Nonlinear Stability Analysis of Active Magnetic Bearings

In order to get a better understanding of the dynamics of active magnetic bearing (AMB) systems under extreme operating conditions a nonlinear model for a radial AMB system is investigated. Instead of the common way of linearizing the magnetic forces at th.e center position of the rotor with respect to rotor displacement and coil current, the fully nonlinear force to displacement and force to current characteristics are used. The nonlinear model employed in this investigation is an advanced version of a simple nonlinear model, which has already been published and discussed in [Steinschaden, Springer, 1999].. Magnetic saturation effects, saturation of the amplifier and limitations of the control currents are additional nonlinear components modeled in this work. The AMB system is excited by unbalance forces of the rotor. Especially for the case of large rotor eccentricities, causing large rotor displacements, the behaviour of the system is discussed. A path-following analysis of the equations of motion shows that for some combinations of parameters well-known nonlinear phenomena may occur.