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

Effective and absolute erosion rates are reported for contact materials from vacuum interrupters used for low voltage contactors. The effective erosion rates were determined from the linear erosion of both contacts with 60 Hz half cycle currents (from 630 to 685 A peak) and maximum gaps of ~2.2 mm. The contacts were opened at a present time in a half cycle, and the drawn arc continued to the next and final current zero. The polarity of the current automatically changed for each operation to ensure uniform erosion history for both contacts during 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. The absolute cathodic erosion rates were determined with applied dc current pulses on the order of 100 A and maximum gaps of 8.5 mm. With ac operation at a gap of 2.2 mm, a significant fraction of the ions and neutral vapor leaving one contact deposits on the other contact, so the effective erosion rate is markedly smaller than the absolute erosion rate. The materials investigated were Ag-WC, Cu-Cr, and Cu-Cr-Bi