变频调速恒压供水系统

基于PLC的变频调速恒压供水系统的研究1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3台电动机的起动、运行与调速，采用循环使用的方式运行。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STEP7MicroWin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。在模拟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基于PLC的变频调速恒压供水系统的研究2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。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。关键词:PLC，变频调速，恒压供水，变频转工频，相位一致基于PLC的变频调速恒压供水系统的研究3AbstractWiththeProgressdevelopmentofsocialeconomicandtheconstructscaleofcitycontinuouslyeularge，increasingofpopulationandstandardoflivingofpeoplecontinuouslyimproved，moreandmoredemandingtoquantity，qualityandstabilityofwatersupplybringforwardincity.Theautomaticdegreeofwaterworksintownismuchlowerthanthecorrespondenceincity.Especiallyinsomeoldwaterworks，theequipmentofautomaticcontrolsystemisdroppedbehind.Thecontrolofunitismainlydependedonmanualoperatingofwatches.Theprocessofcontrolisverycomplicatedandthatmanualcontrolcan’tmeetthechangeofpressureinthepipesandthewaterlevelofthepurepond.Italsocan’tacttheproperfeedbackintime.Forthesakeofwatersupply，theelectromotorunitusuallyworksinthestateofovercharge.Notonlyefficiencyofelectromotorislowandelectricityconsumingisbigger，butalsothepressureofpipesisoverloadinlongterm.Onthebasisofanalyzingstatusinquoofthewaterworksinourcountry，thispaperdesignsasuitofconstantpressurewatersupplyautomaticcontrolsystembyusingvariablefrequencyspeed-regulatingtechnologybasedonPLC.ThesystemismadeupofPLC，transducer，unitsofpumpsandelectromotors，pressuresensor，industrialcontrolcomputerandconsole.Thesystemisusedatransducertomakethreeelectromotorsstarting，runningandtiming.TotuneupthefrequencyofPowersupplybythetransducerinthesystem，thisadjustingmakethespeedofelectromotorandperformancecurveofpumpchange.Atlast，itmakethestatusofapumpalter.Throughanalyzingthefigureofenergywastingofpump，thequalityinpipesisdirectproportionofthespeedofpumpifthespeedofpumpisreduced.Thepowerofelectromotorfellthecubeofthespeedofpump.Sotheenergywastingofconstantpressurewatersupplybasedonvariablefrequencyspeedregulatingtechnologyisenvidentlessthanthetraditionalmodeisusedthrottletosupplywater.ThroughusingSTEP7MicroWinProgramsoftwareofSIMENSCompanyinGermaninthecontrolsystem，aPIDcontrollerofpressurecontrolwithinPLCisdesigned.Thecontrollercancomparethemeasurevalueandthevalueinadvanceofpressure.Throughreal-timecontrollingtheoutputvoltageandfrequencyof基于PLC的变频调速恒压供水系统的研究4transducer，theoutputqualityofpumpischangedalongwiththechangingofpump’sspeed.Itmakesthepressureofpipeself-regulatingandsteadyinthescheduledvalue.Therearesomedrawbackssuchasconversioncurrentisgreat，easytoburnthefuseandswitchifoffinvariablefrequencytoworkingfrequencyofhigh-powerelectromotor.Intheformerresearching，theconversionofvariablefrequencytoworkingfrequencyisonlyemphasizedthespeediness.Theconversionisusuallyfailedbecausethekeyisnotthespeedinessofconversion.Throughanalyzingandresearchingtheproblemofvariablefrequencytoworkingfrequencyofhigh-powerelectromotorintheory，thekeyofconversionispointedoutinthepaper.Onthebasisofanalyzingequivalentcircuitdiagramandvector-diagramofinduction-motor，thePhaseofworkingfrequencypowerwhetherornotconsistentwiththeoutputphaseofvariablefrequencypoweratthemomentofconversionisthekeytodecidewhethertheconversionissuccessfully.Withtheapplicationofphasedetectorinindustrialexperimentonthespot，whenthephaseofworkingfrequencypowersupplyisconsistentwiththeoutputphaseofvariablefrequencypowersupply，PLCsendsouttheinstructiontoswitchofftheoutputoftransducer.Theelectromotorisconnectedwiththeindustrialfrequencyvoltageafteritwasdisconnectedfromtheoutputoftransducer.Theconversiontimeislessthanhalfhundredms.Theswitchoffcurrentofvariablefrequencytoworkingfrequencyisaroundoneandhalftimesofratingcurrentofelectromotorwhenthemeasuresismade.Itissucceedtosolvetheproblemofvariablefrequencytoworkingfrequencyandrealizingthesmoothconversionofhigh-powerelectromotor.Itreducedtheimpactonpowersupplyandequipments，too.Theoperationandexperimentdemonstratesthatthesystemmaketheworkofwatersupplyautomatedinsomedegree.Itisgoodforenergysavingandimprovingthemanagementofwaterworks.It’salsofacilitytomountandmaintain，reliabletooperating，andprecisetosuperviseandcontrol.KeyWords:PLC，variablefrequencyspeed-regulating，constantpressurewater-su