Contact Voltage Drop and Brush Wear Characteristics for Various Silver Content of the Silver Graphite Brush in Slip Ring System

Sliding contact enables the transfer of electrical current between stationary and moving conductors. Typical contacting components are a brush and commutator in direct current machines and a brush and slip ring in alternating-current machines [1]. In recent years, brushes have been installed increasingly severe environments, such as in wind turbine generators and automotive alternators. Furthermore, the brush/slip-ring system for an electrical signal, such as that of radar and computed tomography systems, must have stable conductivity and therefore be reliable and easily maintained. Thus, graphite brushes coated with silver, which has excellent conductive performance, and noble-metal slip rings are widely used. However, the effects of the materials on sliding characteristics have not yet been determined [2]. In this study, we conducted sliding tests for 20 h, using silver graphite brushes (coated with silver contents of 50, 60, 70, 80, and 90 wt%) and a gold-coated slip ring, under a temperature of 20°C and brush current of 9.6 A. Results showed that the contact voltage drop and amount of wear were optimal when using a 60-80% silver brush. In addition, the lubrication effect of carbon is reduced and the amount of brush wear rapidly increased with a carbon occupancy of 40% or less in the contact area of the brush. Then, in the case of the brush current of 9.6A, the 80% silver brush had a ratio of mechanical to electrical wear of about 1:2 while the 90% silver brush had a ratio of about3:1.