A model switch has been developed to investigate the influence of kinetic parameters, as contact velocity and contact force, on contact welding under conditions prevailing in relays, micro switches etc. Both frequency and intensity of the statistically scattering weld forces increased with increasing arc duration and current. Pre-strike and bounce arcs show identical results under corresponding conditions. Both frequency and intensity of the statistically scattering weld forces initially increased and then decreased with increasing impact velocity. Increasing the static contact force hardly influenced the strength of the primary weld after impact but reduced or prevented weakening of the weld by a following bounce process. The results agree with the predictions of a mathematical model proposed. The configuration of the load circuit determining the actual arc current curve essentially influences the weld force values.

Influence of electrical and mechanical parameters on contact welding in low power switches

Welding in closed and opening electrical contacts is described mathematically by three models. 1) a nonlinear model of static welding based on differential equations for contact temperature and electrical potential enables one to get information about the spatial distribution of both variables. 2) a transient model of static welding, on the basis of the axisymmetric Stefan problem, describes the dynamics of phase transformation in the property dependence of contact material, current and other parameters. 3) a satisfactory description of dynamic welding during arcing can be obtained by modelling of heat transfer in a domain with two free boundaries corresponding to melting and evaporation interfaces. In conclusion, the relationship between current, parameters of contact closure and contact material, which define immediate or delayed cases of welding, are found and discussed. Results of calculations are compared with experimental data.

The mathematical models of welding dynamics in closed and switching electrical contacts

A new model switch allows adjusting test conditions to those found in a typical relay with the advantage of more precise measurement of erosion, welding resistance, and temperature operating characteristics. Four silver tin indium oxide materials, which differ in total oxide content and minor additives, have been studied by using this model switch. The results show that the total metal oxide content is the most influential parameter for both electrical erosion resistance and welding resistance and that the minor addition tellurium (Te) has some effects on low metal oxide content material.

A comparison of silver tin indium oxide contact materials using a new model switch that simulates operation of an automotive relay

Microscopic investigations of the molten marks left by metal vapor bounce arcs on both anode and cathode showed characteristic results dependent on the arc mode ("anode arc" or "cathode arc" according to Germer). Differences in arc spot sizes and appearance yielded a key to explain the different weld strength values caused by short time and by long time bounces, respectively, but also to understand the changes of direction of material transfer when the arc mode changes

Short Arc Modes Determining Both Contact Welding and Material Transfer

Silver/tin oxide composite materials have increasingly been used in DC switching because of their good anti-welding properties and resistance to arc erosion. The superior anti-welding and low arc erosion properties are observed to be partly due to the weak interface sticking between arc erosion products on the contacts. The phenomena of contact bounce creates a short arc erosion and material transfer by ions and metal vapor, while the equally important effect of dynamic welding creates a surface structure that is modified by sticking and fracture of weld spots. In this research, we examined the surface microstructure of Ag and Ag/SnO/sub 2/ contacts in various combinations of current, contact force and contact bounce parameters to further understand the complex phenomena of total material transfer in switching.

http://www.ieee-holm.org/h2002/h2002.1.2.pdf

Material transfer in dynamic welding of Ag and Ag/SnO/sub 2/ contact material

Mechanical parameters were correlated with dynamic contact welding for a hard silver contact alloy The study was conducted with a model switch that simulated an automotive relay The switching tests conducted at 13 VDC for rigid and flexible blades correlated contact bounce to weld strength The weld strength was shown to be highest for single bounces of very short duration and contact gap Flexible contact springs were shown to create weak welds by mechanical adhesion

Z. Chen

Papers

Dynamic welding of silver contacts under different mechanical bounce conditions