF shifting.