Optimisation of Magnet Shape for Cogging Torque Reduction in Axial-Flux Permanent-Magnet Motors

This paper presents a new method for reduction of the cogging torque in axial flux permanent-magnet motors based on the shape of the edges of the magnets. First, starting from a previous work, developed for 2D radial flux motors, and extending it to the case of 3D axial flux motors, the new cogging torque cancellation conditions are set. From these conditions, the optimised profile of the magnets is derived, for which an original and precise method based on superposition has been developed. Furthermore, the proposed method does not need Finite Element Method (FEM) iterations to achieve the required precision, which makes it really very fast. The cancellation condition is fully compatible with the optimisation of other characteristics related to magnets, such as the waveform of the induced EMF and the torque ripple. Both the new cogging torque cancellation condition and the magnet profile design method to meet this condition constitute the main contributions of this work. The results obtained are in good agreement with those obtained by FEM simulation. Finally, the proposed method of minimising the cogging torque has been successfully validated experimentally by employing three different configurations of a single-stator single-rotor axial flux motor. The practical reduction of the obtained cogging torque is noticeable even when using non-skewed magnets and when the number of slots is an integer multiple of the number of magnets.