Optimisation of the design parameters of an asymmetric brushless DC motor for cogging torque minimisation

This paper presents an investigative study into minimising the cogging torque of a novel three phase surface mounted PM brushless DC motor. The motor is novel because it has a non-integer stator-rotor pole ratio and also asymmetric stator pole arcs. In the paper, the results of the work demonstrate how the optimum stator pole asymmetric arc ratio and pole shoe radial depth or thickness can be found to minimise the cogging torque and to maintain a trapezoidal back EMF shape. An accurate 2D finite element modelling FEM software was employed to numerically analyse the magnetic field distribution inside the motor. The aim of the work is to optimise the design parameters for minimising the cogging torque. The results show that there is one stator pole arc ratio and pole shoe depth combination that produces the lowest cogging torque whilst maintaining a near trapezoidal back emf shape.