The Effect of Field Damping on Rotordynamics of Non-salient Pole Generators

Abstract

This paper investigates the influence of magnetic field damping on lateral shaft oscillations of non-salient pole generators. Field damping is caused by compensating currents affecting the magnitude and orientation of the magnetic field and resulting lateral forces. These currents can either occur in especially constructed devices, like a damper cage, or simply in the core material as eddy currents. While damper windings are used to reduce torsional shaft vibrations by generating an asynchronous damper torque, this survey reveals that in contrary to intuition, the field damping in general may cause self-excited lateral shaft oscillations leading to noise emission and reliability issues. It is shown that the effect is strongly dependent upon the machine type and the nominal rotational speed compared to the critical speed. The applied approach is analytical taking into account arbitrary lateral rotor motion in the context of linear rotordynamics.