Vibration Sensitivity of Large Turbine Generator Shaft Trains to Unbalance

A shaft train in power plants is comprised of many journal bearings supporting turbines, generator and exciter. Each single rotor may possess one or more natural frequencies below or close to the rated speed. The entire shaft train often possesses many coupled or uncoupled natural modes close to the rated speed which influence the lateral vibration behavior in the normal operation. Some modes are low damped and some highly damped. It is difficult to judge how sensitively the machine reacts to theses modes and to make an acceptance design criteria for them. ISO 21940-31 makes some recommendation about vibration sensitivity of the damped modes of simple systems with rotors having only one resonant speed over the entire service speed range or widely separated resonance speeds. But it cannot be applied to evaluate a large turbine generator shaft train with many interacting resonances. This paper proposes two methods—sum of unbalance responses and sum of modal magnification factors to analyze the vibration sensitivity of a rotor system with many frequencies in the vicinity of the service speed.