Simulations of Brush Contacts of Homopolar Motor/Generators

Naval Research Laboratory, Washington, DC

Saturday, December 01 2007

Time-dependent distributions of electromagnetic force and heating were studied.

A computational- simulation study of distributions of electric current and temperature in brushes and slip rings in two model homopolar- motor/generator configurations was performed in support of the development, by the U.S. Navy, of a superconducting homopolar motor/generator (SCHPMG) machine for ship propulsion. Electrical-contact performance (more specifically, brush/slip-ring contact performance) is a limiting factor in the performance of an SCHPMG machine. The present study and similar studies are needed to gain understanding of brush/slip-ring contact performance in order to enable optimal design of brushes for homopolar machines.

At the interface between a brush and a slip ring, where transfer of electric current occurs, factors that affect wear include the spatial distributions of electric current, magnetic field, temperature, and electromagnetic forces as the brush slides at a given velocity on an imperfect rotor (slip-ring) surface. There is experimental evidence that wear is not only highly asymmetric between anode and cathode brushes but is also nonuniform within the contact area of an individual brush. The asymmetry and nonuniformity of wear limits the life and efficiency of a brush assembly, inasmuch as the full volume of brush fiber material is not fully utilized. Moreover, debris from worn brushes can give rise to internal short circuits and/or other adverse effects on the operation of a machine containing a brush assembly.