Carrier signal injection alternatives for sensorless control of active magnetic bearings

This paper analyzes different forms of carrier (high frequency) excitation for sensorless control of three-pole active magnetic bearings. Three options are compared: rotating injection, pulsating injection at a fixed angular position, and pulsating injection at an angular position synchronous to the minimum reluctance axis. Criteria for comparison include sensitivity to displacement changes, parameter and operating point dependence and signal processing burden for signal isolation. Theoretical derivations, simulation results and experimental results are included. The system model and the practical implementation will be shown to share similarities with high frequency injection sensorless methods for AC machines but also to have significant differences mainly on how pulsating carrier signals are used.