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 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 [ 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.
The electrical erosion of high tungsten content, tungsten-copper (7-10 wt. % Cu) was investigated. The contacts were placed in a vacuum interrupter envelope with a background pressure of about 10 6 torr. The contacts switched one half cycle of 60 Hz current per operation. The polarity of the current was changed on each operation. Six contact pairs were investigated. Each pair was subjected to an ever-increasing number of operations: 1K, 5K, 10K, 20K, 40K, 50K, and 60K. The contact erosion was inferred by measuring the linear position of the moving contact terminal. On completing the electrical testing, the vacuum interrupters were dismantled and the contact surfaces were observed. Unlike the previous work on Ag-WC (50 wt. % Ag) and Cu-Cr (75 wt. % Cu), the W-Cu contacts showed a localized build up of erosion products on the contact surfaces even beginning at 1K operations. The experiments were repeated switching a unidirectional current i.e., the contacts remained at the same polarity throughout the experiments. Here an anode pip and a cathode crater were formed immediately. The difference in the topographies of these contacts is discussed in terms of the metallographic analysis of the deposits on the contacts, the erosion deposits on the shields surrounding the contacts and the expansion of the vacuum arc.
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