Analysis of Axial Vibrations for Bearingless Segment Motors arising from Parameter Excitation

The bearingless segment motor is a fully magnetically levitated drive system. Due to the disc shaped permanent magnet rotor the axial position and tilting are stabilized passively by means of reluctance forces. Therefore, only the radial rotor position and the motor torque require active control. Because the stator of the bearingless segment motor consists of separated elements and consequently has a non unique stator circumference, the axial stiffness depends on the rotor angle. This leads to Hill’s differential equation describing the axial rotor position. The axial deflection cannot be controlled actively, so that the knowledge of its dynamic behavior is crucial. After modeling, the differential equation is analyzed using Floquet’s theorem. Stability charts indicating stable and unstable operating points for the bearingless segment motor are introduced. Furthermore, the effects of external excitation and damping are taken into account. Finally, measurements on a bearingless segment motor prototype are compared with the theoretical results.