Finite element analysis of the contact problem between graphite-brushes and — Slip rings in double-fed asynchronous generators

The slip ring system is used in double-fed asynchronous generators in wind turbines. The rotor current flows through the resulting contact surfaces between the graphite-brushes and slip rings, passing over a frequency converter into the electrical grid. While transmitting power, sometimes micro brush fires in the area of contact occur, which are caused by the separation of the contact components. Influences to that phenomenon are the electromagnetic forces, radial eccentricity and electro hydrodynamic forces. In this paper, the nonlinear characteristics of steady-state contact problems between graphite-brushes and slip rings and the influence of the electromagnetic forces in the contact areas are analyzed. By using ANSYS software, a finite element model (FEM) of the contact problem is established and simulated. For validation, the main parameters of the contact area are also calculated by using the Hertz contact theory. The electromagnetic forces are mathematically determined from the contact areas and current amplitude in the components. The results obtained by simulation and theoretical analysis are both compared and it is shown that the simulation of the contact between graphite-brushes and slip rings and the electromagnetic forces is feasible.