RIGID MODE VIBRATION CONTROL AND DYNAMIC BEHAVIOR OF HYBRID FOIL-MAGNETIC BEARING (HFMB) TURBO BLOWER

Abstract

In this study, experimental and analytical analyses of the vibration stability of a 225 kW class turbo blower with a hybrid foil-magnetic bearing (IIFMB) were performed. First, critical speed and unbalance vibration responses were examined as part of the rotordynamic research. Then, an experimental double suction turbo blower with an HFMB was built. The turbo blower consisted of an impeller at each end and a permanent magnet motor in the center. Its shaft diameter was 71.5 mm, its total length was 693 mm, and the weight of the rotor was 17.8 kg. The air foil bearing (AFB) utilized was 50 mm long and had a 0.7 aspect ratio. The results of analyses indicate that rigid mode (conical mode) occurred close to 8,036 rpm, and the results of natural frequency analysis and dynamic behavior prediction of the rotor-bearing system were similar to those obtained experimentally. However, in the experiments conducted, excessive vibration and rotor motion instability occurred in the range 12,000-15,000 rpm, which resulted from insufficient dynamic pressure caused by the length of the foil bearing being too short. Consequently, as the rotor speed increased, excessive rotor motion attributable to aerodynamic and bearing instability became evident. This study therefore focused on improving rotordynamic performance by rectiing rigid mode unstable vibration at low speed, 20,000 rpm, and asynchronous vibration due to aerodynamic instability by using HFMB with vibration control. Although the normal operating speed is 39,000 rpm, the experiments were conducted at 20,000 rpm. The experimental results obtained were compared for each bearing type (AFB and HFMB) to improve the performance of the vibration in the low speed region. The experimental results show that the HFMB technology results in superior vibration stability for unbalance vibration and aerodynamic instability in the range 12,000-15,000 rpm (200-250 Hz). The remarkable vibration reduction achieved from vibration control of the hybrid foil-magnetic rotor-bearing system show that oil-free turbomachinery can achieve excellent performance.