Application of Perturbation Methodology and Directional Filtering for Early Rotor Crack Detection

This paper documents analytical and experimental research of the lateral and torsional responses of a cracked rotor to different types of excitation. The experimental research has been performed on a rotor rig, which emulate a turbogenerator. It includes driving motor coupled to the main rotor, a lateral nonsynchronous perturbation device, and a generator with an electrical field consisting of a constant component (constant torsional load) and a sinusoidal component, provided by a signal generator. The generator was used as a torsional nonsynchronous perturbation device. The midspan of the rotor was modified so that a section could be changed starting with a circular cross-section (undamaged rotor) to the cross-section with transverse crack. The lateral and torsional responses have been measured at two axial locations. The obtained lateral data was processed using directional filtering into forward and reverse components of the corresponding filtered elliptical orbits. The forward component of the lateral response to nonsynchronous perturbation allows to identify overall stiffness reduction and rotating stiffness asymmetry introduced by the change in midspan rotor cross-section, while the reverse component largly depends on the support asymmetry. The nonsynchronous torsional excitation allows identify of the system torsional dynamic stiffness and it’s reduction due to the crack. The ratio of the filtered to 1× or to the perturbation frequency rotor responses at two axial locations was considered as an indicator of a lateral mode shape change due to the crack. The experimental results are compared with the analytical model of the rotor response, which was obtained by the application of a perturbation method of small parameter to the system of nonlinear equations. The equations describe the rotor system with four lateral (two displacements and two inclination angles) and two torsional degrees of freedom.