The unusual electrical erosion of high tungsten content, tungsten copper contacts switching load current in vacuum

Abstract

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.

Electrical Life of Vacuum Interrupters for Load Current Switching

“…The arc melting patterns are very similar to those see in [41] at lower currents of 600 A rms. In [41], arcing tests at alternating polarities observed the formation of polycrystalline W particles on the contact surface over multiple arcing operations, which then were remixed with the surrounding contact material in later arcing operations. This led to the formation of small, irregular features on the main arcing surface.…”

“…Irregular erosion patterns can appear on Cu-W contacts in vacuum [41], and these patterns are associated with the arc behavior [48]. The arc melting patterns are very similar to those see in [41] at lower currents of 600 A rms.…”

“…Irregular erosion patterns can appear on Cu-W contacts in vacuum [41], and these patterns are associated with the arc behavior [48]. The arc melting patterns are very similar to those see in [41] at lower currents of 600 A rms. In [41], arcing tests at alternating polarities observed the formation of polycrystalline W particles on the contact surface over multiple arcing operations, which then were remixed with the surrounding contact material in later arcing operations.…”

“…Ag-WC also maintains the contact shape over the erosion tests in [28] and [29]. Cu-W had a more complex melting behavior during arcing and produced some behavior similar to W-based contacts in air [41]. Nevertheless, VI with Cu-W was able to maintain the contact shape necessary for low erosion, small increase in contact resistance, and interruption of the expected currents.…”

“…Currents of 450-600 A rms are typical of the load current switching in VI, and these studies measured the contact erosion and the sources. Measurements on Cu-W at 600 A rms studied the erosion rate and the contact mechanical integrity [41]. Ernst et al [35] briefly described the dielectric behavior of VI after 10 000 switching operations at 1250 A rms.…”