Permanent magnets segmentation effect on cogging torque of axial flux machines for wind turbines

Abstract

In wind turbine generating systems, Axial Flux Permanent Magnet Synchronous Generators (AFPMSG) are becoming increasingly popular due to many advantages they offer. Nevertheless, these machines suffer from the so-called cogging torque resulting from the interaction between the permanent magnets and the slotted armature. Not only it causes noise and vibration, this cogging torque affects the selfstart ability of wind turbines at low wind speed. Therefore, the aerodynamic torque •generated by the rotor blades should overcome the cogging torque anytime during the operation otherwise the wind turbine may not come out of stall and never start resulting thus in a loss of electric energy output. Therefore, minimizing its effect is a major design concern for a reliable and smooth operation of small wind turbines. This report presents a new method for reducing cogging torque based on stacking and shifting rotor magnets in the normal direction. First, the exact magnetic field distribution is computed using Maxwell's equations in magnetostatics. This analytical model takes into account the armature slotting effect and the multilayer permanent magnets configuration. Then, the cogging torque is computed by means of Maxwell's stress tensor. The accuracy of the proposed model is validated by finite element analysis. Simulation results show that a substantial peak magnitude reduction can be achieved. Finally, the results of this work are published in the International Journal of Renewable Energy Research (IJRER). This scientific contribution describing the aforementioned model with the corresponding salient points and numerical simulation will appear in Vol. 8. 2018.