| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 99 === Elimination of instabilities existing in rotating mechinery, including fluid whirl, fluid whip and dry whip, is imperative for safe and stable operation of rotating machines with journal bearings. When the plain cylindrical journal bearing in rotating machinery is lightly loaded, perhaps due to radial loads on the rotor that act against gravity, the shaft journal can move to low eccentricity ratios, and the bearing can become fully flooded. When this happens, it can trigger fluid-induced instability. This study develops an electromagnetic actuator with a feedback control system to tackle the kinds of instabilities. The actuator can be used as a generator of variable stiffness. The actuator mounted near the fluid-film bearing at a rotating machine incorporates the fluid-film bearing in parallel configuration to provide higher stiffness for eliminating fluid-induced whirl instability, and it mounted near mid-span at a rotating machine incorporates shaft in parallel configuration to provide higher stiffness for eliminating fluid-induced whip instability. The actuator does not act until the system encounters the fluid-induced instability. When instability occurs, the actuator will be actuated and combined with the fluid-film bearing or shaft stiffnesses to stabilize the rotating machine. For the actuator, favorable force of actuation is reached by changing the control current in the actuator applying the proportional and derivative controls as control algorithm. The proportional gain causes the force to increase as a function of displacements; its function simulates stiffness. The derivative gain causes the force to increase as a function of velocities; its function simulates damping. Moreover, a spring model is used to estimate the stiffness the system needs to stiffen the system and raise the threshold of instability. The proposed method combining the root locus plot is based on an experimental rotor rig model, as well as using MATLAB® for simulation. Experimental results demonstrate that the instability of the rotating machine can be removed effectively. The proposed technique can also be used to diagnose rub and fluid-induced instability existing in this kind of rotating machines.
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