Performance of doubly salient permanent magnet motors for parallel and tapered rotor poles

This paper presents the results of a finite element (FE) analysis carried out to study the effects of variation of rotor pole arc and rotor pole shapes on the performances of a doubly salient permanent magnet (DSPM) motor; based on which a clear comparison can be done on two types of rotor pole geometries namely, the parallel and tapered. Two-dimensional (2-D) FE models of a 8/6 DSPM motor for parallel sided and tapered rotor poles with rotor outer pole arcs varying from of 15 degrees to 38 degrees have been analyzed with appropriate phase excitations. It is observed that in all the cases, the tapered rotor pole gives higher average torque. The maximum average torques in both the cases, occur for an outer pole arc: of 22 degrees, where the maximum torque in tapered case is more by 1.8%. The average torques in both the cases varies similarly for rotor outer pole arcs above 22 degrees and up to 30 degrees. This is also true in case of torque ripples. For outer rotor pole arcs of less than the optimum 22 degrees, it is observed that the tapered case is better; at 15 degrees pole arc, the average torque developed by the motor is 14.8% higher than the corresponding value of parallel-sided case. The magnet utilization in case of the tapered poles is found to be better for rotor pole arcs up to 26 degrees.