Research On Capacitances Calculation And The Bearing Damage Degree In Bearing Currents Of AC Motors

Frequency converters and high-speed power devices are widely used in modern AC transmission system,accompanied with the high frequency bearing current problems.The high frequency bearing current causes electric corrosion of bearing,shortens the service life of bearing,seriously endangers the safe operation of AC motor,and causes unit outage.In this thesis,the stray capacitances and bearing parameters of AC motor are calculated accurately,the damage degree of bearing current to bearing is studied,and the corresponding bearing current suppression strategies are put forward.It has important theoretical significance and engineering practicability for early fault judgment and early warning of bearing and even reducing the incidence of such faults.Firstly,the stray capacitances of the induction motor are calculated and tested.The analytics method and the finite element simulation are used to calculate the capacitances.By comparing the 2D and 3D finite element results with the test results,it is found that the stator end winding has a great influence on the capacitance between the stator winding and the rotor.The improved end modeling method can improve the calculation accuracy.Comparing the calculation and test results of capacitance between the rotor and the stator,it is found that the capacitance formed by bearing labyrinth seal must be considered.The influence of labyrinth sealing parameters on the capacitance and bearing voltage ratio are further analyzed.The results show that the bearing voltage ratio increases when the radial and axial clearance of the sealing cavity increases.Secondly,the parameters of bearing capacitance in healthy state are calculated and tested.Based on the theory of bearing force and elastohydrodynamic lubrication,the thickness of lubricating oil film and Hertz contact area are analyzed,and the bearing capacitance parameters are calculated for the cylindrical roller bearing and the ball bearing respectively,and the variation of bearing capacitance parameters under different speed,load and temperature is analyzed.The capacitance parameters of bearing oil film are tested by voltammetry study,and the accuracy of calculation of the bearing capacitance parameters is verified.Then the bearing damage degree of bearing current is studied.The influence of motor parameters,the roughness and the impurity particles on the electric field strength and breakdown threshold voltage is compared and analyzed.After that,the further influence of bearing failure degree on bearings is analyzed,and the bearing capacitance parameters in non-healthy state are calculated by finite element method.The simulation model of frequency conversion drive AC motor is built,and the bearing voltage prediction of healthy bearing and unhealthy bearing is compared to predict the probability of breakdown of bearing again.Finally,the suppression measures of bearing current are studied.This thesis analyzes the inhibition principle of insulated bearings on bearing currents,calculates the influence of insulation coating thickness and material dielectric constant on bearing voltage ratio,and further compares and tests the motor bearing current experiment of ordinary bearings and insulated bearings.The effect of restraining bearing partial pressure when motor is equipped with insulating bearing is analyzed.Then the principle of common mode chokes suppression is analyzed and tested on the experimental platform.It is found that the common mode choke has more obvious inhibition effect on common mode current.In order to restrain the bearing current better,different number of common mode chokes and the turns of windings are tested.It is found that the more the number of common mode chokes in a certain range,the more the winding turns,the more obvious the inhibition effect on the bearing current.