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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

Active magnetic bearings (AMBs) allow a lubricant and friction free support of structures, e.g. rotors. Important advantages of this ecologically friendly technology are that there is no systematical requirement for maintenance as well as the possibility to adjust bearing parameters to various operating conditions. The most significant drawback of AMB-technology is that even a single failure within one sub-assembly may cause a break down of the bearing load capacity. The risk of an unintentional machine standstill combined with the resulting costs is one major reason that the broad industrial acceptance of active magnetic bearings is quite low at the time. To increase AMB reliability at reduced costs a new AMB concept has been developed by the authors. It comprises a highly integrated DSP controller and a reliable switching amplifier (RSA) along with a spatial and galvanic isolation between the digital controller and the signal conditioning circuitry from the power units. To investigate the increase in reliability using the RSA instead of standard industrial switching amplifier an analysis of the RSA as well as a comparison with industrial switching amplifier concepts is presented in this paper. Considered are the failure rates of the individual components and sub-circuits, the circuit topologies as well as different maintenance strategies. It is shown that the mean time to failure (MTTF) of the RSA is up to a factor of 21 higher compared with conventional industrial switching amplifier concepts.

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.

Active magnetic bearings (AMBs) offer a new way in bearing technology for supporting structures as, for example, platforms or rotors in rotating machinery. Compared with conventional journal bearings or rolling element bearings AMBs possess substantial advantages: No lubricant and no friction due to non-contact suspension; no maintenance required, bearing parameters as stiffness or damping may easily be adapted to various operating states; for example, when passing a rotor through its critical speeds; instantaneous engine start-up without warm-up of bearings etc. However, the most significant drawback of AMB-technology is the breakdown of the load capacity if one single component fails. This may cause an unintentional machine standstill and is one of the major reasons why the industrial level of acceptance for AMBs is quite low at the moment. Some concepts to realize higher operational reliability of AMB-systems may be found in the literature. In general, they are based, more or less, on multiple redundancy of single AMB-components, which has some significant drawbacks: high costs, higher weight and higher space requirements. In spite of these drawbacks there is still a remaining risk of failure, for example, due to a missing concept for safe decoupling of defect modules or coupling of working ones. In this paper an alternative concept is presented which substantially increases the overall reliability of AMBs. It is based on a complete spatial and electrical isolation of the digital controller and the signal conditioning circuitry from the power units (magnetic actors, amplifiers). Only the switching amplifier is implemented in a modular redundant way. In case of a fault the faulty module can be replaced during operation (hot-swap).