Analytical Model for Slotless Permanent Magnet Axial Flux Motors

Permanent magnet axial flux motors are very well adapted to applications requiring a very short active length and a big diameter. They also have been used for micro-motors. Because of their size, these motors require the use of a permanent magnet and their winding is usually located in the air gap between stator and rotor. The size of the magnet is very small and the number of poles can be relatively high to generate a higher value for the Back-EMF at low speed. Therefore the effect of the magnetization process of the permanent magnet cannot be neglected. The paper presents an analytical method for accurately calculating the flux density in the air gap, the back-EMF and torque of such a motor. This method can account for the permeability of the permanent magnet, the finite permeability of iron and the transition zone between two consecutive poles. It provides a quick way of determining and optimizing the performances of a slotless axial flux motor. The results of the model have been compared with measurements done on several prototypes of a micro-motor and there is good correlation.