Investigation of Magnetic Drag Torque in Permanent Magnet Brushless Motors

In permanent magnet (PM) brushless motors, a magnetic drag torque, which results from the effect of magnetic hysteresis, exists at low speeds but is often neglected. However, in some applications it can have potentially serious consequences. In this paper, a practical method for predicting the magnetic drag torque in a PM brushless motor is presented. It is based on the fact that the average drag torque over one mechanical revolution can be determined from the energy associated with excursions around appropriate hysteresis loops of the steel. Finite element analysis is employed to calculate the variation of flux in each element throughout the stator core, as the rotor rotates, and, hence, the hysteresis loss. The method is applied to two PM brushless motors having 18-slots/12-poles, 12-slots/10-poles, respectively. The predicted magnetic drag torques are compared with the measurements