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Flux-switching permanent magnet (FSPM) motor is a novel PM machine with doubly-salient structure, having magnets in the stator and employing concentrated stator windings, which attracts considerable interests due to its essentially sinusoidal phase back-EMF waveform. However, to date, this inherent nature of this outstanding feature is not investigated, and will be studied in this paper. A typical 3-phase FSPM motor with 12-stator-teeth and 10-rotor-poles is designed and prototyped. It is found that there is a significant difference in the magnetic paths for the coil windings composing one phase windings, which causes harmonics in the coil back-EMFs to be cancelled by each other, resulting in essentially sinusoidal phase back-EMF waveform. In addition, the influence of the rotor pole width on the deviation of sinusoidal phase back-EMF waveform is evaluated by finite element analysis and the optimal rotor pole width for minimum back-EMF harmonics is obtained and verified by experiments on the prototype motor.