Active magnetic bearings of high reliability

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).