Research On The Prediction And Suppression Of CM EMI In Permanent Magnet AC Drives

Permanent magnet AC drive system is used more and more widely, such as high precision servo,electric vehicle and wind turbine generation. It has been the important domain of the domestic andforeign researchs now. As the demands for higher power densities and smaller footprints for thesystem, designers usually increase converters’ switching frequencies to provide equipments smallervolume. This makes the EMI problems, particularly common-mode(CM) EMI which includes largecirculating path and strengthens emission intensity with the rise of switching frequency. In this paper,the permanent magnet AC drive system is studied; its design method as well as prediction andsupression of CM EMI are also proposed.The mathematical model of permanent magnet synchronous motor(PMSM) is established andtwo kinds of general strategies are stated. Compared with each other, the SVPWM is used for theimplementation of control strategy. After software and hardware of prototype is designed successfully,the validity of regarding the prototype as EMI experimental platform is comfirmed.As far as the Intelligent Power Model (IPM) equipment in a typical system is concerned, itsvoltage of mid-leg jumps constantly. An equivalent model of CM EMI noise source is presented byanalyzing that phenomenon. The propagation path of CM EMI is stated, and then the high frequencyparameters of the main parts are extracted with different methods.Nanely, high frequency parasiticparameters of power converter are extracted with analytical method; high frequency parameters ofshield cable are extracted with numerical calculation method; equivalent model of CM EMI forPMSM is obtained through measured with impedance analyzer and calculated with resonance unit.Therefore, A method of predicting common mode EMI (CM EMI) is explored and carried outwith Saber. In150kHz~30MHz frequency, actual measurement of CM EMI for prototype iscompleted, compared with the simulation result, the validity of the proposed model is confirmed.A method to suppress the noise is proposed, in which a planar EMI filter will be used. The finalexperimental measurement shows that this filter can greatly reduce the noise in this system.Based onthe current probe, the near field coupling EMI between IPM and EMI filter is measured and its impacton CM EMI of the system is also analyzed. The study of near field coupling EMI is valuable forpredicting the EMI accurately during the earlier stage of development.