Metallic Brushes for Extreme High-Current Applications

Recent studies of homopolar machines and electric vehicles have generated a renewed interest in sliding contacts which can carry a very high-current density. Simple metallic brushes were shown to be capable of the efficient transfer of very high currents under proper conditions of mechanical load and with control of the surrounding atmosphere. In stable operation under a carbon dioxide cover gas, copper fibers have transferred 12.4 MA/m2(8000 A/in2) with contact drops less than 50 mV. During a 240-h test, the dimensionless wear rate was found to be 5 x 10-11. Current densities as high as 23.2 MA/m2were demonstrated in short-term tests. To evaluate the feasibility of practical application a larger copper fiber brush was assembled and tested in a standard holder with a conventional spring loading system. The observed contact voltage drop was quite similar to that found for the smaller brushes. The test results conformed to elastic contact theory and indicated that the controlling resistive component was due to film rather than constriction. The ability to use metallic contacts under a controlled atmosphere provides an opportunity for the investigation of sliding electrical contacts without the complexity introduced by excessive oxide formation and without the added parameters that would be introduced by graphite or other lubricant films.