Research On Variable Frequency Speed-regulating System Based On The Dual-pwm Converter

To achieve motor energy-saving, variable frequency speed-regulating technology is wildly used in industrial production procedure. In normal, uncontrollable diode rectifier is used as the front power supply unit in traditional inverter, which leads to many problems, such as low power factor in grid-side, large current harmonic content, unidirectional power flow. Fortunately, the dual-PWM variable frequency speed regulating system which takes PWM rectifier composed of full-controlled power devices as the front power supply unit not only can achieve good speed-regulating performance, but also can overcome those disadvantages montioned above.Firstly, the circuit topologies and control stratigies of dual-PWM converter wre summarized in this paper. Then, as to the voltage source dual-PWM converter topology used in this paper, its working principles and current changing method were analyzed.As the two units of the dual-PWM variable frequency speed-regulating system, PWM rectifier and inverter are in the same bridge structure, but they take different control objectives. Therefore, it's necessary to design corresponding control system for them. And the overall performances of the dual-PWM system mostly lie on the quality of the separated control system. As to the reason that many advantages of dual-PWM converter are fulfilled by the PWM rectifier, more studies were done in this front power supply unit. Dynamic model is very important for further study. So the PWM rectifier's dynamic model was constructed at first. Then, based on the vector control principle, dual closed-loop control system which composed of outer voltage loop and inner current loop was designed in the two-phase synchronous rotating coordination. And, the vector control method based on the rotor flux orientation was adopted in inverter unit (motor speed-regulating system). The vector control eaquations and rotor flux observer model were derived from the dynamic model of induction motor. Then, the vector control system was built. PWM rectifier dual-loop control system and vector control stratedy of induction motor were both valided by simulation.Based on what were done above, the dual-PWM variable frequency speed-regulating system was built. The working performances of whole system was analyzed through simulation. And it not only could achieve good speed-regulating performance, but also could fulfill unit power factor in grid-side and bi-direction power flow. However, the larger DC-bus voltage fluctuation during the dynamic process of mutation load will bring about a larger threat to the system in practical. Corresponding suppression strategies were proposed from two aspects based on the analysis of this phenomenon. One strategy is adopting direct power control (DPC) method which has a faster dynamic response in the PWM rectifier; the other is power coordination consideration of the whole dual-PWM variable frequency speed-regulating system(which in detail is power feedforward control strategy). Through the superposition of these two strategies, the fluctuation of DC-bus voltage was significantly reduced, which was valid by simulation results.Finally, the experimental platform of PWM rectifier which took TMS320F2812 as its center control chip was constructed. Some experiments of the constructed PWM rectifier platform were carried out. And related experiment waveforms were listed and analyzed at the end of this paper.