This paper deals with a synchronous vibration instability that occurred in a two-stage overhung centrifugal compressor supported by magnetic bearings. The authors encountered an unbalance vibration that increased spirally in a polar plot at/near the first bending critical speed. The concentration of iron loss and thermal bending due to heat have been identified as the causes of the phenomenon, because the vibration stopped increasing when unbalance force rejection control (UFRC) was applied. In this paper, prior to an in-depth discussion of experiments on the above phenomenon, the compressor and magnetic bearing system are described. To provide a theoretical perspective, a model of the thermally induced vibration is presented and the stability is discussed. In the experiments, to exceed the first bending critical speed stably, balancing of the rotor under UFRC was carried out and rapid acceleration/deceleration was applied to the variable-speed drive system. The vibration behaviors around the critical speed were measured and the results verified the theoretical model. To evaluate the stability limit of the thermal bending, a method of measuring the model parameter that determines the stability is proposed and the measured data are compared with calculated results. Finally, methods for improving the stability are discussed.
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