Rotordynamic Stability Evaluation With Constructed Forward Whirl Signals Based on Full Spectrum Technique: Numerical and Experimental Investigation

Abstract

Abstract The rotordynamic stability evaluation by measuring the damping ratio or log-dec of the first forward whirl mode in the shop test is of importance to reduce the risk of instability fault in field. This paper focuses on the signal processing of inverse Fourier transform based on the full spectrum technique to reduce the modal overlap of closely-spaced first order modes (in which the amplitudes of the forward and backward responses are comparable) in the time domain. By using this transform method, forward and backward whirl signals are obtained, and blocking excitations in x or y direction can also achieve the same effectiveness as the forward and backward excitation with the active magnetic bearing (AMB) when there is a significant cross-coupled stiffness (CCS). The envelope of the analytical signal obtained by Hilbert transform is used to analyze the component of the excited vibration and show the effectiveness of weakening the overlap of the closely-spaced twin modes. The result from the numerical investigation indicates that for the rotor-bearing system with a significant CCS, the constructed forward and backward whirl signals are clearly dominated by the respective forward and backward modes with no significant overlap. Besides, experiments are also carried out in a nearly industrial centrifugal compressor test rotor with AMB, which is used as an actuator to apply blocking excitations for rotordynamic stability tests, with result that the unidirectional excitation is enough to estimate the log-dec of the rotor system with a slight CCS. Thus, the traditional circular forward/backward whirl excitation and its associated complicated equipment such as AMB is unnecessary and replaceable, providing a sight to simplify the complicated equipment only by using the signal processing techniques when the rotating machinery has an either slight or significant CCS.