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Vibration characteristic of a rotor supported by a PID controlled magnetic bearing is investigated. The differential equation of motion is derived with nonlinearities of the magnetic force for the gap, the current, and the characteristics of a current ampli?er together with the decrement of magnetic force due to surface eddy current caused by rotation of the rotor. The in?uence of the rotational speed on the magnetic force is approximated by a fractional function and the coef?cients are determined by experiments. By this approximation, the resonance curves for a major critical speed by the shooting method have good agreements with the experimental results.

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Booktitle: Proceedings of ISMB11