Production and performance of metal foil brushes

Abstract

Electrical brushes which were made of silver, copper and aluminum foils of 12.5 and 25 μm thickness and were composed of 15–195 individual foils, were tested in purified argon on a polished copper rotor at a speed of 13 m s −1. Brush pressures varied between 3.1 × 10 3 and 2.8 × 10 4 N m −2 and current densities were up to about 700 A cm −2 (about 4500 A in −2). The observed dependence of the voltage drop across the brushes as a function of the current densities agreed closely with Holm 's contact theory as applied to foil brushes. The film resistivities were found to be near σ F = 10 −12 Ω m 2 for copper and silver and to be about 3 × 10 −12 Ω m 2 for aluminum. The projected performance of foil brushes based on these results is very favorable and the future commercial use of foil brushes appears to be possible. The total loss, electrical and mechanical, through the brushes is independent of current density if the brush pressure is chosen to minimize the total loss. If so, the loss depends only on the brush speed, the hardness of the softer of the two materials involved (i.e. of foil and rotor or slip ring), the coefficient of friction and the film resistivity. Microscopic surface examinations of rotor and brushes show that the brush surface is smoothed through running the brush, whereas the rotor remains almost unaffected or is mildly roughened, as long as no arcing takes place. Arcing causes considerable surface roughening on both the brush and the rotor surface and debris is thus deposited on the rotor; this can score the brushes. Further experiments are required to determine the rate of brush wear.