Effecting Critical Frequency Shift in Rotors Using Active Magnetic Bearings

Abstract

Low stiffness bearings are useful to reduce the force transmitted from the vibrating rotor to the surrounding support structure. However, having low stiffness requires us to cross the low rigid body critical frequency while accelerating to operating rpm. In this work the stiffness of the bearing is changed online during operation by using an active magnetic bearing instead of a conventional constant stiffness rolling element bearing. This methodology is shown for a rigid rotor using both simulation and experimental techniques. During acceleration phase, a high stiffness is maintained, which gives us high critical frequency. After acceleration to operating rpm, the stiffness of the bearing is reduced at run time so that the bearing again becomes a soft support. In this work, a thrust magnetic bearing of variable stiffness is used to show that by changing the stiffness at run time, we can avoid crossing the rigid body critical frequency and hence reduce the amplitude of resonant vibrations.