Conducted Electromagnetic Interference And Suppression In Variable Frequency Motor Drive System

The variable frequency motor drive systems have the advantages of high work efficiency,low system loss and fast dynamic response.With the concept of energy saving and emission reduction being accepted by people gradually and the gradual regulation of relevant national laws and regulations,the use of variable frequency motor drive systems in the household appliance industry is becoming more and more extensive.However,the accompanying problems such as harmonic current and conducted electromagnetic interference have also received increasing attention.In this paper,the pump motor drive unit is taken as an example to study the conducted electromagnetic interference and suppression methods of the variable frequency motor drive systems.First of all,the mechanism of conducted electromagnetic interference in variable frequency motor drive systems is studied.Aiming at the problem of large harmonic current and low power factor of the traditional variable frequency motor drive systems,the overall circuit structure of the variable frequency motor drive systems after adding the Boost power conversion circuit is analyzed.Next,the working principle of the pre-stage Boost PFC and the post-stage three-phase motor drive circuit of the variable frequency motor drive system is briefly introduced.Then,through the analysis of the conduction interference generation mechanism,conduction path and suppression methods of the pre-stage Boost PFC and the post-stage three-phase motor drive circuit,the conduction interference equivalent circuit of the front and rear stage circuits is established.Finally,the general conduction interference equivalent circuit model of the variable frequency motor drive systems is established.Secondly,this paper studies the modeling of key components in EMI filters.The common mode and differential mode high frequency equivalent models of common mode chokes and safety capacitors are established respectively.Considering the frequency nonlinearity of common mode impedance,an improved Foster network cascade model is proposed.Through the comparison tests of multiple groups,the related factors affecting the impedance frequency characteristics of the common mode chokes are analyzed in detail.And several methods for optimizing the impedance frequency characteristics of the common mode chokes are proposed.Finally,this paper analyzes the conducted interference noise of the variable frequency motor drive system through experimental tests.Furthermore,the high-frequency equivalent circuit model of the two-stage EMI filter is established,and its voltage insertion gain expression is derived and verified by simulation.Based on this,an EMI filter design method based on the optimization of the impedance characteristics of common-mode chokes is proposed.Finally,different common mode chokes were made and comparative experimental test was carried out to verify the effectiveness of the design method.