Simulation And Experimental Research On Contacts Welding For DC Relay

The DC relay is a key component for performing functions such as execution,switching,and protection in DC power distribution,and control systems such as electric vehicles.Welding?or adhesion?which causes contact repelling action is one of the most serious failure modes of DC relays,it has gradually become a bottleneck problem restricting the development of related products in the direction of high reliability,long lifespan,and large capacity.The contacts welding forming is a complicated physics which accompanied by the molten bridge,molten pool and droplets sputtering.There is a certain difference between dynamic and static welding,and welding has the characteristics of suddenness and randomness.The existing micro-molten pool and droplets sputtering models were only suitable for vacuum arc,which can not be directly applied to the analysis of atmospheric pressure in relays.There was an insufficient description of the liquid metal deformation for the previous molten bridge.There are relatively few numerical studies on the physical processes of contacts static welding and dynamic welding.The above problems lead to the lack of understanding of the relay contacts welding,and which can not provide an effective theoretical basis for the anti-welding performance evaluation of contact materials and optimization design of relay.In this paper,the key common problems in the formation process of contacts welding,such as deformation of contact area melting,DC electric arc and the molten pool-droplet sputtering behavior simulation model and the uniform analysis model of the temperature rise-melting-solidification process have been established.Combined with experiments and simulation,the static contact welding and dynamic welding of relay contacts were studied from microscopic and macroscopic perspectives.Firstly,the hydrodynamic model of the molten bridge has been established for the melting deformation problem of micro contact area during the formation of contact static welding,which considered the melting phase transition of metal materials and the deformation of liquid metal under the action of Joule heat of load current.The temperature,current density and melting time during the molten bridge forming were calculated,and the quantitative analysis of the physical process of molten bridge formation was realized.Based on the multi-physical field which including mechanical,electric field,heat transfer and laminar,the mathematical model of electrical contact was established for DC relay.The analytical method of the equivalent dynamic viscosity function was proposed for simplifying the metal softening deformation.The initial contact radius was calculated by the finiteelement simulation of solid mechanics.The current density non-uniform distribution caused the ringed Joule heat source in the contact area,which produced a thin metal layer with a thicker edge and it will become the static welding core area,and the melting and softening metal around the edge of the contacts would be the non-core welding area.Local temperature gradient distribution is relatively concentrated in the actual contact edge.With the contact force decreasing,the forming time for the effective welding zone was also shortened.The maximum axial depth of the contact molting region increases as the initial contact radius and load current increasing.Secondly,the DC relay arc model based on magnetohydrodynamic theory has been established for the arc,molten pool and droplet sputtering behaviors during the contacts dynamic welding.Mechanism and physical properties of arc dwelling,motion and re-striking were analyzed according to parameters such as voltage curve,temperature field distribution,and current density distribution.The influence of magnetic field strength,contact speed and voltage-current on the arc characteristics were analyzed.Based on the arc simulation,a 2D axisymmetric hydrodynamic model was established for the micro-molten pool and droplets sputtering under DC arc.The moving mesh and the automatic re-meshing grid were combined to solve the problem of large deformation of droplets sputtering.The calculated results show that the increment of arc spot pressure leads to the decreasing of the sputtering angle.The increasing of the arc heat flux density leads to the increment of the sputtering angle.The arc spot pressure has less influence on the sputtering angle than the arc heat flux density.Based on the simulation results,a new physical interpretation is proposed which arc spot pressure spatial distribution determines the molten pool metal droplets center sputtering or edge sputtering.Thirdly,aiming at the key common problem of liquid metal solidification in the process of DC relay contacts welding,the equivalent heat capacity method was used to solve the latent heat problem during the phase transfer process.The non-isothermal fluid and heat transfer coupling modeling was used to calculate and analyze the solidification of the equivalent liquid metal for molten bridge and the molten pool-sputtering droplets between contacts.Phase change and temperature decreasing during the solidification process were simulated as a common problem of contacts welding.The temperature distribution during the solidification phase transition was obtained.A fraction of solid-phase shows a gradual transition between the liquid and solid.As the length of the equivalent molten bridge between contacts increasing,solidification time decreases;solidification time increases along with the initial temperature.With an increment of liquid metal thickness,the solidification time will be slightly shortened.The Von Mises stress in the equivalentwelding zone becomes larger as the equivalent liquid metal thickness increases after solidification,and the non-uniformity along the axial distribution becomes more and more obvious as the thickness increases.Finally,static welding force and critical static welding of the DC relay contact were also calculated.Contacts dynamic welding force formulas were derived and calculated based on the bounce arc energy,and the relationship between the macro-welding section and the micro-welding spot was analyzed.The experimental system of DC relay contacts welding was carried out,which realizes the accurate measurement of DC relay contacts static welding,contacts closed dynamic welding with bounce arc.The results show that the order of the anti-static welding ability of the contact materials is Ag>Cu>AgNi>AgSnO₂>AgSnO₂ln₂O₃,and the average and randomness of the static welding force increase along with the increase of the load current.The fracture cross-section of the dynamic and static welding expands with increasing load current.Increasing the contacts force is beneficial to reduce the probability of contacts bouncing and the dynamic welding force.The increasing of contacts closing speed will prolong the contact bounce time,which leads to dynamic welding force increasing.This research has important significance for perfecting the basic theory of electric arc and electrical contacts.It can also provide a basis for the analysis of contact welding mechanism and failure mode,evaluation of contact material anti-welding performance and high reliability and long lifespan design of DC relay.It also has certain practical value for the tolerance of short-circuit current impact and anti-welding capability optimization of magnetic latching relays for intelligent watt-hour meters and DC high-power relays with high-voltage level.