Analysis Of Sliding Electrical Contact Characteristics Of Carbon Brush Slip Ring

The function of the carbon brush slip ring is to form a stable and reliable connection for the electric energy and the signal between the rotating equipment and the fixed part of the radar during 360° rotation,which belongs to the sliding electrical contact system.In order to improve the contact area between the carbon brush and slip ring,the manufacturer extended running-in process from 2 hours to 60 hours,then the ablation phenomenon occurred when the device was working,which indicated that on the one hand,it was possible for the change of the running-in process to increase the real contact area so as to reduce the contact resistance,but on the other hand,the excessive of running-in made contact wear,the formation of more abrasive debris,carbon powder and particle oxidation,which led to higher sliding contact resistance,causing welding of the contact area under the impact of large current,destroying contact interface,then aggravating wear and oxidation,thus increasing the possibility of electrical contact thermal failure.This topic started with the failure analysis of the carbon brush slip ring caused by the ablation failure in working after the running-in process’change,established the fault tree to analyze the bottom events that cause the thermal failure.Firstly,the characteristics of the carbon brush’tightened spring were studied and the influence of the spring on the contact force was analyzed.Then,the wear characteristics and mechanism of the contact surface of the carbon brush slip ring were analyzed through the measurement of parameters such as the weight loss,surface roughness of interface,real contact area of the carbon brush slip ring in the running-in process.By the simulation experiments combined with the real-time monitoring of sliding contact resistance,the relationship between sliding wear and sliding contact resistance degradation characteristics was studied.Finally,the possibility of contact welding caused by the degradation of sliding contact resistance of carbon brush slip ring was analyzed by thermal analysis theory of connectors and finite element thermal simulation.The results indicate that the ablation failure of the carbon brush slip ring’s interface shows a trend from the carbon brush wear,interface welding,shearing friction,and slip ring’s ablation after excessive running-in.Analysis of fault tree concludes that the initial failure is due to the thermal failure caused by the welding of the interface material due to the temperature rise of the contact interface.Contact force,contact resistance and current conditions are directly related to thermal failure of carbon brush slip ring system.By testing the elastic properties of the carbon brush’ tightened spring,it was found that when the carbon brush was installed with copper wire,the copper wire and the tightend spring in parallel formed a nonlinear composite spring to apply contact force to the carbon brush and the ring.Under the limit condition,the composite elastic coefficient of the spring is decreased by 13.05%,and the contact resistance is increased by 15%.In the early stage of running-in wear process,the wear of carbon brush is more severe.Due to the formation of surface oxides and the accumulation of sliding wear debris,the frequency of abnormal contact resistance increases and the average value shows an upward trend,and the contact property stability is relatively poor.After that,the wear shows a gentle trend,and the contact resistance stability increases to some extent but still fluctuates.After 60 hours of wear,the real contact area attains to about 63%of the nominal contact area of the carbon brush surface,and the surface roughness Ra is reduced to 50%of the initial state.Based on theoretical thermal analysis and finite element thermal simulation,the interface temperature rise of the carbon brush slip ring contact pair under normal operating current was estimated,and critical impulse current to produce contact welding was estimated.It is indicated that with the increasing of sliding contact resistance,the contact material may reach the welding temperature under the condition of impact current,which makes devices to have the safety risks during their service.