The mounting errors of displacement sensors, the influence of rotor weight and processing quality will make the working position relative to the reference location center of the rotor shift for the magnetic bearings. In different equilibrium positions have different stiffness, so the stiffness namely variable stiffness changes with the actual work situation for the magnetic bearings. In the paper, a kind of AC radial hybrid magnetic bearing is taken as the research object. Different from the traditional expression of the invariable stiffness coefficient, the expression of variable stiffness coefficient is deduced, the influence of the ratios of eccentricity and the bias magnetic field to the stability of magnetic bearing is analyzed, and the stability region of the magnetic bearing is also acquired. And then refer to the analytical characteristics of variable stiffness coefficient, a prototype of AC hybrid magnetic bearing and its control system are designed, and the experiment is done. The displacement waveforms for different stages of the rotor including floating, suspended and suspended with outside interference are tested. The results of the experiment show that the magnetic bearing has good dynamic, static and anti-jamming characteristics, so the digital control system of the radial AC hybrid magnetic bearing can meet performance requirements of the magnetic bearing perfectly, and the stability region of magnetic bearing determined by characteristics of variable stiffness coefficient can supply the theory gist to design and analysis of magnetic bearings.
