Effect of equilibrium position offset on rotordynamics of a rigid rotor supported by hybrid foil magnetic bearings: Numerical and experimental investigations

This study investigates the influence of equilibrium position offset on the stability of a hybrid foil magnetic bearing (HFMB)-rotor system. Experimental tests and theoretical calculations are conducted using an established setup comprising the HFMB rotor system and a PID control system. The effects of equilibrium position offsets on resonance peaks, 1/2X subsynchronous vibration, and whip are analyzed through waterfall plots, synchronous vibration plots, rotor center orbit, and Fast Fourier transforms. The results reveal that the impact of equilibrium position offset on different vibration components varies due to distinct mechanisms of induced vibration. Surprisingly, the offset does not necessarily decrease system stability. Negative offset of the equilibrium position in the vertical direction mitigates tangential friction and 1/2X subsynchronous vibration. Conversely, positive offset of the equilibrium position in the vertical direction enhances system stiffness and the instability threshold of whip. These findings offer guidance for future PID controller designs for HFMBs and demonstrate the exceptional flexibility and controllability achievable with HFMBs.