Rotor Dynamic Behaviour of a High-Speed Oil-Free Motor Compressor with a Rigid Coupling Supported on Four Radial Magnetic Bearings

The combination of a high-speed motor, dry gas seals and magnetic beatings realized in this unit facilitates the elimination of oil. The motor is coupled with a quill shaft to the compressor. This yields higher natural frequencies of the rotor than with the use of a diaphragm coupling and helps to maintain a sufficient margin of the maximum speed to the frequency of the second compressor bending mode. However, the controller of each bearing then has to take the combined modes of both machines into account. The requirements for the controller to ensure stability and sufficient damping of all critical speeds are described and compared with the implemented controller. The calculated closed loop behaviour was confirmed experimentally, except the stability of some higher modes due to slight frequency deviations of the rotor model to the actual rotor. The influence of a mechanical damper as a device to provide additional damping to high modes is demonstrated theoretically. After all, it was not necessary to install the damper, since all modes could be stabilized by the controller.