基于PLC的变频调速恒压供水系统研究

【作者】 顾跃；

【导师】 刘义伦；

【作者基本信息】 中南大学 ， 机械工程， 2003， 硕士

【摘要】 随着社会经济的飞速发展，城市建设规模的不断扩大，人口的增多以及人们生活水平的不断提高，对城市供水的数量、质量、稳定性提出了越来越高的要求。我国中小城市水厂尤其是老水厂自动控制系统配置相对落后，机组的控制主要依赖值班人员的手工操作。控制过程繁琐，而且手动控制无法对供水管网的压力和水位变化及时作出恰当的反应。为了保证供水，机组通常处于超压状态运行，不但效率低、耗电量大，而且城市管网长期处于超压运行状态，曝损也十分严重。本论文结合我国中小城市供水厂的现状，设计了一套基于PLC的变频调速恒压自动控制供水系统。 变频调速恒压供水自动控制系统由可编程控制器、变频器、水泵电机组、压力传感器、工控机以及控制柜等构成。系统采用一台变频器拖动4台电动机的起动、运行与调速，其中两台大机(220Kw)和两台小机(160Kw)分别采用循环使用的方式运行。通过工控机和PLC连接，开发出了用于工控机的集数据采集和通信、设备状态控制和数据管理的监测程序，实现了监测控制。 在变频调速恒压供水系统中，单台水泵工况的调节是通过变频器来改变电源的频率f来改变电机的转速n，从而改变水泵性能曲线得以实现的。分析水泵工况的能耗比较图，可以看出利用变频调速实现恒压供水，当转速降低时，流量与转速成正比，功率以转速的三次方下降，与传统供水方式中用阀门节流方式相比，在一定程度上可以减少能量损耗，能够明显节能。 控制系统中采用了德国SIMENS公司的STEP7 Micro Win编程软件，设计了一个用于供水系统压力控制的PID控制器，PID控制器内置在PLC中，该控制器对压力给定值与测量值的偏差进行处理，实时控制变频器的输出电压和频率，进而改变水泵电动机的转速来改变水泵出水口流量，实现管网压力的自动调节，使管网压力稳定在设定值附近。 大功率电机变频转工频时存在转换电流大，容易烧毁熔断保险或跳闸的情况。在以前的研究中，变频转工频时只是强调转换过程要求在瞬间完成，并没有指出问题的关键所在，因此转换并不成功。本论文针对大功率电机变频转工频转换存在的问题在理论上作深入细致的研究，根据感应电动机的等效电路和相量图分析，指出大功率电机变频转工频能否成功，关键在于变频转工频瞬时，工频电源和变频输出电源是否相位一致。 在现场工业试验引入鉴频鉴相控制器，在工频电源和变频输出电源相位一致时，PLC发出指令切断变频器输出，电机从变频器输出端断开后，马上接入工频电源，转换时间控制在几十毫秒之内。采取这些措施可以使变频转工频的转换电流控制在电机额定电流的1.5倍左右，成功地解决大功率电机变频转工频的问题，实现了大功率电机变频转工频的平稳切换，减少对电机设备的损坏和电网的冲击。中南大学硕十学位论文 摘要 现场调试和运行表明，该系统能够对供水过程进行自动控制，能够有效地降低能耗，保证了供水系统维持在最佳运行状况，提高生产管理水平。监控系统安装维护方便，运行稳定、可靠；监控软件功能齐全，人机界面友好，使用方便。更多还原

【Abstract】 With the progress development of social economic and the construct scale of city continuously enlarge, increasing of population and standard of living of people continuously improved, more and more demanding to quantity, quality and stability of water supply bring forward in city. The automatic degree of waterworks in town is much lower than the correspondence in city. Especially in some old waterworks, the equipment of automatic control system is dropped behind. The control of unit is mainly depended on manual operating of watches. The process of control is very complicated and that manual control can’t meet the change of pressure in the pipes and the water level of the pure pond. It also can’t act the proper feedback in time. For the sake of water supply, the electromotor unit usually works in the state of over charge. Not only efficiency of electromotor is low and electricity consuming is bigger, but also the pressure of pipes is overload in long term. On the basis of analyzing status in quo of the waterworks in our country, this paper designs a suit of constant pressure water supply automatic control system by using variable frequency speed-regulating technology based on PLC.The system is made up of PLC, transducer, units of pumps and electromotors, pressure sensor, industrial control computer and console. The system is used a transducer to make four electromotors starting, running and timing. The two high-power electromotors(220Kw) and low-power electromotors(160Kw) is partly circulated. To connect control computer with PLC and realize supervisal, a real time supervisal control software of water supply system is developed. The software has the function of data connection, communication, state control of facilities and data management.To tune up the frequency of power supply by the transducer in the system, this adjusting make the speed of electromotor and performance curve of pump change. At last, it make the status of a pump alter. Through analyzing the figure of energy wasting of pump, the quality in pipes is direct proportion of the speed of pump if the speed of pump is reduced. The power of electromotor fell the cube of the speed of pump. So the energy wasting of constant pressure water supply based on variable frequency speed regulating technology is envident less than the traditional mode is used throttle to supply water.Through using STEP7 Micro Win program software of SIMENS Company in German in the control system, a PID controller of pressure control within PLC isdesigned. The controller can compare the measure value and the value in advance of pressure. Through real-time controlling the output voltage and frequency of transducer, the output quality of pump is changed along with the changing of pump’s speed. It makes the pressure of pipe self-regulating and steady in the scheduled value.There are some drawbacks such as conversion current is great, easy to burn the fuse and switch if off in variable frequency to working frequency of high-power electromotor. In the former researching, the conversion of variable frequency to working frequency is only emphasized the speediness. The conversion is usually failed because the key is not the speediness of conversion. Through analyzing and researching the problem of variable frequency to working frequency of high-power electromotor in theory, the key of conversion is pointed out in the paper. On the basis of analyzing equivalent circuit diagram and vector-diagram of induction-motor, the phase of working frequency power whether or not consistent with the output phase of variable frequency power at the moment of conversion is the key to decide whether the conversion is successfully.With the application of phase detector in industrial experiment on the spot, when the phase of working frequency power supply is consistent with the output phase of variable frequency power supply, PLC sends out the instruction to switch off the output of transducer. The electromotor is connected with the industrial frequency voltage after it was disconnected fr更多还原