ANALYTICAL AND EXPERIMENTAL EVALUATION OF INSTABILITY IN ROTORDYNAMICSYSTEM WITH ELECTROMAGNETIC EDDY-CURRENT DAMPER

There have been a number of papers published that concern the design a nd operation of electromagnetic, eddy-current dampers for controlling lateral vibration of rotating machinery. Many of these papers have inc luded analysis approaches and all have been generally effective for lo w-speed operations. There have been a few reports concerning high-spee d (supercritical) operations and many of these have indicated instabil ity problems, but none of these have provided a valid analysis to acco unt for instability. That is, all of the analytical approaches have ig nored the disk rotation, relative to the magnetic field, and no obviou s sources of instability have been found. In this paper, we will prese nt our work in which we have rederived the analyses of this system in which we have not made the common assumption of no rotation between th e disk and the magnetic field. In this case, the potential of instabil ity for supercritical speed operation is clear and, in fact, the equiv alent negative damping contribution of the eddy-current damper, under these conditions, has a negative effect on the system even if not full y unstable. We have carefully performed a series of experimental tests which corroborate this analytical approach. Finally, we briefly discu ss alternative eddy-current damper design approaches that could be con sidered to provide effective damping at all speeds and avoid these ins tability problems.