This paper is concerned with the dynamics of a rigidrotor supported by journal and thrust active magneticbearings. For small perturbation, the action of the thrustbearing can be described by a series of stiffnesscoefficients. The system equations are formulated bycombining the equations of motion of the rotor and theequations of the decentralized PID controllers. Theeigenvalues of system are calculated and examined. Theresults indicate that the thrust magnetic bearing has greatinfluence on the eigenvalues corresponding to lowfrequencyconical whirls, and causes the system stabilityto degrade.
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