Simplified mathematical model of stiff rotor supported by controlled active magnetic bearing (AMB) is studied with the main attention to the influence of two types of strong nonlinearities. Great displacements cause impacts in emergency retaining bearing (hardening nonlinearity). Limited output current of power amplifier introduces the softening nonlinearity into the control feedback loop. After ascertaining the stability domain for small oscillations, the behaviour at high amplitudes, when strong nonlinearities come into action, is analysed by means of method of equivalent linearization and of numerical simulation. The mutual effect of both strong nonlinearities and existence of quasiperiodic and chaotic oscillations etc. are shown on examples and will be illustrated by response curves, phase-plane trajectories, time histories and Poincare mapping.
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