High Speed, High Current, Copper-Finger Brushes for Pulsed Homopolar Generator Service
ln continuing research into higher energy density, higher current rated pulsed homopolar generators (HPG's), solid copper fingers are explored as sliding electrical contacts to slip speeds of 300 m/s and collector area current densities above 75 MA/m2. Achieving increased compactness in HPG's is easily accomplished by spinning the inertial energy storage flywheel to higher tip speeds but generally involves brush operation at higher Slip speeds. Sintered copper-graphite brushes presently limit slip velocity to 220 m/s and collector area current densities to 7-15 MA/m2, if a reasonable brush wear rate is expected. Development of copper finger brushes began in 1981 by Marshall who performed low speed (15 mm/s) testing at current densities above 450 MA/m2. Based off encouraging results obtained by Marshall, the present investigation focused on characterization of copper finger brushes in an actual HPG operating environment. Experimental data were collected using the Compact HPG System Tester, a working 5-MJ HPG which can deliver current pulses in excess of 1 MA into resistive loads for the evaluation of brush performance. The copper finger brushes tested successfully transferred 1.15 MA at a slip speed of 100 m/s and currents of 600 kA at 180 m/s. Experimental evaluation of effective contact resistance as functions of slip speed, Current transferred, and brush normal force was performed, and the results are discussed. Interfacial heat fluxes due to friction and contact voltage drop for the test conditions were also determined, and then extrapolated to operating levels of 300 m/s and 3 MA. These projections were used to determine the feasibility of rejecting the heat flux through direct injection (transpiration) cooling methods. Although problems with coolant delivery and hydrodynamic forces do exist, the approach holds promise in the continuing effort to extend pulsed HPG brush operating limits.