Key Problems Of High Frequency Bearing Currents In Variable Frequency Induction Motor Fed By PWM Inverter

Bearing current of the variable frequency speed regulation system has become one of the main issues for the PWM(Pulse Width Modulation) inverter supply. The bearing failure caused by bearing current has serious effects on the stability and security of the system. The follow aspects, calculation model of discharging bearing current and circulating bearing current, influences of system factors on the rule of bearing currents, measurement method of bearing currents and suppression technology of bearing currents, were researched on, which is the basic theory for the bearing current issue of variable frequency motor fed by PWM invertor. The concrete research content is as follows:Initially, the measurement of bearing current is the key step to evaluate the calculation model and analyze the rule of bearing currents. Therefor the experiment platform for menasurment of bearing currents was set up with the test induction motors, induction motor YVP3, induction motor YVP11, middle voltage induction motor YSY-75 I and high voltage induction motor YSY-75 II. The modified process of the test motors was presented, which enable the measurement of the parameters, such as the bearing current, the bearing voltage, the grounding current and the common mode voltage. According to the frequency domain characteristics and order of magnitudes of the bearing current, the bandwidth and measuring span were given to reduce measuring error.According to the lumped parameter common mode equivalent circuit, the analytical expression of discharging bearing current was deduced. The analytical solution of the parasitic parameters was given. The bearing voltage coefficient, tooth slot ratio and slot gap ratio were presented to illustrate relationship between dimension of slot and teeth of the motor and bearing voltage. The model of distribution parameters common mode equivalent circuit was presented that distribution impedance characteristic of the winding was considered in this model. And the parasitic capacitance of the winding overhang was taken into account to improve the rationality of the model. The impedance separation method was present to measure the parasitic parameters. This method took the frequency domain characteristic of the parasitic parameters and medium in the motor into account. The calculation accuracy of the bearing current was improved.Accoring to the characteristics of the high frequency grounding current, the distribution paramenters network was present to calculate the grounding current. The calculation method of parameters in the network were given and grounding current was simulated. The anlatical sultion of the common mode flux was deduced and the relationship of the common mode voltage, the common mode current and the end to end shaft voltage was analyzed. The phenomenon of circulating bearing current was equivalent to the current transformer model and the inductance and resitance in the path of the circulating bearing current were calculated. The expression about the ratio of circulating bearing current and the grounding current was obtained, which offered the theory basic for the quantitative calculation.Based on the experiment platform, influence of the system hardware and parameters on the rule of the bearing current was analyzed. The beairng current was meaured in the condition of diffenent speed, switching frequency of inverter and bearing temperature. The influence of factors, such as the cable shield, cable length and grouding type, on the bearing current was analyzed. The research integrated the theory of the bearing current and the whole variable frequency speed regulation system.Finally, the suppression effectiveness of the bearing current short circuit ring, electromagnetic shield slot wedge and insulating were analyzed respectively. Based on the shielding theory, electromagnetic shielding slot wedge(ESLW) was proposed to suppress high frequency discharge bearing current. The structure design method of ESLW was presented. An YVP3 induction motor with ESLW was designed and manufactured. Shield effectiveness was evaluated by element finite method and bearing current experiment.