Simulation and Experimental Study of Thermal Effects on Composite Contacts in Electrical Life Cycle Tests of DC Relays

Composite contacts used for dc relays can help reduce their cost and the amount of precious metals used in their construction. Because of their electrical properties, silver metal oxide-copper (AgMeO-Cu) composite contacts are widely used; however, at heavy loads, the bonding strength of the composite contact will change due to the influence of uneven temperature and stress distributions, which may even cause failure in the form of oxide layer peeling. Here, typical AgSnO2In2O3-Cu composite contacts are studied. A 3-D finite-element simulation model that considers contact making, arcing, and breaking was established to calculate the contact temperature and thermal stress after any operation cycle. The model can be used to determine the variation of the contact temperature and thermal stress distributions versus operation time during an electrical life cycle test. For different AgMeO layer thicknesses (0.25 and 0.38 mm), the temperature and thermal stress distribution of composite contact's bonding area were calculated under different current load levels: 30, 45, and 60 A, while the load voltage is a constant 17 V. Results show that the thickness of the AgMeO layer has a significant effect on the temperature and stress distribution of the bonding area in the moveable contact and the composite contacts may more easily curl and crack with an increment of the AgMeO layer thickness under the same current condition. The simulation results agree well with the experimental data and the relative error is less than 10%. The validity of the model and method was verified. The numerical model in the paper may be used to analyze a composite contact's properties and is therefore useful for design optimization and the selection of composite contact materials.