Reducing Torque Ripple in PM Synchronous Motors by Pole Shifting

An inherent drawback of permanent magnet synchronous motors is the torque ripple caused by stator and rotor anisotropy and non sinusoidal distributions of the PM flux-density and stator mmf. A classical method to minimise the torque ripple consists in skewing the stator or the rotor. However such a method causes difficulties in motor manufacturing and it is generally avoided. Also alternative techniques as fractional-slot windings, diverse pole widths, dummy slots or teeth cause manufacturing troubles or are well-suited only for low power motors. A technique based on an appropriate shifting of identical width rotor poles is presented in this paper. It can be applied to both surface-mounted PM and interior PM sinusoidal-fed motors of any size. Hints to design rotors of any number of pole pairs are also given. The results achieved by the proposed technique are verified by a finite element analysis of the motor performance and their validity is confirmed by experimental tests on a prototype.