A Reliable Switching Amplifier driving an Active Magnetic Bearing - Experimental Results

Active magnetic bearings (AMBs) offer some advantages over conventional bearings (contact-free suspension, variation of suspension parameters like stiffness and damping during operation, etc.). A lower reliability caused by a fairly complex assembly of the controller, sensors, electromagnets and amplifiers, however, is one major reason for the rather low industrial acceptance of AMBs. Based on a study on failures in AMB-suspended turbomachines a concept for an AMB with high reliability has been developed by the authors, providing a spatial and galvanic isolation between the controller and the power elements along with a reliable switching amplifier (RSA), implemented in a modular redundant way. Depending on the required reliability, a RSA consists of two or more left and right hand switching amplifier half-bridges. These so called "hot-swap modules" (HSMs) are connected in parallel, each with its own error detection and decoupling circuitry. If an error occurs in a HSM, it is autonomously switched off and disconnected from the AMB-electromagnets, 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 AMB test rig and measurement results of the complete laboratory version of the RSA during induced component errors and the hot-swap of a HSM are presented. Severe half-bridge component errors (IGBT short circuit, open circuit of a free-wheeling diode) were induced for demonstration purposes, normally causing a breakdown of the complete AMB when conventional switching amplifiers are used. An analysis of the RSA output currents of this new concept shows, that there nevertheless remains an undisturbed bearing functionality despite of these manually induced errors