Drive characteristics of slotless PM motors

The rare-earth magnet whose BH-product is more than 30 MGOe makes the permanent magnet (PM) motor store attractive from a viewpoint of power density and efficiency. However, as the flux density becomes higher, the magnetic saturation in the stator teeth of the conventional slotted PM motor becomes severe. To overcome this problem, the slotless PM motor is expected to be one solution. In this paper, the basic design procedure of the slotless PM motor and the drive characteristics of the designed motor are presented. The designed motor has a rare-earth magnet on the rotor and the resin-molded helical windings are provided over the inner surface of the stator yoke. Since the inductance is small, the current is distorted, thus increasing the copper loss. The experimental studies of the effect of the current waveform on the performance are presented. By improving the current waveform, the designed slotless motor allows higher efficiency at higher speed.