Magnet asymmetry in reduction of cogging torque for integer slot axial flux permanent magnet motors

Cogging torque component is one of the key issues in the design of radial and axial gap permanent magnet motors and the geometry and the position of the magnets plays a critical role in this process. This study presents the influence of magnet asymmetry in minimisation of the cogging torque for axial flux permanent magnet (AFPM) disc motors. Cogging torque minimisation techniques, such as magnet skewing and magnet shifting or groping are examined in detail by using three-dimensional finite element analysis with various types of magnet skewing alternatives since the shaping and the positioning magnets are low cost approaches to minimise or eliminate the cogging component. A prototype AFPM disc motor is built with shifted magnets based on the models which are the results validated with the experimental results for the minimum cogging component and the sinusoidal back-EMF waveform. It is shown in this study that the cogging component in AFPM motors can be practically eliminated by using magnet asymmetry and perfectly sinusoidal back-EMF can also be obtained even for integer slot/pole/phase motors.