Simulation Computation Of High Frequency Circulating Bearing Current In PWM Inverter-Fed AC Motors

With technological advancement, the PWM control strategy and the performance of semiconductor switching devices has been quickly improved in recent years. Then PWM inverter fed asynchronous motors, due to its accuracy controls, energy savings and torque oscillation reduce, has been vastly used in various industrial fields in our country.Nevertheless, the circuit structure of PWM inverter control system determines the common mode voltage at motor terminals. And the high oscillation frequency characteristics of the common mode voltage have been caused by semiconductor switching devices’ fast switching capacity. That is, high dv/dt of the common mode voltage. The high oscillation frequency common mode voltage lead to a series of influences on asynchronous motors, and finally causes the high frequency circulating bearing current which harm to the stable operation of the motor. This thesis analyzed the series of influences on asynchronous motors and find out the calculation method of high frequency circulating bearing current. The specific analysis is as follows.Firstly, the theory of the common mode voltage at motor terminals and the high oscillation frequency characteristics of the common mode voltage have been analyzed. The value of the oscillator frequency has been calculated. Then, the motor model has been build by Ansoft software and simulated the capacitive coupling parameters of the common mode motor model. Moreover, the error rate of different motor model’s capacitive coupling parameters has been analyzed in this thesis.Secondly, the circuit model of common mode current based on the common mode motor model and the path of the current has been build. Doctor Petri Maki-Ontto’s method of calculate input impedance of the common mode current has been analyzed. Based on this, this thesis put forward a new method to calculate the input impedance according to the theory of electric network knowledge. Then, the input impedance of the300kW induction motor has been calculated use Matlab software.Thirdly, the common mode flux induced by common mode current has been analyzed. The eddy current model of the stator lamination has been built to calculate the common mode flux according to maxwell equations. And the physical properties of the common mode flux have been analyzed.Finally, the circuit model of circulating bearing current has been build combined with the path of the circulating bearing current and the electromagnetic induction relationship between the circulating bearing current and the common mode current. Then the whole model which the circuit of circulating bearing current feed by impulse voltage has been simulated by Matlab/Simulink software. And physical properties of the circulating bearing current have been analyzed.