Research On Contact Erosion Mechanism And Material Transfer Characteristics Under Direct Current ARC

When the electromagnetic relay operates, the arc burns between the contacts, accompanying evaporation and sputter. Evaporation and sputter can cause contact erosion and material transfer, which lead to the contact system failure. The ability of the contact to resist the evaporation and sputter erosion directly determines the reliability and lifespan of the electromagnetic relay. Current evaporation models are suitable for vacuum arcs, which are not accurate for normal pressure conditions in relays. Sputter erosion models can not give the relationship between sputter phenomenon and arc characteristics. No theoretical research is conducted on material transfer phenomena. As a result of the above situations, the contact erosion degree by the arc can not be evaluated, and no theoretical evidence can be offered for contact anti-erosion ability design and optimization. Through the establishment of three-dimensional molten pool magneto-hydro dynamic(MHD) model and the arc-contact unified model, the evaporation and sputter erosion and material transfer mechanisms are studied under steady and breaking arc. The effect of material properties on the contact ability to resist evaporation and sputter erosion is analyzed.Contact erosion and material transfer due to evaporation lead to the contact performance degradation and failure when breaking small current. For heat transfer and fluid flow phenomena inside the contact, a three-dimensional MHD model is established to get physical characteristics of the molten pool. Based on gas dynamics, the evaporation model above the molten pool is set up including the Knudsen layer, the vapor layer, the transient arc layer and the steady arc layer. The relationship between the evaporation flux and the surface temperature of the molten pool is achieved by solving gas dynamics equations. The evaporation erosion rate is achieved under static arc. Under the assumption of zero material loss, the material transfer direction and rate are obtained. Based on the above models, the influence of the thermal conductivity, electrical conductivity and other material properties on evaporation erosion characteristics is analyzed. Experiments arc carried out under small currents with a gap-adjustable arc experiment system. Mass variations and surface conditions are acquired. The evaporation model is verified through compare with the experiment results.Research on contact erosion and material transfer mechanisms due to sputter is essential for the contact failure mechanism analysis under high current. Through the analysis of the arc force, the physical model is established for the sputter process due to the arc force, and the relationship between the sputter form and the arc spot evolution is analyzed. Based on the principle of mechanical energy conservation, arc force energy expression is derived. Based on the relationship between the arc force energy and surface tension bounding energy, the sputter criterion is proposed. Combining with the equivalent model of the arc spot, sputter erosion rate is calculated under static arc. The effects of spot features such as spot current density, spot evolution cycle and the movement rate on sputter erosion characteristics are analyzed. Due to the sputter randomness, probability distribution assumptions are for the initial state of sputter droplets. The movement trace of the droplet is simulated to achieve the relationship between material loss-transfer ratio and droplet initial status. Based on the above models, the influence of the thermal conductivity, electrical conductivity and other material properties on sputter erosion characteristics is analyzed. In order to verify the sputter erosion and transfer model, experiments are carried out under large currents. Compare and analysis arc carried out for the experimental and theoretical results.The equivalent model of all stages during the contact breaking process is established. Combining the molten pool simulation model, the contact erosion and material transfer characteristics are investigated. Simplified models for the contact spot, liquid bridge and the arc are established. The contact voltage-current predicting method is proposed, and the current density and heat flux into the contact during the breaking process are achieved. On the above basis, the transient simulation model of the molten pool for the breaking process is established, and the physical field distributions inside the molten pool are obtained. Through the analysis of evaporation and sputter erosion and transfer characteristics, the erosion and transfer mass are achieved for single operation. Contact breaking experiments are repeated for 10,000 times in a 30V/50 A resistive circuit. Contact voltage and mass variations are obtained. Reasons for the difference between simulation results and experiment results are analyzed.According to the material and energy transfer process between the arc and the contacts, the MHD model including the cathode, the anode and the arc column is established to study the contact erosion and material transfer characteristics during the breaking process. The mathematical model for near-electrode region is set up including the cathode, the sheath, the pre-sheath and the arc column. The relationship between the current density and the temperature on the surface is obtained. Based on the cathode and the anode surface energy balance model and the near-electrode region mathematical model, a solution method for the coupled boundary between the arc and contacts is proposed. Combining with dynamic mesh technology, the coupling multi-physical fields are solved in the arc and contact regions for the breaking process. The contact erosion and material transfer process due to evaporation and sputter characteristics is analyzed.The above research is of great theoretical value for the analysis of the contact failure mechanism, the evaluation of the contact system, and the high-reliability and long-lifespan design of the electromagnetic relay. It is also valuable for the anti-erosion ability design and optimization of the contact under different arcing conditions.