Analysis And Measurement Of Bearing Current In Varied Frequency Supply AC Motors

Variable frequency drive technology is widely applied to motor drive systems due to the rapid development of high-performance semiconductor switching devices and the advanced control strategies. However, the negative effects such as the bearing currents exist in the variable frequency driving systems. The characteristics of the common mode voltage generated by inverters are non-zero and high-frequency. The common mode voltage can induce bearing current problem through the common mode paths and motor stray capacitances, and finally it can cause bearings early failure. The problems are closed concerned by motor users, motor manufecturers and bearing manufacturers. The equivalent model of bearing current, the calculation of bearing capacitance and the measurent of bearing current are investaged in this thesis. The content is consist of following four parts.Firstly, the bearing current problem of properly grounded motors is analyzed. The equivalent bearing current model is established. The stray capacitances of a 5.5kW squirrel-cage motor are calculated through the finite element method. The bearing voltage and the bearing current are calculated through MATLAB simulation.Secondly, the bearing current caused by rotor grounding is investigated. Based on the Wheatstone bridge theory, a comprehensive equivalent circuit considering stator and rotor grounding impedances is built, through which the influence of motor grounding states on bearing voltage and bearing current is analyzed. Related experiments are performed and the results agree with the analysis. The results show that bearing voltage changes with stator or rotor grounding impedances in a V shape curve rather than a monotonic curve. This means that the bearing voltage and bearing current can be reduced by adjusting the stator or rotor grounding impedances.Thirdly, after analyzing the difference between calculated value and actual value of the bearing capacitance, a new calculation method on bearing capacitance is put forward. The central film thickness of bearing oil is acquired from the elastohydrodynamic lubrication analysis. Then the bearing model is established in ANSYS Maxwell finite element analysis software. Related experiments are performed to verify the calculated results.Finally, the bearing current measure methods and measure results are analyzed. The complexity of bearing current measurement is discussed, and the advantages and disadvantages of three methods on bearing currents measurement are compared. In this thesis the bearing current is measured on a modified test motor. The analytic expressions of measuring bearing current and real bearing current are derived through analyzing the equivalent circuits of bearing current measerment. The conversion ratio between measuring current and real current shows that the measurement method is reasonable.