Impacts of cogging torque and its reduction for an external rotor permanent magnet alternator

The outer rotor radial flux permanent magnet alternator (PMA) is more suitable for low-speed applications than axial flux or inner rotor radial flux PMA. With the reduction of rotational speed the cogging torque and ripple torque magnitude increases significantly which is known as pulsating torque. The increment of these torque factors superimpose on generated torque thereby degrading the quality of the output power. This superimposed torque increases body vibration and creates extra noise. This paper examines the impact of these torques especially the cogging torque, and extracts it from the actual torque to minimize its impacts on the performance of the machine. By using analytical method, total cogging torque has been derived and correlated from the cogging torque by Fourier series expression of a single stator slot. Some factors or parameters like fractional slotting, cogging torque goodness factor have been considered, examined and modified by using these derived expressions which are directly involved in cogging torque manipulation to minimize the cogging torque from an extremely low-speed outer rotor PMA arrangement to improve the performance and stability of the machine. The design aspects of the PMA have been examined using finite element method (FEA) for its higher accuracy.