运行工况对碳刷/集电环摩擦副表面膜形成的影响

Effect of Operating Conditions on Friction Surface Film Formation of Carbon Brush/Slip Ring Assemblies

目的解决水轮发电机组碳刷/集电环组件接触界面表面膜形成困难和容易破损的问题。方法在分析载流摩擦表面膜层成因的基础上,采用某水轮发电机组所用的碳刷及集电环配副,在同尺寸水轮发电机组碳刷/集电环模拟试验台进行相似性试验,用四探针方阻仪表征表面膜层的氧化情况,研究电流密度、速度、温度、载荷对集电环表面膜生成的影响。结果碳刷与集电环形貌磨合大约需要200 min,随后表面膜逐渐形成。在4、8 m/s转速下,正极集电环表面方阻的最大值可达到0.6 mΩ,在12 m/s转速下正极集电环表面方阻的最大值可达到25 mΩ。电流提高了摩擦副的温度,有利于氧化反应的进行,但过高的温度会导致摩擦表面水分子脱附,从而严重磨损表面膜。负极碳刷的温度普遍高于正极碳刷。弹簧压力分布不均会导致碳刷载流分配不均,更易引发负极碳刷出现高温,损坏表面膜。在12 m/s转速、800 A电流工况下,表面膜的生成速度相对最快。结论表面膜主要由石墨层和氧化物层组成,在表面膜的形成过程中,碳刷与集电环间的接触电阻在前期以收缩电阻为主,在后期以膜层电阻为主。膜层电阻主要受到膜层厚度的影响。转速是影响表面膜生成速率的主要因素。高温是造成表面膜破坏的直接原因。

In order to solve the problem that the film on the contact interface surface of the carbon brush/slip ring assembly of the hydro-generator set is difficult to form and easy to be damaged.Similar tests are carried out on a current-carrying friction and wear test machine with the same size and structure as the one in service,to explore the mechanism and key factors working on the formation and destroy of film on the surface of carbon brush and slip ring.Based on the analysis of the causes of the film on the surface of the current-carrying friction pairs,the effects of current density,velocity,temperature,and load on the surface film formation of the collecting ring were studied.The surface square resistance of the slip ring was detected with a four-probe square resistance tester to understand the formation of the surface film layer without damaging the collecting ring.The maximum square resistance of the positive electrode surface could reach 0.6 mΩat 4 m/s and 8 m/s,and the maximum square resistance of the positive electrode surface can reach 25 mΩat 12 m/s,and the high rotational speed tearing the surface carbon film promoted contact of the base metal with oxygen.The current increased the temperature of the friction pair,which was conducive to the oxidation reaction,but too high temperature would lead to the desorption of water molecules on the friction surface,resulting in serious wear and damage to the surface film.The temperature of the negative carbon brush was generally higher than that of the positive carbon brush.Uneven spring pressure would lead to an uneven current carrying the carbon brush,which was more likely to cause high-temperature damage to the surface film of the negative carbon brush.Film generation was the fastest at 12 m/s and 800 A current.It took about 200 minutes for the topography of the carbon brush and the collector ring to be run in,after which the surface film began to form gradually.The surface film was mainly composed of a graphite layer and an oxide layer.The size of the film resistance was mainly affected by the film thickness.During the formation of the surface film,the contact resistance between the carbon brush and the collector ring is mainly based on shrinkage resistance in the early stage,and film resistance in the later stage.The rotational speed is the main factor affecting the rate of surface film formation.High temperature is the direct cause of damage to the surface film.In this paper,the current-carrying operation of the carbon brush/slip ring under actual working conditions is restored to the greatest extent,and the formation mechanism of the surface film is analyzed,which has a certain guiding significance for the formation and maintenance of the surface film of the collector ring.