A hydrogen-cooled turbogenerator operating at 3600 rpm showed the phenomenon of spiral vibration with a forward rotating unbalance vector. The relative shaft vibration measured at the NDE-bearing was close to the trip level. Spiral vibration is observed at various types of rotating machinery with both rotation directions of the unbalance vector, forward and backward. Spiral vibration is caused by a vibration-induced hot spot on the shaft surface generated by friction. The turbogenerator has three bearings: two main bearings and the brush gear bearing. The carbon brushes sliding on the slip rings were identified as the hot spot location. Potential modifications were studied using hot spot stability analyses with a rotor dynamic model of the generator rotor on three journal bearings. The applied method, introduced by J. Schmied [1], allows the handling of general systems. The hot spot model is based on the theory of W. Kellenberger [2, 3] using a thermal equation between the shaft’s thermal deflection and the shaft displacement at the hot spot location. Three different relations between the heat input and the shaft vibration were modeled: heat input proportional to the shaft displacement, to the shaft velocity and to the shaft acceleration. The model in which the heat input is proportional to the velocity is the most suitable variant for slip rings. This was confirmed by comparison with the measured vibration behavior. A modification of the shaft line was selected based on the calculation results and successfully implemented. This generator and other generators with the same modified brush gear unit are in operation since more than four years.

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