Abstract: In this study, we investigated the torque characteristics of large low-speed direct-drive permanent magnet synchronous generators with stator radial ventilating air ducts for offshore wind power applications. Magnet shape optimization was used first to improve the torque characteristics using two-dimensional finite element analysis (FEA) in a permanent magnet synchronous generator with a common stator. The rotor step skewing technique was then employed to suppress the impacts of mechanical tolerances and defects, which further improved the torque quality of the machine. Comprehensive three-dimensional FEA was used to evaluate accurately the overall effects of stator radial ventilating air ducts and rotor step skewing on torque features. The influences of the radial ventilating ducts in the stator on torque characteristics, such as torque pulsation and average torque in the machine with and without rotor step skewing techniques, were comprehensively investigated using three-dimensional FEA. The results showed that stator radial ventilating air ducts could not only reduce the average torque but also increase the torque ripple in the machine. Furthermore, the torque ripple of the machine under certain load conditions may even be increased by rotor step skewing despite a reduction in cogging torque.

Key words: Permanent magnet synchronous generator (PMSG), Radial ventilating air duct, Torque ripple, Step skewing, Magnet shape optimization, Finite element analysis, Wind power

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