Modelling and Control of Power-Converter-Fed Electromagnetic Bearing

Control of electromagnetic bearings (EMB), which are used to support the shaft of a high-speed rotating machine, involves stabilization of an unstable system and high-bandwidth position control of the rotor being supported. The control should be immune to significant vibrations that are encountered in high-speed machines, satisfying stringent International Organization for Standardization (ISO) standards. The controller should also reject disturbances due to high centrifugal forces on the shaft due to inevitable rotor mass unbalance. Also, the control should be robust to wide variations encountered in the plant parameters. A systematic design procedure for the PID controller is presented. This includes identifying possible space of controller specifications, closed-form expressions for the parameters of a PID controller for given specifications, and selecting a controller that satisfies all requirements in the presence of actuator bandwidth limit and plant parameter variations. The proposed procedure is used to design a position controller for a radial electromagnetic bearing of 180-N load capacity. The same is verified through simulations to satisfy stability, bandwidth, disturbance rejection and ISO standard requirements. The controller is shown to perform satisfactorily over the complete range of possible plant parameter variations. The performance is satisfactory even in the presence of actuator bandwidth limitation. The controller design procedure is further validated through simulation and experiment in the context of one-degree-freedom position control of a shaft of mass 2.285 kg.