A hybrid foil-magnetic bearing (HFMB) was successfully studied as a vibration isolator by introducing a sudden imbalance or an unexpected disturbance during turbine/rotor operation. This HFMB is used to achieve stability during transient vibration behavior. The HFMB consists of two oil-free bearing technologies: an active magnetic bearing (AMB) and air foil bearing (AFB). Using both technologies takes advantage of the strengths of each bearing while compensating for their inherent weaknesses. In addition, the HFMB has good dynamic characteristics, and the damping can be adjusted using the appropriate gain selection for the AMB controller. Based on these unique features, dynamic stability can be enhanced, even if a sudden imbalance occurs while the rotor is operating. In this study, a rigid rotor was operated at up to 12,000 rpm and tested using a control algorithm to reduce the sudden imbalance vibration amplitudes. The experiment was conducted under the situation that the mass dropped out at 6,000 rpm. In order to validate the stability performance of the HFMB with a sudden mass loss, the vibration response results for the AFB and HFMB were compared. When applying the HFMB, the asynchronous vibration was suppressed, and the 1x vibration results showed reductions of almost 30%. When the sudden mass loss occurred, the magnetic control force was remarkably effective at reducing the asynchronous vibration of the rotor supported by the HFMB. In conclusion, it was experimentally verified that using the HFMB made sudden imbalance vibration control possible during rotor operation with an air foil bearing. In this respect, the HFMB has the characteristics of high stiffness/damping, which prevent rubbing and suppress excessive vibration due to a sudden imbalance event.

A hybrid foil-magnetic bearing (HFMB) consists of an air foil bearing (AFB) and an active magnetic bearing (AMB). The HFMB, inherently proposed as a backup bearing for an AMB, has many advantages, such as good controllability and the ability to exhibit preload sharing with the two types of bearings (i.e., the AFB and AMB) in high-speed turbomachinery. However, because the bearing has a limited clearance, the eccentric position of the rotor affects its stability and the reliability parameters of the AFBs such as the initial preload rub. In this study, a rigid rotor supported by an HFMB was operated at speeds of up to 18 kr/min and was tested using a proportional-derivative control algorithm, in order to reduce the vibration amplitude. In addition, to elucidate the effect of the initial eccentric position of the rotor, the control algorithm was started from the initial position of the rotor (X: from –100 to 100 µm and Y: from –80 to 200 µm) using a constant gain value. When the HFMB was active, the magnetic control force was remarkably effective in reducing the subsynchronous vibration of the rotor supported by the HFMB. Eccentricities of 0.2–0.5 corresponded to appropriate rotor positions for the hybrid bearing, and the corresponding load distribution of the AFB was found to be the optimal one. In addition, the proportional-derivative control gain was not very high. The performance of the bearing could be improved further by controlling the eccentricity. An HFMB was tested experimentally, and it was verified that it is possible to determine the effective load carrying capacity for a specific load distribution of the AFB.

In this study, experimental and analytical analyses of the vibration stability of a 225 kW class turbo blower with a hybrid foil–magnetic bearing (HFMB) were performed. First, critical speed and unbalance vibration responses were examined as part of the rotordynamic research. 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. 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 rectifying rigid mode unstable vibration at low speed, 20,000 rpm, and asynchronous vibration due to aerodynamic instability by using HFMB with vibration control. 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 HFMB–rotor system shows that oil-free turbomachinery can achieve excellent performance.