Electrical contact behavior of silver-graphite (75 wt% Ag) brushes

The objective of the research for this dissertation was to investigate the influence of temperature and surface films on the electrical contact behavior of silver-graphite (SG 142, 75w% Ag) brushes. To this end, the brushes were tested in ambient air at constant settings of the test conditions, i.e., current density, sliding speed, and brush load. The electrical and mechanical losses through the brushes, as well as the bulk rotor and brush temperatures, were monitored as functions of the elapsed testing time. Contact temperatures, not directly measurable, were estimated using the voltage-temperature relationship.
The performance of the brushes was strongly affected by the nature of the surface films present at the brush-rotor interface. Test data showed evidence for the build-up and breakdown of surface films due to graphite lubrication and also for a persistent surface film of about 10~¹² ohm-m² film resistivity. The coefficient of friction of the brushes was found to increase with elapsed testing time, as the measured contact voltage drop and calculated film resistivity decreased. Brush wear was observed to increase with increasing current through the brushes, The observed brush behavior is explained in terms of an increase in the metallic properties of the surface films during testing.