Flux-modulated-type magnetic gears are expected to be applied as a step-up gear for wind-power generation because of their advantages, including high torque density and maintenance-free operation. Furthermore, the flux-modulated-type magnetic gears with an axial-flux structure have attracted great attention recently since they have a smaller axial length and are relatively easy to assemble. In some cases, magnetic gears are required to have an integer gear ratio based on the design requirements of the entire system, which results in larger cogging torque in the high-speed (H-speed) rotor that causes vibration, acoustic noise, and startup error. A skewed rotor structure is well known to be capable of reducing the cogging torque; however, it is complicated and poor in assembling, especially in a large-scale axial-flux magnetic gear (AFMG) used for the wind-power generation. To resolve the above problem, this article presents two new types of pole pieces for reducing the cogging torque: one is an unequal-space type, and the other is an unequal-width type. The validity of the two newly proposed types of pole pieces is proved by both a 3-D finite-element method (3D-FEM) and experiment.
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