Study on Enhancement and Weakening of Permanent Magnet Air Gap Flux Density

In a conventional permanent magnet (PM) machine, the air gap flux produced by the PM is fixed. It is difficult to enhance the air gap flux density because of limitations of the PM in a series magnetic circuit. However, the air gap flux density can be weakened by using power electronic field weakening to the limit of demagnetization of the PMs. This paper presents an analytical study for controlling the PM air gap flux density through a stationary brushless excitation coil. The air gap flux density can be either enhanced or weakened. There is no concern with demagnetizing the PMs during field weakening. The leakage flux of the excitation coil through the PMs is blocked. The prototype motors built on the principle of this analytical study confirm the capabilities of significant flux enhancement and weakening.