Study of anti-demagnetization property for a flux-shunt permanent magnet DC motor

Rare earth magnets of high-energy products have been widely used to improve the efficiency, as well as the power and torque per unit weight, of permanent magnet machines. In addition to limited resources and high prices, the rare earth magnet may risk irreversible demagnetization during field weakening or malfunctioning in machine operation. To solve this problem, this research investigates the anti-demagnetization property due to the flux shunt mechanism and hybrid magnetomotive force in a permanent magnet direct-current motor. The hybrid magnetomotive force is provided by low-grade permanent magnets and additional field windings, between which are ferromagnetic media that shunt the adjustable magnetic flux in the machine. An optimal machine design is performed and verified with a finite element analysis. A prototype is fabricated, and the experimental results show that machine efficiency and torque are increased by increasing the current of field winding and that machine speed can be extended by reducing the field current. Finally, the anti-demagnetization mechanism is proved effective by investigating the reduction of back electromagnetic force after demagnetization.