en-Circuit Field istribution in a

An analytical model has been developed for calculating the open-circuit time-varying magnetic field distribution in the airgaplmagnetlretaining sleeve region of a brushless permanent magnet motor equipped with diametrically magnetised magnets. It accounts for the stator slotting and curvature, as well as for parasitic eddy currents induced in the retaining sleeve and magnets. The analytically predicted time-varying field distribution in the different regions is compared with corresponding finite element predictions. I. INTRODUCTION Due to their high efficiency, small size and light weight, high-speed electrical machines are likely to be a key technology for many future applications of motion control and drive systems. Fig.l(a) shows a 20,000rpm brushless permanent magnet motor with a 3-slot stator carrying a concentrated winding, and a 2-pole rotor with diametrically magnetised sintered NdFeB magnets which produce an essentially sinusoidal airgap flux density distribution. Because of the high speed operation, the permanent magnets are usually retained within a stainless steel sleeve 111. However, both the sleeve and the magnets are exposed to high order stator slotting harmonics which cause. eddy current losses. In order to calculate these losses, the accurate prediction of the time-varying magnetic field distribution in the airgap/magnet/sleeve regions is essknzal (2) (3). Boules (3) presented an analytical rectangular model for the prediction of both the time-varying field distribution and the corresponding eddy current losses in the sleeve/magnet region by use of an equivalent current sheet to represent the MMF of radially magnetised magnets. However, such a representation is inappropriate for a rectangular model of motors having diametrically magnetised magnets. Therefore, in this paper an alternative 2-d rectangular analytical model, Fig. 2(b), for calculation of the open-circuit time-varying field distribution is proposed. It can account for stator slotting effects and curvature, and predicted results compare well with corresponding finite element predictions.