| Using a single variable-frequeccy driver to drag multi-motor variable-frequency operation and then conducting automatic control of constant pressure water supply, which is a technical problem for many water supply enterprises. Output conversion of variable-frequeccy driver is a problem which has not yet gotten enough attention. If adopting hard switching mode directly, not only cannot achieve soft starting of the convertor, but also would cause serious consequences. Especially for high-power motor,direct switching would create a great impact even danger on motor, electrical control equipment, power network. This paper is focused on the conversion problems of High-power three-phase induction motor, it analyses and studies the theories and implementation methods of output switch,specific studies are as follows:1,According to an induction motor stator voltage equation and the phase diagram, discussion is made in static analysis about the problem that exists in the dynamic switching course of high-power motor,this paper studies the effect that switching process of power grid and the inverter output voltage while the both voltage's phases are inconsistent .The result shows that the smaller the initial phase difference is, the lesser the current impact is.2,This paper makes the dynamic analysis about conversion of high-power motor, creates a matlab mathematical model and then discusses the method of solving mathematical model so that to simulate the system of the course of switching. According to a time-varying characteristics of AC motor'inductance matrix,the stator coordinate system useαβcoordinate system ,and the rotor coordinate system use d q coordinate system which is converted from rotor to stator system. A mathematical model is set up,so that the inductance matrix and the rotation motor inductance matrix are constants,achieving decoupling and making elements become constant. Then the fourth-order Runge - Kutta equation is modified to simulate the system by using the different initial phase between the inverter output voltage and power grid in the course of switching.3,Under the way of theoretical analysis of the inverter output switching,this paper explores the transfer method of high-power motor. The phase frequency controllers is designed and made,by using microcontroller to detect and track phase sequence,phase and frequency differences between inverter input voltage and output voltage. When the phase sequence,frequency and phase would become identical,the switching process may be carried on instantly by PLC.This paper analyzes PLL principle in theory and uses it simulate the Locking phase process.4,Experimental study of inverter output switching.First,to use the phase frequency controller,PLC modules and small power motor make the switching experiments in the laboratory. Second,to use high power motor make the switching experiment in the industrial field. Experimental results show: when the inverter output voltage and frequency is same as the power grid's ,switching is done,which can effectively reduce the impact of current and switching current is about 1.5 times of the rated current.Therefore,to switch timely when the inverter input voltage and output voltage have the same frequency and phase,which can effectively reduce or eliminate mechanical shock and impact of voltage and current during the startup process ,and protect electrical equipment and facilities and power grid. This method has a strong practice Promotional value. |
