Evaporation Erosion of Contacts Under Static Arc by Gas Dynamics and Molten Pool Simulation

Two mass erosion mechanisms or mass loss modes for electrodes during the repetitive arcing processes should be responsible for the failure of electromagnetic relays, and there are evaporation erosion and sputter erosion. Of these two, the evaporation erosion is believed to be the main failure mechanism for the electrodes used in low-current relays. Unfortunately, there is still no straight-forward model to predict the characteristics of the evaporation process on the electrodes. In this paper, heat transfer and fluid dynamics in the molten pool in silver contacts are coupled with the gas dynamics in the vicinity of arc spot to determine the evaporation rate due to static arc. The influences of input parameters, such as arc current and material properties, on the evaporation rate are analyzed. Of all those parameters, boiling point and thermal conductivity of the electrode are found to be the most significant influences on the evaporation rate. The transfer direction of mass due to electrode evaporation is discussed on the assumption of zero net material loss, and it is found to be directly related to the current and heat flux from the arc to the electrodes. The method used in this paper to evaluate the evaporation erosion characteristics can serve for strategies to alleviate evaporation erosion for relay electrodes.