Analysis of Axial Leakage in High-Speed Slotless PM Motors for Industrial Hand Tools

This paper presents an analysis of axial-leakage effects in high-speed slotless permanent-magnet (PM) motors for industrial hand-tool applications. Analytical predictions based on a 2-D model and results from a 3-D finite-element (3DFEM) simulation model are compared with experimental results from three series-produced PM motors. The reduction in flux density (resulting from 3DFEM simulations and measurements) outside the motor's axial ends is substantial; the flux density has practically vanished only a few millimeters outside the stator lamination stack. To predict the resulting flux linkage, a simple winding model is presented where the winding is made up of a discrete number of winding layers. Using this winding model, it is verified that the reduction in flux linkage, manifested as a reduction in the back electromotive force, is not only dependent on the motor's axial length but also that the effect is minor for slotless PM motors with dimensions suitable for hand-tool applications. The results are also supported by no-load line-to-line voltage measurements on the three PM motors.