Research On Contact Welding Characteristics And Failure Mechanisms Of Electromagnetic Relays

Electromagnetic relays,which are widely used in aerospace,communication and electric energy,are the most basic automatic control component.There are inevitable occurrences of contact bounce,molten metal bridge and arc discharge during the switching process of electromagnetic relay.Contact welding failure has become the key restrictions for longer electrical life and better reliability of electromagnetic relays.The contact welding,which involves the interaction of material deterioration,arc erosion and mechanical behavior,are complex and variable.So far,the theoretical and experimental researches on the welding characteristics of contact materials are still insufficient,and it is still unclear for the effects of mechanical parameters and load types on contact welding characteristics.Therefore,the understanding of the contact welding mechanism in the electrical endurance test of electromagnetic relays is far from reaching.In this paper,the welding characteristics of contact materials and the failure mechanism of electromagnetic relay contact are studied.First,the heat transfer model of the welding characteristics for contact materials under the conditions of resistance heat and arc heat is established,and the simulation method of contact welding is proposed.Then,the in-situ test and analysis method of contact welding characteristics is proposed,the influence of operation modes,mechanical parameters and load types on the contact welding are clarified explicitly.Finally,the welding failure process and failure modes of contacts are analyzed,and the failure mechanism of contact welding based on the competition of welding ability of bounce arc and molten bridge and the anti-welding ability of contact materials is revealed.First,the heat transfer model of the welding characteristics for contact materials is established,and the key parameters to determine the welding force are obtained.Then a mathematical model for calculating the threshold welding current,welding area and welding coefficient is proposed based on elastic contact conditions,and the anti-welding ability of contact materials is analyzed by welding coefficient.Furthermore,the relationship between the electrical parameters,the physical properties of material and the welding force is explored,and the key parameters to determine the welding force are obtained,including arc voltage,arc current,thermal conductivity,tensile strength and proportion coefficient,which lays a theoretical foundation for analyzing the welding characteristics of contact materials.Second,the method of simulating welding phenomenon under the conditions of resistance heat and arc heat is proposed,and the welding force,welding area and nugget tensile strength of contact materials are tested.A novel test rig which can control contact force,load current,load voltage,bounce time and bounce energy is designed.Then,the welding force and welding area of Ag,Cu,AgSnO₂ and AgNi contact materials under different load currents are observed,and the nugget tensile strength is calculated.Meanwhile,the heat transfer model of welding characteristics for contact materials is verified.The roles of resistance heat and arc heat in welding are clarified by analyzing the morphology and element composition of the contact erosion area,which provides a way for evaluating the anti-welding ability of contact materials.Next,the in-situ test and analysis method of contact welding characteristics for electromagnetic relay is proposed,which realizes the simulation of switching process of electromagnetic relay and the online testing of electrical parameters,mechanical parameters and surface profile information for contacts.In order to clarify the influence of operation modes,mechanical parameters and load types on contact welding characteristics,the effects of bounce energy,contact resistance,molten bridge energy and arc energy on contact welding for different operation modes are discussed respectively with the using of the in-situ test and analysis method,and it’s found that the bounce arc and molten bridge are the root reason of contact welding.The mechanical characteristics of spring system are analyzed theoretically,and the effects of the contact gap,over travel and free travel on the contact welding force are investigated experimentally.The influence of electrical load on the electrical characteristics in the contact switching process is studied,and it is confirmed that the contact bounce behavior and the inrush current in the making process could aggravate the contact welding phenomenon.Finally,the welding failure process and failure modes of contacts for electromagnetic relay in electrical endurance test are clarified,and the welding failure mechanism is revealed.The degradation processes of electrical characteristic parameters such as bounce energy,contact resistance,contact force,arc energy,contact gap and sliding distance in the electrical endurance test are analyzed respectively.The scanning electron microscope,metallographic microscope and energy dispersive X-ray spectrometer are used to analyze the surface morphology,microstructure and element composition of the failed contacts.Then,the contact failure process of electromagnetic relay has been clarified,and three types of welding failure modes have been defined.The contact welding failure is the result of the coupling of arc erosion,material degradation and mechanical parameter changes,when the welding ability of bounce arc and molten bridge is stronger than the anti-welding ability of the contact material,the contact welding fail ure occurs.