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Cogging torque, as one of the main parasitic demerits of permanent magnet brushless machines, is of particular importance and primary concern during the machine design stage in many high-performance applications. Hence, numerous design techniques have been proposed and employed to effectively alleviate the cogging torque in permanent magnet brushless machines. The effects of rotor step skewing techniques including both conventional and herringbone styles on the cogging torque of permanent magnet brushless machine are comprehensively investigated and compared by synthesized 2-D and 3-D finite-element analysis in this paper. The results have revealed that both the conventional and herringbone rotor step skewing techniques can reduce the cogging torque significantly, but the latter is less effective than the former especially with small skewing step numbers. Moreover, the machine with herringbone rotor step skewing technique has rather peculiar and asymmetric cogging torque profiles, while the machine with conventional rotor step skewing technique exhibits normal and symmetric ones. The validity of the obtained results and findings is underpinned by the experiments on the prototype machine.Index Terms-Axial electromagnetic force, back electromotive force, cogging torque, conventional step skewing, finite-element analysis (FEA), herringbone step skewing, permanent magnet brushless machine, three-dimensional.

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A novel axial flux permanent magnet (AFPM) machine with a Segmented-Armature Torus (SAT) topology is investigated. The machine's key feature rests on a new and simple configuration of laminated stator poles, which allows high flux density similar to that of the radial flux machines to be established. A full analytical model of the proposed AFPM machine is first developed. To overcome the analytical complexity arising from the novel stator configuration, an efficient and yet realistic approximation is used. Then a three-dimensional (3-D) finite element analysis (FEA) model of a 6kW AFPM machine is developed for the validation of the analytical model. Finally, experimental results are obtained from an AFPM prototype motor, which has been designed by the sizing equations developed in the analytical model. The results from the analytical model are then compared with those from the FEA model and the tests of the prototype motor. The validity of the analytical model and the viability of the proposed AFPM machine as a leading contender for in-wheel direct drive traction applications are then confirmed.