A Reliability Analysis of Switching Amplifier Concepts for Active Magnetic Bearings

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.