Influence of contact geometry and current on effective erosion of Cu-Cr, Ag-WC, and Ag-Cr vacuum contact materials

Effective ac erosion rates were measured for Cu-Cr, Ag-WC, and Ag-Cr butt-type contacts in vacuum contactors, using half-cycle current pulses of 450-600 A rms. The polarity was changed for each operation to ensure uniform effects for both contacts. The contacts parted during the rising current, with the full gap set to 10-30% of the contact diameter. The effective linear volume erosion rate [cm/sup 3//C] was determined by measuring the axial erosion of the contacts versus the number of operations. This was converted to an effective mass erosion rate [/spl mu/g/C], which was significantly smaller than the reported absolute cathode erosion rate based on measured loss of cathode mass with long square current pulses at large fixed gap. The effective erosion rate increased when spiral-slotted contacts were used. The dependence of the effective erosion rate on the gap was studied, and also the distribution of metal droplets on the arc shield. Most of the droplet flux from the gap was close to the plane of the cathode, while a large fraction of the ionized vapor from the cathode spots was deposited onto the anode. The droplets were a significant fraction of the cathode material loss and the overall effective erosion.