INTEGRATED HYBRID AIR FOIL-MAGNETIC THRUST BEARING (I-HFMTB) PART I: PRELIMINARY EXPERIMENTAL TEST FOR ROTORDYNAMIC PARAMETER IDENTIFICATION AND BEHAVIOR WITH PD CONTROL

Abstract

The selection of a thrust bearing for the operational reliability of turbomachinery is very important because it supports axial loads and influences the lateral vibration caused by the thrust collar tilt. A type of integrated hybrid air foil-magnetic thrust bearing (i-HFMTB) is hereby proposed to overcome the limitations of the air foil thrust bearing (AFTB) and the active magnetic thrust bearing (AMTB). The AFTB is composed of four top foils and three bump strips. It is spot-welded on a non-magnetic Inconel X-750 (relative permeability approximate to 1) pad so that it does not affect the magnetic flux when combined with AMTB. A slot is made so that the pad can be coupled to the outer pole of the AMTB, and the nominal air gap does not increase in the axial direction because of the inserted pad. i-HFMTB is used to design a different area to compensate for the slotted area and the nominal air gap. Moreover, it is unaffected by the flux imbalance owing to flux density differences because of the design. In the experiments involving the i-HFMTB, the test apparatus is supported by an active magnetic journal bearing (AMJB) to evaluate its performance. A PD (proportional and differential) feedback control system was applied to the i-HFMTB, and a Bode plot was obtained using an external excitation signal to determine the efficacy of the design. An experimental percentage overshoot method was used to validate the theoretical dynamic coefficient of the i-HFMTB. In addition, the effect of reducing the lateral vibration through the i-HFMTB at the critical speed of the rotor was experimentally determined. The proposed i-HFMTB can effectively control the rotor system.