A reliable switching amplifier for active magnetic bearings error detection strategies and measurement results

Active magnetic bearings (AMBs) offer some advantages over conventional bearings such as contact-free suspension and variation of suspension parameters as stiffness and damping during operation. A lower reliability caused by a fairly complex assembly of the controller, sensors, actuators and amplifiers, however, is one major reason for the rather low industrial acceptance of AMBs. A study on failures in 200 magnetically suspended turbomachines clearly states that almost exclusively the switching amplifiers and their DC/DC-converters caused substantial electronic component failures. Based on the results of this study an AMB is being developed by the authors, providing a reliable and economic solution by means of spatial and galvanic isolation between the controller and the power elements along with a reliable switching amplifier (RSA). Depending on the reliability required, the RSA consists of two or more left and right switching amplifier half-bridges. These so called "Hot Swap Modules" (HSMs) are connected in parallel, each with its own error detection and decoupling circuitry (EDC). If an error occurs in an HSM, it is autonomously switched off and disconnects itself from the AMB-actuator, while the remaining, fully functioning HSMs take over the required current. The faulty HSM can be replaced by a functioning one during full operation of the AMB in a hot-swap procedure. In this paper the error detection and decoupling strategies along with explanatory measurement results of the first RSA-prototype are presented. All considered fault-scenarios (malfunctioning of switching elements, errors within the control signals, etc.) as well as the tasks of the EDC are explained. Measurement results show the reliable detection of induced component errors and the nevertheless almost undisturbed functionality of the RSA.