Rotation of vibration vector caused by thermally induced unbalance changes is a frequently observed phenomenon in large rotating machinery. The heat arising from the friction losses, which are generated at the interfaces between rotating and statoric components of the machine, is partly absorbed by the shaft. This heat input is typically not uniform around the shaft circumference and the resulting temperature difference causes the rotor to bow. The excitation resulting from the sum of mechanical unbalance and thermal bow will lead to a slowly rotating (in the synchronously rotating coordinates system) whirl vector, whose magnitude can decrease or increase in time.
A generic understanding of this effect (B.L. Newkirk in 1926, [4]) had been followed by a number of physical models representing specific heat exchange mechanisms (W. Kellenberger [3], J. Schmied [6], P. Morton [11]). A hot spot on the shaft surface can be generated at various locations of a shaft-line. Typical components responsible for thermally induced modulation of vibration vector are journal bearings, seal rings, labyrinth seals (in case of a soft rubbing). Furthermore carbon brushes sliding on the slip ring, supplying the DC current to the field winding of the generator rotor, were identified as a source of nonuniform heat input that may excite spiral vibrations (L. Eckert and J. Schmied in [7], [8]). These local heat input phenomena affect consequently the vibration behavior of the overall shaft train.
This paper provides a new approach to the quantitative description of a heat exchange mechanism which leads to the hot spot generation on the surface of a slip ring. A new thermal equation has been formulated, which determines the stability and frequency of the thermal mode. Characteristics of spiral vibration are discussed based on the analytical solution of the Jeffcott rotor model coupled with the proposed thermo-elastic equation.
The implementation of the described method to a full shaft-line model of a combined cycle, single shaft power train was done using the Finite Element Method. The results of this calculation were validated against measurement data. The paper shows how the applied computational approach can be used to extend stability margin of the spiral vibration in turbo-generator shaft trains.
