The Research And Application On The Key Technologies Of Variable-frequency Drives

Ac drive, developed fast in recent years, is a new kind of technology and one of resource saving technique implemented by nation. Today, with the resource all over the world becoming less, the research and application of ac drive have an important significance and huge social and economic efficiency. Studying the various ac drive control strategies and technique, and improving the ac drive to a new level has become a hot topic in the drive field at home and abroad. How to improve the performance and credibility of the system with maximum limit becomes the main target in motor control in future, while maintaining the cost. Improving the performance and credibility of digital ac drive system as the target, deep analysis and research is made on the several key problems, such as low speed performance improvement, modulation strategy, over voltage and so on.Enhancing the low-speed performance of induction motor is still a difficult problem, and especially how to improve the motor's loading ability and the steady speed precision in low or zero speed with no speed sensor is a huge challenge. Some kinds of open loop enhancing methods are analyzed and compared in their merits and demerits when implemented. The method, which is based on nonlinear gap power compensation, has been researched in detail. It combines the merit of vector control and scalar control and utilizes the least motor parameters as possible. By analyzing the pole locus of characteristic equation of the system, the influence of stator resistance voltage-drop compensation and low-pass filter on the system's stability is discussed.The nonlinear characteristic of inverter influences the low speed performance of ac motors deeply. The nonlinear characteristic and influence factor are analyzed in detail in this paper. The main works is emphasized on the zero-current clamping phenomenon and its mechanism. A novel compensating method is provided utilizing the stator currents decoupling and the linearization of the angle interval of current vectors.Based on the performance index of voltage utilization ratio, total harmonic distortion (THD), switching loss and etc, various PWM methods have been introduced. The characteristics of PWM methods have been analyzed theoretically and comparison research is introduced between the space vector PWM (SVPWM) and discontinuous PWM1 (DPWM1). The conclusion that different modulation strategies in different modulation depth with fragment can be used is drawn. It can guide the reactivity of engineering proceeds.Many different random PWM methods have been developed to reduce noise and suppress electromagnetic interference (EMI). A new dual randomized modulation of zero-vector and variable-delay is presented based on analysis and comparison of random PWM technologies. The introduced method has the merit in simplifying digital implementation and the special characteristic of harmonic spectrum while operating at low modulation index.The over voltage induced in motor terminal has great hazard on safety and reliability of system operation. The simulated platform has been established, which includes high frequency model of electric cable and low-high frequency general model of induction motor based on the research of over voltage mechanism. The over voltage phenomenon with influence factors less than 2pu and more than 2pu has been analyzed and tested by means of the platform. The corresponding software and hardware methods of suppressing over voltage have been discussed.The experiment platform, which is based on TMS320LF2406, and the general emulating platform, which is based on Saberdesign frame, have been constructed. The related conclusions have been verified with emulation and experiments on the above platforms.