A Numerical Approach for Accurate Prediction of Magnetic Field in Permanent Magnet Motors

To support very dense data storage on recording disks, the spindle motor is to provide high precision rotation with minimal jitter in speed and minimal departure from perfect rotation. In designing such motors, accurate prediction of the magnetic air gap field is essential for analysis of cogging effect and validation of design choices. The analytical solutions for instantaneous air gap magnetic field in PM motors were discussed using the relative permeance function. However, the relative permeance function only accounts for the peak value of flux reduction in the slot center. Therefore, whilst the prediction of the radial forces is usually of satisfactory accuracy, the tangential flux density derived was not accurate especially in the area of magnet pole transition passing over slot opening. As a result, the prediction of cogging effect can be very poor for some pole and slot number combinations. From the flux density predication between FEM calculation and previous analytical solution, it is observed that the radial field component of the existing analytical solution has good accuracy. However the tangential field component is not so satisfactory, especially over the region when the magnet transition is close to slot opening. Such inaccuracy is the main cause of inaccuracy in cogging torque calculation. In this paper detailed analysis of the tangential force due to pole transition passing over slot opening is given. It is shown that such component predominantly contributes to cogging effect. A practical approach based on a combination of numerical and analytical methods is introduced for accurate prediction of the magnetic field and cogging effect.