Active Magnetic Bearings used in BW/IP Centrifugal Pumps

Prediction of the dynamic behaviour of pump rotors requires a profound knowledge of fluid excitation and motion dependent forces on all rotor components. In order to characterize these forces at the impellers, a test rig was built, in which a multistage boilerfeed pump is suspended in active magnetic bearings (AMB). External signals fed into the electronic controller of the AMB excite the rotor following well defined orbits. Force and displacement measurements at the bearings provide on-line FRF's which are the basis for identification of fluid forces, for health monitoring / diagnostics, and for modal analysis. This paper describes the AMB's and the control loop. Two excitation techniques are briefly discussed. The parameters characterizing the dynamics of the AMB's are identified. They are related to the layout of the electro magnets and the electronic controller. Extensive lensive calibration of the magnetic forces was necessary to generate correct FRF's. Finally, some motion dependent forces acting on an impeller are shown. These forces are obtained from the FRF's in combination with an indirect identification technique on 11 the flexible pump rotor.