Cogging torque is a common design issue in Permanent Magnet Synchronous Machines (PMSMs). Altering the magnet shape is one of the solutions in reducing the cogging torque. This paper addresses the variation in the rotor magnet pole arc which would affect the cogging torque of the external rotor permanent magnet synchronous motor (ERPMSM). Analytical calculations to design a three-phase, 12slot/8pole (12s/8p) ERPMSM with consideration of major parameters and material properties are presented. A two-dimensional (2D) layout of the ERPMSM is constructed and analysed. The variation in reducing the magnet pole arc is modelled and simulated in a commercial finite element software i.e. OPERA 2D, to illustrate its effects on the cogging torque generated in the machine. After which, the paper describes the possible reasons behind the trend of the changing cogging torque exhibited by variation of the magnet pole arc, and its effects on the phase-back electromotive force (EMF) produced by the ERPMSM.
