绝缘轴承对变频电机高频轴电流的抑制机理与效果

The Suppression Mechanism and Effects of Insulated Bearings on High Frequency Bearing Current

随着功率半导体器件的发展,脉宽调制技术被广泛应用于变频电机调速系统,显著提高了电机的动态性能,同时也带来了严重的轴承电蚀问题。绝缘轴承具有安装便捷、可靠性高的优点,是目前轨道交通、风力发电等行业主要采用的电蚀抑制措施。但是绝缘轴承对于变频器引起的高频轴电流的抑制机理、抑制效果及其与绝缘涂层参数之间的关系,有待于进行更深入的理论和实验研究。该文分析了变频电机轴电流的产生机理,建立了考虑绝缘涂层参数的变频电机轴电流等效电路模型;分析了全膜润滑稳态过程和放电击穿瞬态过程中,绝缘轴承对于轴电压和轴电流的抑制效果,以及涂层厚度、介电常数对抑制效果的影响。搭建了变频电机轴电流测试平台,进行了绝缘轴承和普通轴承的对比实验。仿真和测试结果表明,采用绝缘轴承可以降低润滑油膜上所承载的一部分电压,可以大大减小轴电流的击穿次数及幅值。

Due to convenient installation and high reliability,insulated bearings are the main electric corrosion suppression measure used in rail transit,electric vehicles,and many other industries.However,high-frequency bearing currents caused by frequency converters pose challenges due to the capacitor effect of insulation coatings,which constitutes the coupling path of the bearing current,resulting in bearing electrical corrosion failures.Therefore,a bearing current equivalent circuit model of motor-bearing coupling is established.The inhibition mechanism,effect,and influencing factors of insulated bearings on high-frequency bearing current are analyzed,providing a theoretical basis for the design of insulating bearing coating parameters.Firstly,the mechanism of the high-frequency bearing current in variable-frequency motors is analyzed,and a coupling model between the motor and bearing is established.The bearing model considers the dynamic parameters of oil films,the equivalent impedance of insulation coatings,and the transient process of discharge breakdown.Secondly,taking a 190 kW induction motor as an example,stray motor capacitance and equivalent insulation coating capacitance are extracted by impedance test curves.Then,based on the equivalent circuit model,the suppression effect of insulating coatings on bearing voltage and bearing current under full film lubrication and discharge breakdown process is analyzed.The influence of insulating coating thickness and dielectric constant on the inhibition effect is studied by simulation.Finally,a bearing current test platform for variable frequency motors is built.Variable frequency power supply and external bearing voltage tests are designed to verify the model and analyze the actual suppression effect.According to the simulation results,the bearing voltage and bearing current can be reduced by decreasing the dielectric constant or increasing the thickness of the insulating coating.However,in full film lubrication,insulated bearings cannot reduce the bearing partial voltage ratio(BVR)by more than 10%.The inhibition effect on bearing current is affected by the breakdown resistance of the bearing.When the breakdown resistance is above 100Ω,the reduction in bearing current is not more than 20%.The test results show that BVR is 5.35%~5.97%using ordinary bearings,and BVR_(iso)is 5.04%~5.51%using insulated bearings.The peak value of bearing current is distributed between 40~100 mA with ordinary bearings.The bearing current is all suppressed below 70mA using insulated bearings,significantly lowering discharge activity compared to ordinary bearings.The following conclusions can be drawn from simulation and test results:(1)When the lubricating oil film of the bearing is not broken down,the capacitance of the insulating bearing coating can share part of the bearing voltage,thus reducing the probability of the breakdown of the oil film.However,the inhibition effect is limited in full film lubrication.(2)Insulating coating thickness and dielectric constant influence the inhibition effect of insulated bearings on bearing voltage,and their design should consider the breakdown threshold of bearing voltage.(3)Increasing the capacitance of the insulation coating reduces bearing current after oil film breakdown and discharge activity.(4)The suppression effect of insulated bearings on bearing current is affected by the breakdown resistance.If the breakdown resistance is tested in different working conditions in advance,coating parameters can be designed according to the corresponding suppression target.