Analysis Of Coupling Capacitances And Bearing Currents Of Asynchronous Motors Fed By PWM Inverters

ABSTRACT: In recent years, the power electronics technology accelerates the development and application of frequency control technology in industry and energy fields. But some negative effects appear in the electrical drive systems with the increasment of the switching frequency of the power electronics devices. The inverter generates high frequency common-model voltage,which leads to shaft voltage and bearing current and induces bearing failure. It affects the safety and reliability of the electrical drive system seriously. Therefore, the research on investigating the coupling capacitance parameters of the motor and analyzing characteristics of the shaft voltage and bearing current is very important to the safety and reliability of the whole system. The thesis includes following several parts.Firstly, through analyzing the mechanism of bearing current, the significance of calculating coupling capacitance parameters of the motor is pointed out. Combined with actual motor structure, analytical calculation and finite element method are adopted to calculate coupling capacitances of the motor. An improved capacitance calculation model is proposed. Measuring methods are applied to obtain coupling capacitances which are compared with calculated results.Secondly,computation formula of bearing capacitance and resistance are developed based on elastic deformation and fluid mechanics. Then the characteristics of the film and bearing capacitance which are affected by temperature, force and speed of motor are acquired.Thirdly, based on joint modeling of whole PWM inverter motor drive system, coupling paths of bearing current are discussed. The effects of the characteristics of the inverter and the motor capacity on shaft voltage, dv/dt current and EDM current are also analyzed. After that, using filter design theory, in view of long cable, the new type filter is designed to restrain common mode interface. The simulation results show that the new filter has good performance.Finally, the experimental platform of PWM inverter power supply motor drive system is built to measure shaft voltage and common mode voltage. Comparison with experimental results, the results are shown in accordance with the theoretical analysis.