Resonance mode detuning in rotor systems employing active and passive magnetic bearings with controlled stiffness

Abstract

In the paper, the investigator offered a method of reducing the amplitude of vibrations of turbo machinery rotors with passive and active magnetic bearings in resonances and resonance zones corresponding to one of the critical speeds ranging from zero to operating rotational ones. The method was based on the ability to vary the nonlinear force characteristic and the damping properties of active magnetic bearings and passive magnetic bearings of a novel design by changing the electric parameters of electromagnet circuits. The offered design of a passive magnetic bearing with permanent ring magnets and a control winding enabled a short-time change of rotor bearing stiffness. Such rotor bearings can realise the method of detuning from critical speeds during rotor speed-up and run-down. Earlier, this method was feasible only when active magnetic bearings were used. However, a complete magnetic suspension of a rotor with the employment of two radial passive bearings and one axial active magnetic bearing was advantageous as compared to three active magnetic bearings. The feasibility of the detuning method had been substantiated by study results. The study presented the results of numerical analysis, which simulated the process of transition through resonances of the initial system with a significant reduction of vibration amplitudes. The nonlinear system of differential magnetic-mechanical equations was solved using the 5th-order Runge-Kutta method with a validation for the duality of solutions. The results were shown as 3-dimensional spectra of displacement of bearing points and amplitude-frequency responses. These results had been confirmed by experimental data obtained for a laboratory setup that implemented a combined passive-active magnetic suspension of a rotor. The overall total relative error of measurements was maintained at less than 0.5%. The discrepancy between the experimental and design data for amplitudes was within 2-3%, and for resonance frequencies, it was less than 0.2%. Hence, these studies substantiated the feasibility of passive magnetic bearings with controlled stiffness. They also confirmed the possibility of using passive-active magnetic suspensions for lightweight rotors (e.g., expanders and compressors) with the implementation of the suggested method of detuning from resonance modes.