A new technique of cogging torque suppression in direct-drive permanent magnet brushless machines

In direct-drive electric propulsion systems, where there is no reduction gear to minimize and absorb the adverse effects due to cogging torque generated by the permanent magnet (PM) machine, minimization of cogging torque generation is of particular importance. In this paper, a novel axial magnet pole pairing method is proposed to minimize cogging torque generation in a special three-phase outer-rotor PM brushless machine, which uses uneven stator poles to enhance back EMF. Analytical formulas of the machine's cogging torque are first derived. The new technique is compared with conventional cogging torque suppression methods by means of analytical models and comprehensive two- and three-dimensional finite element analysis (FEA). The FEA results show that the new method not only achieves effective cogging torque reduction, but also results in less magnet material with no compromise on machine performance. Finally, the validity of the FEA model is verified by experimental results from the prototype machine.