Switching Behavior of Ag/SnO2 Contact Materials at High Operating Overload Currents

High efficiency motors lead to increased currents on make operations which in turn requires contactors with improved contact materials. Especially welding forces at elevated make currents and after electrodynamic lift offs with short circuit withstand currents are of interest.The present paper compares different silver / tin oxide (Ag/SnO$_{\mathbf {2}}$) contact materials with additives of bismuth oxide (Bi$_{\mathbf {2}} \mathbf {O} _{\mathbf {3}}$) and copper oxide (CuO): a commercial material with 14 % total oxide content and three materials from a family of a briefly developed type of Ag/SnO$_{\mathbf {2}}$ with improved dispersion of oxide particles, total oxide contents 10, 12 and 14%.Tests are performed in a commercial contactor (AC3 power 110 kW at 400V) according to IEC 60947–4-1.Results reveal that the improved material with 14 % oxide content leads to substantially decreased welding forces compared to the standard material. The improved material with reduced oxide content of 12 % gives results close to standard 14 % materials while with just 10 % oxide content there are higher welding forces.Results obtained are discussed based on microstructure of the materials in new condition as well as after switching operations.