Experimental Study Of Dynamic Welding Behavior Of Contacts Within Electromechanical Relay

Dynamic welding, being the principle failure mechanism for contacts, correlates closely with the reliability and electrical life of electromechanical relay applications. The corresponding experimental study of dynamic welding not only helps to better understand the electrical characteristics of contact material in the electrical endurance test, but to investigate the degradation physical mechanism of contact welding. In this paper, a test rig for measurement of the dynamic welding force is designed, then we mainly investigate the detailed relationship between the dynamic welding and making bounce.Firstly, a novel test equipment for in situ and real time measurement of dynamic welding force within the electromagnetic relay is developed. Its main functions include: variable mechanical parameters of the test relay, such as magnetic gap, contact gap, mechanical spring force and stationary contact force obtained by associated adjustment of the liner translation stages with five-degrees-of-freedoms; the measurement of dynamic contact force is achieved by using a piezoelectric force sensor, and the dynamic displacement of contact, contact voltage, contact current are also obtained by the corresponding measurement units, respectively; data acquisition and logging process are controlled through a PC with the help of LabVIEW software specifically programmed for this purpose.Secondly, the impact force and stationary force are well defined with the typical waveforms of contact voltage and dynamic contact force during making process, and a special experiment is designed, where a strain gauge force sensor is used to measure the stationary contact force with the piezoelectric one at the same time, to assess the stationary contact force; the welding force is also defined with the typical waveforms at breaking process; a physical model of contact separation is built for explaining the contact welding process at break, and the welding force algorithm is also designed.Thirdly, the physical model of contact at make is built for illustrating the relationship between the impact velocity, stationary contact force and bounce width, bounce height; the bounce parameters, such as bounce time, bounce number and last bounce duration, are introduced for describing the bounce behavior at make; this paper also investigates the effects of over-travel and magnetic gap on the making bounce characteristics; a function of the contact voltage and bounce duration is developed in order to investigate the effect of electrical load on the bounce behavior.Finally, the dynamic welding formation process is depicted according to the relationship between bounce time, bounce numbers, last bounce duration and relevant welding force. It is noted that the dynamic welding is caused by the solidification of molten metal occurred in bounce arcing. This paper also investigates the failure mechanism of electromechanical relay and the variation features of welding force, stationary force and bounce in the electrical endurance test. Moreover, the welding behaviors in different load current are investigated.The various capabilities of the test rig could help to evaluate the electrical characteristics of different contact material, and provide guidelines for the mechanical parameters optimization of relay design.