The author studies the nonlinear spning characteristics of a magnetic bearing by expeirments and numerical simulations. The experimental system consists of a radial magnetic bearing stator which is located horizontally，a cylindrical iron rotor supported by steel balls，analog PID control circuits and an exciter. The influences of the static load，the exciting force and feedback coefficients of the PID control circuits on the resonance curve of the system are investigated．
As results followings are clarified. Due to the characteristics of linearpower amplifier in the control circuits，the spring constant of the magnetic bearing becomes smaller when the control current exceeds the bias current. Consequently，the characteristic of the bearing becomes soft spring type and inclination of response curve is observed．