Identification of Various Frequency Response Functions for Levitating Rotor System using Active Magnetic Bearings

Frequency response functions (FRFs) of rotor systems can be used as indicator functions for condition monitoring. Component-wise FRFs are of high interest to locate errors in the fault case. To enable continuous monitoring, measurements should be taken during operation. This contribution shows methods of using active magnetic bearings (AMBs) for deter- mining different FRFs of a rotor system. The AMBs are used as combined sensors and actuators and at the same time as support for the rotor. Two different types of FRFs, namely for the free rotor and the supported/bounded system dynamics can be determined from a single experiment. This procedure does not need any change in the assembly, be- cause the AMBs are simultaneously used as bearing and exciter. Using the ground reaction force of the magnetic bearings gives the free rotor's FRF. The supported system's FRF is determined by considering the excitation force only. As show-case, an academic rotor test rig is used with and without rotation to show the general feasibility of the method. To evaluate and interpret the results of the experiments, a numerical model of the rotor using finite-element formulations is used.