Motor Castellation for Enhancing the Passive Stiffness of Bearingless Slice Motors

In bearingless slice motors, the passive axial and tilt stiffnesses acting on the rotor are important for constraining rotor motions. This paper presents a novel approach to increasing these stiffnesses by using axial castellation. In this approach, the stator and rotor surfaces facing the working airgap are formed with teeth (castellation) in the axial direction. This tooth structure enhances magnetic energy variation with respect to axial and tilt motions, and consequently raises the stiffness in these degrees of freedom. The paper also shows that a further increase in these stiffnesses can be achieved by including small radially magnetized inter-tooth permanent magnets located in the slots between the teeth. To support this new design, a reluctance-based circuit model is created to elucidate the mechanisms contributing to enhanced passive stiffness. Finite element simulations are then used for the detailed motor design, and to optimized the passive stiffness via parametric studies. A representative design has been constructed to allow experimental measurement of the axial and tilt stiffnesses. These experimental results demonstrate a significant improvement in the passive stiffness for bearingless motors with the castellation, and with the use of inter-tooth magnets. This paper also discusses alternate motor modifications to enhance the passive stiffnesses.