Torque Interaction of the Drive and Active Radial Magnetic Bearing in an Ultra-High Speed Spinning Ball Motor

The ongoing miniaturization trend of electric machines increases the demand for higher rotational speeds to provide a required power level at decreased size. The goal of this project is to push the limits of rotor miniaturization by researching new concepts for ultra-high speed machines achieving rotational speeds above 20 million rotations per minute (Mrpm). Such high rotational speeds can only be attained by limiting the centrifugal loading on the rotor, which can be accomplished by decreasing its size to less than 1 mm in diameter. Furthermore, the rotational losses have to be minimized. This requires precise and fast frictionless magnetic bearings for stabilization of the rotor in all dimensions, which interfere with the drive system of the motor by generating a breaking torque and torque ripples. The interaction is studied in this paper based on an analytical model and transient finite element method (FEM) simulations. The occurring torque components are identified and guidelines for the design of the radial magnetic bearing are provided. By incorporating these guidelines into the machine design, acceleration of the rotor to the desired rotational speeds can be achieved.