Permanent-Magnet Demagnetization
Permanent-magnet demagnetization is a reduction in a motor magnet's retained magnetization after temperature or an opposing magnetic field moves its operating point away from the intended region.
Reversible and irreversible loss
Magnet flux normally changes with temperature and can recover as the magnet cools. Irreversible demagnetization occurs when heat and an adverse field push part of the magnet beyond the recoverable portion of its magnetic characteristic. The result can be local rather than uniform across the rotor.
In an EV permanent-magnet motor, high rotor temperature, strong negative direct-axis current, a short-circuit event, or another severe opposing field can combine to reduce margin. Controlled field weakening is therefore not the same as demagnetization, but the controller must keep its current command inside the motor's temperature-dependent limit.
Material grade, magnet shape and segmentation, rotor magnetic circuit, cooling, current limits, and fault strategy all affect resistance to demagnetization. An irreversible loss can reduce back-EMF, torque per ampere, and available performance.
For motor topology and thermal protection, see Permanent-Magnet Synchronous Motors and Electric Motor Cooling in EVs.