Research On EMC And Reliability In The Motor Drive System Based On A Three-Phase Four-Leg Inverter

With the development of power electronics technology and the drop in the cost of elements, the speed regulating system of the motor driven by the inverter is being more and more widely used in various fields. It is a matter of vital importance that the motor works securely and reliably in some fields such as astronautic, aeronautic, military, mining, medical, etc. In these cases, the motor must run uninterruptedly on the rails even if some faults occur. In the speed regulating system, the power semiconductor element and the control circuit of the inverter are the weakest sectors which tend to break down easily. Thus to increase its reliability in work is very necessary. Besides, the pulse width modulation (PWM) technology is adopted in almost all speed regulating system. The PWM technology can simplify the structure of the inverter, improve the dynamic response performance of the system, decrease the harmonic wastage of the motor and increase its utilization rate of energy. But the high speed pulses cause the large dv/dt and unbalance voltage in inverter output, which will lead to the huge common-mode (CM) voltage and couple to overload bearing voltage and bearing current. This is harmful to the motor system itself and will shorten its service life. Apart from that it also brings about serious electromagnetic interference (EMI) to affect the working reliability of the surrounding equipments. The research in this dissertation is about improving the electromagnetic compatibility (EMC) of the inverter and increasing the reliability of the motor speed regulating system based on three-phase four-leg structure. The works mainly include four aspects: study on reducing the inverter output CM interference by using three-phase four-leg structure, strategy of optimizing the difference-mode (DM) performance in the three-phase four-leg inverter, reliability analysis of three-phase inverter and fault tolerant operation of motor speed regulating system based on three-phase four-leg inverter.On the basis of the inverter CM model, the generation of huge CM voltage in the traditional three-phase inverter is attributed to the unbalanced output voltage. The three-phase four-leg topology can balance the output voltage by the fourth leg if the zero-state doses not appear in the first three legs. A novel method to control three-phase four-leg inverter named jump-edge delay strategy is proposed. This strategy is based on SPWM carrier phase-shifting control technology, and breaks through its modulation index limitation which must be less than 0.666 to avoid the extremely low utilization ratio of direct current (DC) voltage. This strategy can let inverter to avoid the zero-state in any conditions and reduce the CM voltage and CM current greatly. Compared with several SPWM and space vector PWM (SVM) strategies which are ordinarily used in three-leg or four-leg inverter by simulating and experimenting, the three-phase four-leg inverter controlled by SPWM jump-edge delay strategy has overwhelming better CM output performance.Based on the DM model of the three-phase four-leg inverter, the DM performance of SPWM jump-edge delay strategy is analyzed. Its DM performance becomes worse because of the alteration to the local volt-second equivalent between the pulse and sine wave. Two optimized strategies are put forward. One is named SPWM pulse delay strategy which does not increase the number of switching. The other is named shortest interval state-exchange strategy that has the best DM characteristic. The simulation and experiment results prove that the above two strategies have the excellent CM restraint ability as SPWM jump-edge delay strategy and the better DM performance than jump-edge delay strategy. The shortest interval state-exchange strategy is the best choice for EMC characteristic in the three-phase four-leg inverter.The failure rates of the elements in the main circuit of the inverter are analyzed in details under the reliability theory. The reliability prediction model of three-phase inverter is proposed. The quantitative analysis between three-leg and four-leg inverter is processed from the aspects of reliability function and mean time between failures (MTBF). Although some elements are added in four-leg inverter, the reliability is proved to be increased greatly compared with three-leg inverter. Thus the three-phase four-leg topology is quite valid to improve the reliability of the inverter.The fault tolerant operation is the main measure to increase the reliability of the motor speed regulating system. It is achieved through fault diagnosis and the circuit reconstruction. The common faults and diagnosis methods of the inverter are introduced. Through the analysis of the inverter output current after failure, a fault diagnosis method is proposed. The relative DC components and working frequency components of three-phase currents are adopted as eigenvector in this method. The simple isolating circuit and the fault switch circuit composed of TRIACs are selected to be used in the high reliability principle. By the sinmulation of three typical strategy controlling three-phase four-leg inverter respectively, it is proved that the speed regulating system is working well in fault tolerant mode.