Research On High Frequency Bearing Current Model And Bearing Discharge Mechanism Of AC Motors

AC motors are used in various fields,and their lifetime and operational stability have also received widespread attention.Bearing faults are the main cause of abnormal motor operation.There are many reasons for bearing failure.With the development of power electronics technology,the high frequency bearing current existed in the variable frequency driven AC motors has become an important cause of bearing failures.The bearing current will cause electric corrosion pits on the bearing surface,which will cause early premature of the bearing,the unit to stop running and bring huge losses in serious cases.However,the bearing breakdown process is a multidisciplinary and complex problem,and its research has not been fully developed.Therefore,it is important to analyze the high-frequency bearing current model of AC motor,explore the breakdown mechanism of bearing and study the bearing current suppression methods and suppression standard.Firstly,the high frequency bearing current model of AC motors is analyzed.The analytical calculation formulas of bearing voltage and bearing current in the lumped parameter model and the three-phase distribution model are given respectively.The bearing voltage and bearing current amplitude of the two models are equal.The frequency of 3-phase distribution model is(?)times of the lumped parameter model,and the above conclusions are verified through the simulation in MATLAB SIMIULINK.Then the influence in the axial distribution model of the number of segments on the bearing voltage and bearing current is analyzed.The simulation results of the axial distribution model and the lumped parameter model are compared with the measured data.It is found that the axial distribution model is closer to experimental results.Secondly,the breakdown mechanism of the bearing is studied and the variable resistance model of bearing is proposed.The experiment to test the electrical properties of the bearing is designed.Combined with the structure of the bearing and the arc breakdown theory,the variable resistance model of the bearing breakdown is proposed.Later,the parameter in the variable resistance model is determined according to the experimental data.In addition,the variable resistance model and the constant resistance model are compared and analyzed.The results show that the bearing voltage and the bearing current waveforms of the variable resistance model are closer to the experimental results.Thirdly,a preliminary study of the relationship between bearing current and bearing damage was made.A three-dimensional model of the bearing was built and the static structural simulation of the model was performed to obtain the minimum thickness of the bearing oil film.The electric-thermal coupling model of the bearing is established,and the breakdown path is set in the model.Through the coupled simulation of the electromagnetic field and the thermal field,the temperature rises at the breakdown point under a certain current excitation is calculated.Then,based on the determination of the melting point temperature of the bearing material,a method for determining the critical current of the electrical corrosion of the bearing is proposed.Finally,this thesis analyzes and tests the suppression measures of the bearing current.Based on the bearing current equivalent circuit of the doubly-fed wind turbine,the suppression principle of the carbon brush and the magnetic ring is analyzed.The mitigation effects of several measures were tested on the experimental platform.The results show that the carbon brush has the best inhibitory effect on the bearing voltage,and the more carbon brushes,the better the suppression effect.And the magnetic ring has a better suppression effect on the common mode current.