Improvement the Prediction Accuracy of Critical Speed and Damping Ratio by using Modal OpenLoop Characteristics

Abstract

In a rotating machine, when the frequency of an unbalanced force acts on the rotating shaft, a resonance phenomenon occurs in which the vibration amplitude of the rotating shaft is maximized, where the rotational speed at this time is called the critical speed. In most cases, rotor designers layout the rated speed to be under a certain critical speed with the smallest margin possible. Therefore, the vibration at the rated speed becomes highly sensitive to the unbalanced force, inducing extremely large unbalance vibrations. We compensate this abnormality through field balancing to achieve safe rotational operation within small vibrations. In these balancing procedures, we recognize that the critical speed certainly exists near the rated speed within small margin, but the exact value remains unknown from these test runs and even after shipping. This paper proposes a method to predict the critical speed and damping ratio by the modal open-loop characteristics rearranged around the critical speed from unbalance response vibration (closed-loop) data measured in the balancing runs. The proposed method was verified using the results of a Hardware in the Loop simulation to show that the critical speed and damping ratio can be predicted without surpassing through the critical speed.