Active Magnetic Bearings for the Identification of Dynamic Characteristics of Fluid Bearings-Calibration Results

Active magnetic bearings, a typical mechatronic product, are getting more and more common in turbomachinery. Besides their function of an oil-, contact- and frictionless levitation of the rotor (Schweitzer et al, 1993), they are best suited to be used as a measurement instrument to extract more information from the system under observation. The aim of the presented DFG-project (German Research Council) is to use active magnetic bearings as an accurate measurement and excitation device for the identification of linear and nonlinear dynamic characteristics of flow medium lubricated joumal bearings. This type of bearing is often used in centrifugal pumps as an alternative to conventional oil-lubricated bearings. Contrary to oil lubrication, the flow medium often has a very small viscosity, which leads to a turbulent lubricating film. This turbulence causes essential changes in the dynamic characteristics of the bearings. The influence of joumal bearings on the dynamics of the pump system is caused by Fluid- Structure-Interactions (FSI) (Gasch, Pfiitzner, 1975, Matros et al, 1994). FSI occur in the presence of relative motion between rotating and stationary part of the bearing. In the presented project the active magnetic bearings are on the one hand used to levitate the rotor and to operate as actuators to generate the relative motion. On the other hand, the magnetic bearings are used to measure the enforced displacements and the resulting forces induced in the rotating structure in a very accurate way.