Research On EMI Filter Optimization Method For AC Motor Drive System

Using the fully-controlled power electronic devices to achieve high frequency switching power converting is the main way of modern AC drive.Pulse width modulation based inverter is the core device in motor drive system.However,during the operation of the motor drive system,due to the high-speed switch of the power module in the inverter,severe electromagnetic interference(EMI)problem will be generated.EMI has a series of negative effects for the system.EMI filter is an effective way to suppress the EMI in the system.For many years,scholars have studied the EMI filters and proposed the optimization method.However,the current research on EMI filter has certain limitation and there is more room for the optimization in power density and integration that has not been utilized well.Therefore,EMI filter for the motor drive system is studied in this dissertation and several optimization design methods of the EMI filter in motor drive system are proposed.The main contents of this dissertation are the following:The generation mechanism and conduction path of the EMI in the motor drive system are first analyzed in this dissertation.EMI test system is first introduced to establish related concepts about EMI test,and then the structure of the power module and motor are displayed and the generation of the leakage current is analyzed.Finally,the interference source and the EMI conduction path in the motor drive system are analyzed based on the analysis above,and the equivalent impedance network used to characterize the common mode(CM)impedance of the motor is obtained by motor CM impedance measurement and impedance curve fitting.Based on this,the optimization idea of EMI filter is proposed.The inductor winding in traditional EMI filter is connected to the main circuit,in addition to filtering the high-frequency EMI noise,the winding has to withstand the current stress of the main circuit which cause the volume and weight of winding and magnetic core large,resulting in an increase of volume and weight for the EMI filter.To reduce the current that CM inductor needs to withstand in the motor drive system to achieve CM EMI filter volume reduction,an active CM EMI filter added in the grounding circuit based on current cancellation is designed in this dissertation.By analyzing the equivalent circuit of this active EMI filter,the element parameters in this active EMI filter are determined.Since the active EMI filter suppresses the system CM EMI by cancelling out the two output current of the active EMI filter instead of increasing the system impedance value,the problem of high motor frame voltage caused by adding CM inductance to the grounding circuit will not occur at the same time when the system CM EMI is suppressed.Finally,the model of the whole system is established in the simulation software and the related experiments are conducted.The simulation and experimental results show that the active EMI filter can suppress the system CM EMI and limit the motor frame voltage to a low value.Most of the existing active CM EMI filters reduce CM EMI filter size by increasing the corner frequency of the passive CM EMI filter.However,most of these active CM EMI filters need transformers to detect the interference current or inject the compensation voltage in order to achieve the CM EMI suppression.Transformers that use magnetic components generally occupy the most volume and weight in active EMI filters.In order to omit the transformer and simplify the structure of EMI filter,a transformerless motor CM impedance based voltage sensing current compensating active EMI filter is designed in this dissertation.By constructing the AC side CM impedance in the system,the sensed CM voltage can be converted into compensation current,and the CM EMI can be suppressed.The value of each component in the active EMI filter can be determined according to the system equivalent circuit.Finally,the simulation and experimental results show that the active EMI filter can suppress the DC side CM EMI in the system well.Through improving the design and manufacturing methods,the integration of inductance,capacitance and other components in EMI filter can be realized which can reduce the volume of EMI filter.By combing the new motor controller topology with the modulation method and reusing the coupled inductor,an EMI filter integrated design method based on zero-CM modulation algorithm and coupled inductor reuse technology is proposed in this paper.The CM EMI can be suppressed by using the zero-CM modulation algorithm,so no additional CM EMI filter is needed.By reusing the leakage inductance of the coupling inductor in the paralleled inverter as the differential mode(DM)inductor and the additional DM inductor can be omitted.Therefore,the DM inductor is naturally integrated in the coupled inductor and no special design and manufacturing methods are needed to achieve the DM inductor integration.At this time,the DM EMI filter can be formed by just adding the DM capacitors and the DM EMI can be suppressed.The coupled inductor which is the core component in this EMI filter is designed and checked in this dissertation and the final experimental results show that the EMI filter obtained by this integrated design method can effectively suppress the AC side EMI in the system.The topology of paralleled inverter and the coupled inductor are fully utilized through this integration method and the EMI filter structure of this integrated design is relatively simple and requires less additional components.Combing the above several innovative methods can suppress the EMI generated in the system and optimize the power density and integration of the EMI filter in motor drive system effectively.