Magnet Eddy Current Loss Reduction in Permanent Magnet Machines

Magnet eddy current loss is very important for any types of permanent magnet (PM) machines, especially for high-speed PM machine since it increases significantly with rotating speed and has a great effect on the temperature of a magnet and, hence, the electromagnetic performance, as well as irreversible demagnetization of the magnet. In this paper, a new method is proposed to reduce the rotor PM eddy current loss. The auxiliary slots with an optimized size and position are proposed. By this mean, the asynchronous spatial harmonics caused by the armature reaction and the conventional slots can be partially decreased by the harmonics produced by the introduced auxiliary slots. It is proved that the rated on-load magnet eddy current loss can be reduced as much as 81.5% by this method, whereas the rated on-load torque is only decreased by 4.8% in the 3-slot/2-pole machine. In addition, it also shows that the proposed method could offer a better performance in a machine with relatively larger air gap length and it is also effective for other slot/pole combinations.

I. Introduction

The interest of a high-speed permanent magnet (HSPM) machine has recently grown significantly due to its high efficiency, high power density, as well as small size [1]–[8]. However, as a consequence of asynchronously rotating harmonics, the magnet eddy current loss is caused by the spatial harmonics as well as the time harmonics, which have significant influence on the machine performance [9]. This part of loss is much more important for high-speed machines since it increases with rotating speed significantly, and affects the temperature of magnet and, hence, the output torque and efficiency, as well as the potentially irreversible demagnetization of PMs.