53. Shaft Vibration Suppression by Manipulating the Poles

Strong shaft vibration has been recorded at the 40 MW bulb unit A of the Dubrava HPP on the Drava river in Croatia. In order to identify and eliminate the cause, air-gap and magnetic flux distribution data were examined in detail. The understanding of the rotor kinematics and dynamics thus obtained enabled modifications of critical mechanical details, which in turn resulted in substantially lower shaft vibration levels. An alternative solution for suppressing vibration was proposed and preliminarily checked. It consists of bringing the rotor magnetic forces into balance by bypassing the windings of one or two suitably selected poles. Here, a report is given on the full-scale experiment in which this approach was verified. Shaft vibration in a hydro-power unit caused by the spider-to-shaft misalignment can be effectively suppressed by manipulating the magnetic field through bypassing a part of one pole or a pair of poles. The method of selecting the poles that need to be bypassed has been devised and verified. The deterioration effects of bypassing the poles have not yet been studied satisfactorily.