Based on the principle of the permanent magnet brushless DC motor, the armature reaction magnetic field rotates hoppingly in the armature circumference, and it moves relative to the rotor, so that the permanent magnet and the yoke of the rotor will inevitably generate induced eddy currents. However, the rotor eddy current loss is generally considered to be insignificant, because the space harmonics of the magnetomotive force of the armature reaction are small in the case of integer slots. However, studies have shown that concentrated winding and fractional slot motors are extremely likely to produce significant eddy current losses in the permanent magnets. This is because the armature reaction magnetomotive force (MMF) contains abundant spatial harmonics, and the MMF harmonics rotating forward and backward generate eddy currents in the rotor magnet and core yoke. This situation is further exacerbated by the time harmonics of the phase currents. Due to the relatively low resistivity of rare earth magnets, the resulting eddy current loss may be very large, leading to a rise in temperature and even irreversible demagnetization of some magnets. This situation may occur especially in high-speed, high-speed, or high-load motors.

　　This article is taken from ------"Permanent Magnet Brushless DC Motor Technology"