High-Current Brushes, Part IV: Machine Environment Tests

Brush performance has been investigated with a view to establish whether or not continuous very high-current density operation is feasible for many parallel brushes running under machine environment conditions. The ultimate objective is the development of sliding-brush contact systems for advanced powerful electrical machines that require large currents in order to achieve high efficiencies and small volumes. Using a homopolar generator designed for that purpose, the performance of a full complement of 92 brushes (46 in parallel per polarity) was evaluated under conditions that included electrical loads to 9500 A (1.6 MA/m²) and a slip ring speed of 42 m/s. Based on test data presented, asymmetry in the anode and cathode brush electrical contact resistances and wear rates of about 2:1 were found. Under the maximum imposed load conditions, the anode and cathode brush interface contact voltage drops are 0.17 and 0.08 V, respectively. Typically, the highest dimensionless linear wear rate (anode brush) is 1.7 x 10^-11. This wear rate is about one-sixth that of diesel-electric locomotive motor brushes, which slide on contact surfaces of similar peripheral speed but with less than one-tenth the electrical load imposed here. The high feasibility demonstrated for multiple-brush high-current operation is attributed to the use of silvergraphite brushes (0.75 mass fraction silver), a nonair (e.g. humidified carbon dioxide) environment, and cooling of the brush holders and slip rings.