Current Tracking Performance of an SiC Power Amplifier for Control of Electromagnetic Bearing With Good Disturbance Rejection

Electromagnetic Bearing (EMB) is an electromechanical system that provides contactless support to the rotor of a high-speed rotating machine through magnetic forces. Excitation coils of an EMB are fed from current-controlled power electronic converters for high-bandwidth closed-loop control of the radial disturbance forces on the rotor. Accurate current tracking by the power electronic converter is crucial in EMB systems to reject disturbance forces. This paper reviews the modeling of EMB and several disturbance forces acting on rotor. Simulation results are presented with a nonlinear model of EMB under transient and periodic disturbance forces. Reference coil currents required to be tracked by the power amplifier are generated using simulations. Closed-form expressions for current controller parameters are derived, considering control loop delay and bandwidth requirements. The current tracking capability of the converter and controller for generated current references is tested on an EMB prototype. Experiments show that the designed power amplifier and current controller can accurately track the reference currents required to reject disturbance forces due to mass unbalance, coupling misalignment and those encountered in applications like machining tools and centrifugal pumps. Further, the current controller is capable of tracking these currents at different airgaps, demonstrating its robustness to variation in plant parameters due to variation in airgap. Additionally, the force generated by the EMB coil on the rotor sleeve while tracking such current shapes is measured and presented. The proposed testing approach can evaluate the performance of the converter and controller for various EMB applications without requiring a complete physical setup.