Energy and Power Engineering, 2011, 3, 221-226 doi:10.4236/epe.2011.33028 Published Online July 2011 (http://www.SciRP.org/journal/epe) Copyright © 2011 SciRes. EPE Experimental Modal Analysis of Stator Overhangs of a Large Turbogenerator Ram Sewak, Rajesh Ranjan, Vivek Kumar Heavy Electrical Equipment Plant (HEEP), Bharat Heavy Electricals Limited, Haridwar, India E-mail: {ramsewak, rranjan, vivek}@bhelhwr.co.in Received March 12, 2011; revised April 10, 2011; accepted May 3, 2011 Abstract Modal analysis of engineering structure for comprehending/resolving the vibration related issues/problems are well known. Two classical techniques-analytical (Finite Element Method-FEM) and experimental (Im- pact 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 pre- sent study, experimental technique was used in mitigating the endwinding vibration problem of a turbogen- erator. In one of the Turbogenerators of 50 Hz variant, an increasing vibration trend was observed with sys- tem frequency sweep in almost whole of the endwinding basket particularly more on exciter end during sus- tained 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 indi- cated 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. Keywords: Turbogenerator, Endwinding Vibration 1. Introduction Modal analysis is a well known technique for analysis and resolution of vibration problems of engineering structures. However, this has recently found use in ana- lyzing the turbogenerator stator endwinding vibration. Its principal co-ordinates—natural frequency, damping and associated mode shape are the functions of structure’s geometry, mass, stiffness, temperature, and boundary conditions. Generator stator endwinding is one of the most inten- sively stressed unit components of a turbogenerator. Endwindings can be excited by electromagnetic forces due to the stator and rotor currents in stationary and tran- sient conditions at simple and double system frequency, core vibrations at double system frequency, unbalances in the rotating shaft at system frequency being transmit- ted to the endwinding via the housing and core [1]. The present study relates to a Turbogenerator of 50 Hz variant, where increasing vibration trend of nearly whole endwinding basket was observed during sustained short circuit and open circuit conditional runs, particularly when the system frequency of prime-mover was varied from 47.5 Hz to 51.5 Hz. The trend was more dominant on Exciter End (EE), compared to Turbine End (TE) (Figures 1 and 2). In view of this, it was decided to carry out investigation of such a vibration behaviour of end- winding structure by experimental modal analysis. This involved: 1) Impact testing of overhang structures on specific points and generation of FRFs in standstill condition in local and global mode; 2) Extraction of modal parameters from FRFs gener- ated-frequency, damping and mode shape; 3) Validating the results during operation of the gen- erator by actually tuning the prime-mover’s frequency. This papers deals with experimental study conducted on overhang structures in a limited frequency span, con- cept of theoretical deformation shape of 4-node mode/ 2-lobe mode for a two-pole generator, its practical im- plication on overall vibration behavior of endwinding basket, rectification measures implemented and their effectiveness validated using measured data. 2. Vibration Monitoring Largely, piezoelectric accelerometers were used for endwinding vibration monitoring. However, lately, Fi-