Andreas Kraus

Development of silver zinc oxide for general-purpose relays

The switching behavior of silver cadmium oxide (Ag/CdO), silver tin oxide (Ag/SnO/sub 2/) and silver iron oxide (Ag/Fe/sub 2/O/sub 3/) contact materials is compared to silver zinc oxide (Ag/ZnO) materials having different amounts of additives. Endurance tests were performed in a commercial general-purpose relay, subjected to resistive, inductive and inrush current loads. Results showed the performance of Ag/ZnO to be significantly dependent on the amount of additives and their grain size. The optimum Ag/ZnO was found to be equivalent or superior to other silver metal oxides depending on the applied load. Thus a substitution of Ag/CdO by an environmentally harmless Ag/ZnO material is possible.

Guidelines for the use of Ag/SnO/sub 2/ contact materials in switching devices for low-voltage power engineering

A detailed account is given of the performance of Ag/SnO/sub 2/ contact materials in a large number of switching devices for a wide variety of applications. Contactors of various sizes and design principles, as well as power relays for use in AC and DC circuits, are considered. The differences in switching behavior observed are interpreted with the aid of metallurgical model concepts for the interaction of the switching arc with the contact material. On the basis of the metallurgical considerations given and the application experience discussed, it becomes possible to derive a practical guide for the use of Ag/SnO/sub 2/ materials in switching devices. The results of this process are shown in a generalized schematic presentation of fields of applicability for Ag/SnO/sub 2/ materials. Once the important parameters for each application are defined, this schematic can be used to find the variety of Ag/SnO/sub 2/ contact material that can be expected to yield the appropriate switching performance.<<ETX>>

Tungsten and tungsten carbide based contact materials used in low voltage vacuum contactors

By means of a single phase vacuum model switch with a current range from 50 A (for current chopping) to 1500 A (for AC4 contact erosion) contact materials of the group of tungsten carbide/silver, tungsten carbide/copper and tungsten/copper are evaluated. The properties focused on are current chopping in new state as well as during service life, contact erosion due to AC4 life due to cracking and reignition behavior. It is found that in regard to current chopping best results are obtained with WC/Ag and WC/Cu, while W/Cu and even an optimized W/Cu having antimony additives show substantially higher chopping currents. In regard to AC4 life and especially to reignitions the copper containing materials were found to be superior to the silver containing materials. In the case of WC/Cu there are indications of poor contact resistance during service life which is attributed to the evaporation of the copper from the arc treated area in combination with a poor wetting behavior of the molten copper to the WC particles.

Influence of the contact material on the performance of temperature-dependent switching controllers in household appliance

Due to their design, the switching behavior of temperature-dependent electro-mechanical switching controllers is completely different to electromagnetically driven devices, especially with regard to opening operation. Usually the expansion of a medium (e.g. liquid, bimetallic strip) is used to initiate the switching operation and thus the break operation is slow. As a result, sticking or welding of contacts can occur, leading to troublesome exceeded temperature or reduced service life. Electrical tests with various contact materials were carried out in a model switch designed to meet the switching conditions in switching controllers. It could be shown that silver/tin oxide containing merely 2% by weight of metal oxide (tin oxide+additives) has significant lower opening forces. Tests in a real power controller could be clearly improved by the application of this special silver/tin oxide contact material. The way this doped tin oxide works is tried to be explained by means of metallographic investigation.