Mitigation of Cogging Torque in a Double-Stator Single-Rotor PMSG for Direct Drive Offshore Wind Turbines

Double-Stator-Single-Rotor (DSSR) surface-mounted Permanent Magnet Synchronous Generators (PMSG) have less active weight in contrast to the traditional direct drive high-power generators used in offshore wind turbines. But due to the insertion of an extra stator in a DSSR PMSG, cogging torque becomes more prominent, as it is produced due to the interaction of the rotor magnets with slots and teeth of the stator. Therefore, there is an imperative need to explore the most effective methods to diminish cogging torque in a DSSR PMSG. This paper investigates different geometry parameters including the ratio of magnet pole arc to pole pitch, eccentricity of PM poles and relative positioning of inner and outer stator slots for reduction of cogging torque in the generator. The influence of these cogging torque mitigation methods is analyzed analytically. Moreover, the geometry of the DSSR PMSG is optimized to minimize cogging torque by employing Genetic Algorithm (GA) based on the analytical results. To demonstrate the accuracy of the proposed analytical calculation of cogging torque, 2D Finite Element Method (FEM) of DSSR PMSG has been performed, which yields equitable results.