Diagnostic and control features with magnetic bearings

The authors investigate the use of a rotating force field applied to a stationary rotor to investigate the critical speeds, amplification factor, rubs and other rotor dynamic phenomena while the shaft remains stationary. A laboratory rotor is supported by magnetic bearings and the rotating force field is implemented using the magnetic bearing coils. This approach could easily be employed for diagnostic work on either new machines or machines with field problems without high-speed operation. By using various input perturbation signals, such as white noise, sine wave, or step function, and a rotating force field, knowledge of the characteristics of the magnetic bearing properties and of the overall system is obtained. Optimal setting of proportional and derivative gains, which control the stiffness and damping, critical speeds, amplitude response, clearance magnitudes, amplification factor, log decrement, damping factor and mode shapes, can all be experimentally obtained from the magnetic bearing system before the rotor is run up in speed. Diagnostic data are shown to be very similar to data obtained from actual run-up information