Study On Dynamic Welding Of High Voltage DC Power Relay Contact

Statistics shows that the contacting failure is the most common failure mode for electromagnetic relays, in which more than 60% contacting failure is caused by contact welding. The main reason for contact welding failure is that ‘bridge rebounce’ and ‘arc rebounce’ occurs due to contact rebounce during its making process. Local molten area will occur in both the stable contacts and the movable contacts because of the high temperature generated by the arc igniting during the rebounce process. The molten area will finally solidate once the contacts are in stable making state, hence the welding occurs. During the consequent opening process, the contacts cannot break if the drive force exerted onto the welded contact is smaller than the welding force. Eventually, the relay fails. Therefore, simulation analysis and experimental investigation are carried out on dynamic welding mechanism and welding force. The numerical prediction method for the welding force is proposed, and the threshold value when welding occurs is calculated. The influence of different factors on the welding force is also acquired during the breaking and making process.Volume of the welding area is the precondition to predict the welding force. Therefore, it is necessary to determine the volume of the welding area caused by arc rebounce under certain arc voltage and current. Based on the material melting theory, a Magneto-hydrodynamic(MHD) model of the cathode and the anode molten pool formation process is established under the electrical and thermal effect of the rebouce arc. The model is solved by using FLUENT software based on finite volume method. The ultimate molten pool diameter and volume are acquired under thermal dynamic equlium condition, which is taken as the volume of the welding area.An equivalent model of the welding area of the contact is established based on the theory of solid mechanics. First, geometric model of welding area is built according to the volume and size obtained previously. Physical properties, including the density, the yield strength and the Young’s module, for contact materials are set for the simulation model. Then, boundry and load conditions are determined for the welding area as well. Dynamic meshing techniques combined with re-meshing and self-adaptive are used to solve deformation of the welding area. At last, the welding force is obtained by using Newton-Raphson iterative algorithm when the welding area satisfied the Von Mises yield criterion. On the basis of the model, three cases are analyzed: weld volume is variable, diameter-height ratio is variable at constant volume, and yield stress is variable at constant volume.A welding force experiment system and its analysis methods are developed. A piezoelectric transducer is installated behind stable contacts to measure welding force for its high response speed and thermal resistance. Output signals from the thansducers are amplified by a charge-voltage transform circuit and then sampled by using an A/D card. A laser displacement transducer with high accuracy is applied to investigate the relationship between welding forces and rebounce process, the time evolution of displacement can be recorded by using a high-speed A/D card. Finally, an optimization is teken to inprove the insulation and mechanical stability.At last, experiments are carried out under different currents, and the influence of current on welding force is studied. The influence of contact gap, the strength of the magnetic field, and the breaking velocity on welding force is investigated through experiments. To acquire the effect of single factor on the welding force, experiments are carried out when those three facters are independently adjusted. Orthogonality experiment is also designed to investigate the order of influence degree on the welding force. The results can provide instrucitons for design of a HVDC relay on anti-welding failure.The investigations and results acquired in this paper, such as the welding force prediction modle and method, the welding force threshold value, and the relationships between different factors and the welding force, can be applied to guide the anti-welding design of electromagnetic relays, which is of practical value for enhancement of the reliability and the lifespan of relays.