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 and results from a finite-element simulation model are compared to experimental results from three, series-produced PM motors. The reduction in predicted and measured flux density 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 (EMF), is 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.