Effective erosion rates for selected contact materials in low-voltage contactors

Effective and absolute erosion rates (g/C and cm³/C) are reported for contact materials from vacuum interrupters used for low-voltage contactors. The effective erosion rates were determined with applied 60 Hz currents (630 A to 685 A peak, alternating polarity) and maximum gaps of 2.1 to 2.3 mm. The absolute cathodic erosion rates were determined with applied dc currents on the order of 100 A and maximum gaps of 8.5 mm. The measured effective erosion rate is markedly smaller than the absolute erosion rate, since a significant fraction of the ions and neutral vapor leaving one contact deposits on the other contact. The materials investigated were: (1) Ag-WC (40/60 wt. %), (2) Cu-Cr (60/40 wt. %), and (3) Cu-Cr-Bi (66/32/2 wt. %). In the ac experiments, a controller allowed a preset number of single-phase current cycles through the sample contactor. The contacts were opened at a preset time in the last half cycle, and the drawn are continued to the next and final current zero. The polarity of the current in the arcing half-cycle automatically changed for each operation to ensure uniform erosion history for both contacts during the many thousands of operations. Afterwards, the contacts were removed and placed in a demountable arcing chamber for determination of the absolute dc erosion rate of the cathode material.