Cogging torque reduction in axial flux machines for small wind turbines

Abstract

This paper presents a technique for cogging torque reduction in an axial-flux permanent-magnet (PM) generator suitable for use in a small wind turbine. These machines can be compact and have high power density; they are axially short with larger diameter which makes then very suitable for a wind turbine. The inherent cogging torque can cause problems during turbine start-up and cut-in. In this paper, a turbine is characterized via the measurement in a wind tunnel. The characteristics are then used to determine the specification for the generator. A method called “hybrid skew” is proposed for cogging torque reduction. 3-D finite element analysis is applied to evaluate the method and a 88 % reduction of peak cogging torque was obtained when combined with another known technique. This significantly improves the turbine performance at low start-up speed while maintaining high power density. The technique can reduce manufacturing confusion in N/S pole placement and allow a simple magnet shape for lower cost manufacturing.