Timo Mützel

Contact Material Combinations for High Performance Switching Devices

Latest developments in the fields of regenerative energies, industrial automation and automotive application generate new technical requirements for switching devices and contact materials. Rising energy densities in such devices were simulated by contactor endurance tests under heavy duty load. The results show a strong exponential relationship between arcing energy at break and contact material erosion; this effect has to be considered carefully in device design. Opportunities for precious metal reduction and/or enhanced switching capacity were found for Ag-SnO2 materials with increased total metal oxide content. The dynamic sticking behavior at make operation is a second important characteristic of electrical contacts. Especially the application of asymmetric material combinations for cost saving issues - e.g. replacing one side of a symmetric silver metal oxide combination by silver nickel - can lead to unexpected device failure. Appropriate model switch tests and corresponding metallurgy can demonstrate that behavior.

Development of Contact Material Solutions for Low-Voltage Circuit Breaker Applications (2)

The focus of the experimental studies was in the influence of contact material composition, production parameters and the contact material - breaker interaction, as the performance of the breaker is a function of breaker design, contact material and especially their interaction. Therefore, typical material combinations used for low voltage protection devices were compared by means of their switching behavior and performance. Model-switch tests have been carried out to show this performance under stable and well defined boundary conditions. The chosen test parameters simulate device tests, excluding the influence of switching device kinematics and tolerances. Focus of the tests was to scrutinize the influence of the material composition on the erosion behavior and contact resistance for AgWC materials. Additionally a comparison on the performance of different silver refractory metals like AgW or AgMo at comparable vol.-% refractory metal was made. Furthermore, the impact of different graphite contents, manufacturing parameters and fiber orientation of AgC materials on weld break forces have been worked out. This experimental quantification of influences can be seen as a basis for contact material selection during protection device design.

The Effect of Material Composition on Dynamic Welding of Electrical Contacts

The dynamic welding behavior at make operation is one important characteristic of electrical contacts and a key parameter to ensure secure usage of electricity. Model switch and device tests have been performed to work out the main factors influencing contact sticking. The behavior of different silver metal oxides (Ag/CdO, Ag/SnO2, Ag/ZnO), including various groups of additives like low melting and boiling metal oxides (Bi2O3), high melting oxides (CuO, WO3), and combinations thereof were studied under different loads. Several working mechanisms - that significantly influence and define the sticking tendency - have been found for different arcing loads. They can be explained by metallurgical investigations and divided into mechanisms under low (below 0.3 Ws), medium (0.3 - 10 Ws), and high (above 10 Ws)arcing load. The paper presents a scientific basis to explain contact sticking failures under different types of loads and can be seen as a guideline for appropriate contact material selection.

Silver tungsten carbide contacts fork circuit breaker applications

Silver tungsten carbide (AgWC) contacts are often applied to circuit breaker applications due to their good arcing erosion properties. The low material loss is beneficial for repeatable high interruption capacity. This paper characterizes the influences of contact material microstructure and sintering agents on the switching properties of AgWC40 material. A detailed description of the microstructure by quantitative metallurgy methods is a key aspect of this work. Furthermore, the influence of contact material microstructure and composition on the sintering shrinkage are evaluated. Model switches are applied to study the switching performance of AgWC40 materials under defined and reproducible boundary conditions. Performance parameters studied are dynamic sticking tendency at make, arcing erosion and contact resistance after break operation. Cross sections after switching are used as basis for interpretation of achieved test results. The studies show that effects of sintering agents dominate the resulting contact resistance after arcing, while no differences in erosion can be observed at chosen overload conditions. In addition dynamic sticking tendency could be reduced by finer microstructure and reduced amounts of sintering agents.

Thermo-mechanical stresses within switching contact systems after arcing events

Silver tin oxide (Ag/SnO2) contact materials are widely used in contactor applications. A general trend in this application field is the steadily ongoing miniaturization of switching devices, resulting in growing energy densities to be handled by the contact system. The higher arcing energy densities in such new designs are inducing increased thermo-mechanical stresses in the contact system. As these stresses cannot be measured, FEM simulation was applied to make them visible for heavy duty break arcs. Focus of the presented studies is the temperature dependent elastic and plastic modeling of the intermediate Ag layer and the braze filler. Based on these simulations, improvements in material stress behavior of the complete system (sub-assembly) should be realized.

The influence of switching arcs on contact resistance of Ag/SnO2 materials

Silver tin oxide (Ag/SnO2) contact materials are widely used for relays and contactor applications. Device approbation in accordance to standards requires several types of temperature rise test in new and switched conditions. Significant differences in temperature rise after the various switching sequences are experienced. In addition, differences between individual phases of one device - especially in switched condition - are known from test. In this work, the contact resistance evaluation under different load conditions has been studied. Therefore the arcing events at make and break operation during different endurance tests (AC-3 and AC-4) have been analyzed and correlated to the contact resistances at rated current level during temperature rise test. Surface layer changes of contact materials after load are characterized by metallurgical methods. The paper shows the formation of different surface layers as a function of arcing energies during make and break operation. The results demonstrate that make and break arc together are creating the surface layers defining the final contact resistance.