Experimental Modal Analysis of Stator Overhangs of a Large Turbogenerator

摘要

Modal analysis of engg. structure for comprehending/resolving the vibration related issues/problems are well known. Two classical techniques-analytical (Finite Element Method-FEM) and experimental (Impact testing/Natural Frequency Test-NFT/Bump test) are generally used as complementary as well as stand-alone depending on the time, nature of structure, availability of the analysis tools, cost etc. In the present study, experimental technique was used in mitigating the endwinding vibration problem of a turbogenerator. In one of the Turbogenerators of 50 Hz variant, an increasing vibration trend was observed with system frequency sweep in almost whole of the endwinding basket particularly more on exciter end during sustained short / open circuit conditional runs. Experimental modal analysis was carried out of the overhangs. Frequency response functions (FRFs) were generated in local and global modes. The analysis thereon indicated global resonance of stator overhangs. Accordingly, appropriate remedial measures were planned and implemented. Consequently, global resonance frequency was shifted to higher zone, which in turn, resulted into substantial reduction in endwinding vibration levels.