Selective Elimination of EMF Harmonics with Stepped Skewing for Ripple Torque Reduction in Surface-Mounted Permanent Magnet Motors

Abstract

This paper proposes a design method for stepped skewing of rotor magnets to selectively eliminate the harmonics of the no-load electromotive force (EMF) and the ripple torque, which is generated by the interaction of the stator current magnetomotive force with the rotor magnet flux distribution. The method is based on the theory that a harmonic component of the ripple torque is generated by the corresponding two harmonics of the no-load EMF. An elementary unit of the proposed skewing for canceling out one harmonic EMF consists of two identical magnet blocks skewed axially. For suppressing a harmonic ripple torque, the corresponding two harmonic EMFs are canceled out by skewing two identical elementary units or a four-step skewing. In the paper, the skew angle for canceling out each harmonic EMF is theoretically obtained. It is shown that one harmonic ripple torque is almost completely suppressed by the proposed four-step skewing through two-dimensional and three-dimensional finite element method (FEM) analyses. In addition, experiments are performed to confirm the selective elimination of one harmonic EMF and to validate the FEM analyses. By using the proposed method, a multiple-step skewing can be designed to simultaneously suppress more than one harmonic component of the ripple torque.