Research On The Suppression Methods Of Conduction Common-Mode EMI For PWM Motor Drive System

PWM motor drive system is widely applied to industrial systems and civil devices because it has excellent operating characteristic; however, it also brings severe EMI problem. The EMI affects the safety and stabilization not only of the motor drive system itself but also of other surrounding electrical system. The conducted common-mode EMI has enhanced systematic radiated interference because its circulating path includes the earth. At the same time it also brings conducted interference to other electrical system in the form of interference current. Emission intensity will gradually strengthen with the rise of switching frequency, voltage and current ratio of power converter. So this dissertation investigates conducted common-mode EMI suppression techniques in the PWM motor drive system and the modeling method of related equivalent circuit. Taking this as a base, some methods to solve this problem are proposed.The common-mode equivalent circuit of induction motor in conducted interference frequency band is proposed based on deep research of the formation mechanism and the time/frequency domain characteristics of common-mode voltage and formation mechanism of common-mode current at induction motor side in PWM motor drive system. In this model, equivalent circuit parameters are obtained according to measurement values of high frequency common-mode characteristic of induction motor and measurement values of amplitude and frequency characteristic of impedances which are winding to the earth and to the neutral point. This equivalent circuit model can be used to analyze and predict common-mode EMI emission intensity and common-mode EMI current at motor side in PWM motor drive system. The consistency of simulation and experimental result is good.It is hard to reduce effectively conducted common-mode EMI emission at power convert side, in order to solve the problem this paper propose a common-mode EMI suppression method by analyzing the formation mechanism and propagation characteristic of common-mode EMI at this side. The suppression is implemented by increasing the common-mode impedance between heat sink and the earth. By adopting FEM-BEM coupling method, the paper simulates numerically the surface electric field of heat sink with and without the presented method separately. The simulation and experimental result validate that the presented method can effectively suppress common-mode EMI emission intensity at power convert side, eventually reducing the EMI emission intensity of the whole system.Whereas common-mode voltage and common-mode current problem exists commonly at induction motor side in PWM motor drive system, this paper proposed a novel structure of passive common-mode EMI filter placed at power converter output side according to the formation mechanism of common-mode voltage and common-mode current at induction motor side. This passive filter is composed of a common-mode voltage detecting element and a four-winding voltage compensation transformer. The common-mode voltage detecting element adopts inductors, the structure of which is similar to an unilateral transformer in Y connection, and capacitors, paralleled with each phase winding. The proposed filter overcomes the disadvantage of conventional filter which has either poor high frequency characteristic or poor low frequency characteristic and has too large bulk. Both simulation result and experimental result prove that the proposed passive filter can effectively suppress common-mode voltage and common-mode current at induction motor side, realizing effective suppression of emission intensity of conducted EMI of the whole system.Insert losses of active EMI filter and optimal match problem in the design and application are researched. And a novel structure of active EMI filter is proposed to restrain common-mode EMI current in the system according to the conclusion that current detection and compensation mode active EMI filter adapts to suppress common-mode current noise and leakage current. In the proposed active filter, compensation current is directly injected into system, thus, propagation path of common-mode EMI at source side of the system is blocked and the aim to effectively suppress common-mode conducted EMI is achieved. Compared to typical active filter, compensation current of which is directly injected into the earth, the proposed filter overcomes the shortcoming of small insert loss caused by deficiency of compensation current. In the source design of the proposed filter, much attention is paid to suppression of common-mode EMI current. Both simulation result and experimental result prove that the proposed filter can effectively reduce emission intensity of conducted EMI in system.