Rotor Eccentricity Effect on Cogging Torque of PM Generators for Small Wind Turbines

This paper discusses the effect of rotor eccentricity on cogging torque of a permanent-magnet (PM) generator applied to small wind turbines. The 1.4 kW radial-flux PM generator was designed to improve the start-up wind speed ($<$3 m/s) by reducing the cogging torque. Finite element analysis (FEA) is first used to simulate the generator for a range of eccentricities and their corresponding cogging torques. In experiment, the cogging torque of a prototype generator with a particular eccentricity of 0.25 mm (33% of the air-gap length) is measured and the result matched that of the FEA. This gives confidence in terms of using FEA for further studies. The relationship between the starting torque provided by turbine blades and the cogging torque is also discussed. From the simulation, it is found that the dynamic eccentricity can be detected by measuring the cogging torque waveform.