Study On The Temperature Distribution And Contact Characteristics Of The Carbon Brush-slip Ring Friction Pair Of Hydro Generator Set

With the increasing wisdom of hydropower plants,the operational reliability and intelligence of hydro generator sets and accessories have become more and more demanding.However,the carbon brush/slip ring assembly,as an important part of the excitation system of hydro generator set,has frequent failures due to carbon brush overheating,firing and arcing,which will lead to damage on the surface of the slip ring and micro cracks on the surface material and inside,shortening the service life of the slip ring in the long run.Over the years,the problem has been alleviated by installing dust removal devices and machining spiral grooves on the surface of the slip ring,but the resulting temporary shortage still occurs from time to time.In this paper,the temperature distribution and contact characteristics of the carbon brush/slip ring assembly are studied by simulation and theoretical calculation,taking the carbon brush/slip ring assembly as the research object and combining with the actual operating conditions.The specific research contents and conclusions are as follows:The temperature field of the carbon brush/slip ring assembly was simulated based on COMSOL Multiphysics finite element analysis software,and the temperature characteristics of the carbon brushes were analyzed under the conditions of uniform and uneven overcurrent.The results show that when the overcurrent of a single brush increases or decreases,its temperature rises or decreases,while the temperature of the brushes on both sides is less affected by the current,and the critical value of the overcurrent is 84 A.When the overcurrent of multiple brushes increases or decreases,its temperature rises or decreases further,and the temperature of the adjacent brushes changes in the same trend as the previous condition,and the critical value of the overcurrent is 74 A.At the same time,the lower ambient temperature will reduce the temperature of the brush set and prevent the overheating of the brushes caused by the increased overcurrent to a greater extent.A mathematical model of contact resistance versus carbon brush temperature is developed by combining electrical contact theory and heat transfer science,aiming to estimate the contact resistance between interfaces by carbon brush temperature to evaluate the contact state between friction subsets.The relevant parameters in the model are obtained by the current-carrying friction test,and the accuracy of the model is verified by using COMSOL Multiphysics finite element analysis software.The results show that the contact resistance reaches a peak at the beginning of the system operation,and then gradually tends to level off.When the interface contact pressure is certain,the contact resistance value decreases with the increase of rated current;under the condition of a certain rated current,the contact resistance value of carbon brush with high interface contact pressure is small.When the collector ring speed increases,the effect of spring pressure on the contact resistance between friction pairs further increases.The transient contact characteristics of the carbon brush set during the eccentric rotation of the collector ring are studied based on the ADAMS mechanical system dynamics analysis software.The contact stability of the carbon brush/slip ring system is analyzed under different eccentric distance and spring stiffness conditions,and the spring parameters are optimized.The results show that when the collector ring eccentricity is constant,the increase of spring stiffness makes the contact stability between carbon brush and collector ring worse.When the spring stiffness is constant,increasing the eccentricity of the collector ring makes the contact stability between the carbon brush and the collector ring worse.When the spring stiffness and the eccentricity distance increase simultaneously,the contact stability between the brushes and rings further deteriorates.In addition,the optimum spring stiffness is 1.2 N/mm when the collector ring eccentricity is 0.5 mm and 1.4 N/mm when the collector ring eccentricity is 0.8mm.