Halbach Magnetization in very Thin NdFeB Rings

Due to advances in computational power of digital signal processors it becomes possible to replace drives with mechanical bearings by electrical machines featuring a magnetically levitated rotor. Such machines are more and more favored due to their unique properties, for instance no wear, low friction and the possibility to achieve very high rotor speeds. A bearingless slice motor is an embodiment which allows for a very compact design of the overall drive. While the motor torque and the radial deflection of the rotor are controlled by a single winding system, the axial deflection and tilting are solely passively stabilized. Many applications require a large outer diameter on the rotor. This follows that the radial dimensions of the electromagnetic drive components should be minimized. This paper deals with the research on a bearingless motor with a rotor that shows no back iron but only a very thin NdFeB ring with Halbach magnetization. As such Halbach magnet rings are not for sale off the shelf, a magnetizing coil was optimized to ensure ideal magnetization characteristics. Thereby, the limits of the magnetizing facility in terms of maximum energy storage, and maximum terminal voltage and current had to be considered. A prototype of the magnetizing coil and the considered bearingless slice motor are presented. Measurements verify the calculation results and the impression of the Halbach magnetization to the magnet ring.