[131] Torsional vibration encoder-based monitoring methods and their experimental validation

J Liska, J Jakl and S Kunkel

University of West Bohemia in Pilsen, NTIS – European Centre of Excellence, Technicka 2, Plzeň 30100, Czech Republic. 
Tel: 420 377 632 597; Email: kunkel@ntis.zcu.cz 

Excessive torsional vibration of turbine generator rotors and torsionally induced blade vibration result in fatigue cracks arising at the shaft surface or blade roots. If not detected at an early stage, severe damage to the machine can occur. Torsional vibration, however, is not measurable with standard vibration sensors and thus a specialised measurement must be used. In this paper, two encoder-based measurement methods are discussed. The first method uses an optical sensor together with a reflexive zebra-tape encoder. The second method uses an eddy current sensor together with a copper incremental encoder. For both methods the measured signal quality and its usability for torsional analysis is compared. Experimental data were obtained from a test-rig and from a steam turbine generator. Both methods prove to be suitable for obtaining the times of arrival of encoder markers used for instantaneous angular velocity and torsional vibration evaluation. However, the eddy current method is better suited for practical applications where dirt and contamination by oil is a concern.