《自動化專業(yè)英語》03 Electric Control Block(電氣控制模塊)

III.ElectricControlBlock

(電氣控制模塊)Lesson1ElectromechanicalRelays(機電式繼電器)Lesson2Contactors(接觸器)Lesson3HowtoWireaMotorStarter(電動機啟動器如何接線)下一頁III.ElectricControlBlock

(電氣控制模塊)Lesson4HardwareofaPLCSystem(PLC硬件系統(tǒng))Lesson5PLCApplicationExample(PLC應(yīng)用實例)Lesson6HowFieldbusWorks(現(xiàn)場總線的工作原理)上一頁Lesson1ElectromechanicalRelaysRelayConstructionAnelectriccurrentthroughaconductorwillproduceamagneticfieldatrightanglestothedirectionofelectronflow.Ifthatconductoriswrappedintoacoilshape,themagneticfieldproducedwillbeorientedalongthelengthofthecoil.Thegreaterthecurrent,thegreaterthestrengthofthemagneticfield,allotherfactorsbeingequal(Figure3-1-1):Themagneticfieldproducedbyacoilofcurrent-carryingwirecanbeusedtoexertamechanicalforceonanymagneticobject,justaswecanuseapermanentmagnettoattractmagneticobjects,exceptthatthismagnet(formedbythecoil)canbeturnedonoroff

byswitchingthecurrentonoroffthroughthecoil.①下一頁返回Lesson1ElectromechanicalRelaysIfweplaceamagneticobjectnearsuchacoilforthepurposeofmakingthatobjectmovewhenweenergizethecoilwithelectriccurrent,wehavewhatiscalledasolenoid.Themovablemagneticobjectiscalledanarmature,andmostarmaturescanbemovedwitheitherdirectcurrent(DC)oralternatingcurrent(AC)energizingthecoil.Thepolarityofthemagneticfieldisirrelevantforthepurposeofattractinganironarmature.Solenoidscanbeusedtoelectricallyopendoorlatches,openorshutvalves,moveroboticlimbs,andevenactuateelectricswitchmechanisms.However,ifasolenoidisusedtoactuateasetofswitchcontacts,wehaveadevicesousefulitdeservesitsownname:therelay.下一頁返回上一頁Lesson1ElectromechanicalRelaysRelaysareextremelyusefulwhenwehaveaneedtocontrolalargeamountofcurrentand/orVoltagewithasmallelectricalsignal.Therelaycoilwhichproducesthemagneticfieldmayonlyconsumefractionsofawattofpower,whilethecontactsclosedoropened

bythatmagneticfieldmaybeabletohundredsoftimethatamountofpowertoaload.②Ineffect,arelayactsasabinary(onorof)amplifier.Intheaboveschematic(Figure3-1-2),therelay'scoilisenergizedbythelow-Voltage(12VDC)source,whilethesingle-pole,single-throw(SPST)contactinterruptsthehigh-Voltage(480VAC)circuit.下一頁返回上一頁Lesson1ElectromechanicalRelaysItisquitelikelythatthecurrentrequiredtoenergizetherelaycoilwillbehundredsoftimeslessthanthecurrentratingofthecontact.Typicalrelaycoilcurrentsarewellbelow1amp,whiletypicalcontactratingsforindustrialrelaysareatleast10amps.Onerelaycoil/armatureassemblymaybeusedtoactuatemorethanonesetofcontacts.Thosecontactsmaybenormally-open,normally-closed,oranycombinationofthetwo.Aswithswitches,the"normal"stateofarelay'scontactsisthatstatewhenthecoilisde-energized,justasyouwouldendtherelaysittingonashelf,notconnectedtoanycircuit.下一頁返回上一頁Lesson1ElectromechanicalRelaysRelaycontactsmaybeopen-airpadsofmetalalloy,mercurytubes,orevenmagneticreeds,justaswithothertypesofswitches.Thechoiceofcontactsinarelaydependsonthesamefactorswhichdictatecontactchoiceinothertypesofswitches.Open-aircontactsarethebestforhigh-currentapplications,buttheirtendencytocorrodeandsparkmaycauseproblemsinsomeindustrialenvironments.Mercuryandreedcontactsaresparklersandwon'tcorrode,buttheytendtobelimitedincurrent-carryingcapacity.Shownherearethreesmallrelays(abouttwoinchesinheight,each),installedonapanelaspartofanelectricalcontrolsystematamunicipalwatertreatmentplant(Figure3-1-3).下一頁返回上一頁Lesson1ElectromechanicalRelaysAsidefromtheabilitytoallowarelativelysmallelectricsignaltoswitcharelativelylargeelectricsignal,relaysalsoofferelectricalisolationbetweencoilandcontactcircuits.Thismeansthatthecoilcircuitandcontactcircuits)areelectricallyinsulatedfromoneanother.OnecircuitmaybeDCandtheotherAC(suchasintheexamplecircuitshownearlier),and/ortheymaybeatcompletelydifferentVoltagelevels,acrosstheconnectionsorfromconnectionstoground.下一頁返回上一頁Lesson1ElectromechanicalRelaysNewWordsandExpressions1.electromechanicalrelay機電式繼電器2.conductorn.導線，導體3.coiln.線圈4.magneticadj.有磁性的5.permanentmagnet永磁鐵6.solenoidn.螺線管7.directcurrent(DC)直流電流8.alternatingcurrent(AC)交流電流9.irrelevant[i'relivant]adj.不相關(guān)的10.attractvt.吸引下一頁返回上一頁Lesson1ElectromechanicalRelays11.armaturen.電樞12.polarityn.極性13.valvesn.閥，閥門14.binary[‘bainari]adj.二進制的，二元的15.amplifiern放大器，擴音器1G.single-pole單刀，單極的17.single-throw單擲的，單拐的18.mercurytubes汞氣整流管，水銀充氣整流管19.open-aircontacts(暴露在空氣的)觸點20.corrode[ka'raud]vt.使腐蝕21.paneln.面板下一頁返回上一頁Lesson1ElectromechanicalRelaysNotes①Themagneticfieldproduced

byacoilofcurrent-carryingwirecanbeusedtoexertamechanicalforceonanymagneticobject,justaswecanuseapermanentmagnettoattractmagneticobjects,exceptthatthismagnet(formedbythecoil)canbeturnedonoroffbyswitchingthecurrentonoroffthroughthecoil.句中produced

byacoilofcurrent-carryingwire作為定語修飾句首的themagneticfield。譯文:通電導線產(chǎn)生的磁場可以對磁場中的任何磁性物體產(chǎn)生一個作用力，這就好像我們用永磁鐵來吸引磁性材料一樣，不同的是線圈的磁場可以通過控制其中的電流來控制它作用與否。下一頁返回上一頁Lesson1ElectromechanicalRelays②Therelaycoilwhichproducesthemagneticfieldmayonlyconsumefractionsofawattofpower,whilethecontactsclosedoropenedbythatmagneticfieldmaybeabletoconducthundredsoftimesthatamountofpowertoaload.句中which…引導的是一個定語從句，while為轉(zhuǎn)折語氣，表示“卻”的意思。譯文:產(chǎn)生磁場的繼電器線圈，它的功耗可能僅有數(shù)瓦，但這個線圈磁場控制觸頭通或斷的時候，它控制的負載功率能夠達到其本身的數(shù)百倍。下一頁返回上一頁Lesson1ElectromechanicalRelaysTime-delayRelaysSomerelaysareconstructedwithakindof"shockabsorber"mechanismattachedtothearmaturewhichpreventsimmediate,fullmotionwhenthecoiliseitherenergizedorde-energized.Thisadditiongivestherelaythepropertyoftime-delayactuation.Time-delayrelayscanbeconstructedtodelayarmaturemotiononcoilenergization,de-energization,orboth.Time-delayrelaycontactsmustbespecifiednotonlyaseithernormally-openornormally-closed,butwhetherthedelayoperatesinthedirectionofclosingorinthedirectionofopening.Thefollowingisadescriptionofthefourbasictypesoftime-delayrelaycontacts.下一頁返回上一頁Lesson1ElectromechanicalRelaysFirstwehavethenormally-open,timed-closed(NOTC)contact.Thistypeofcontactisnormallyopenwhenthecoilisunpowered(de-energized).Thecontactisclosedbytheapplicationofpowertotherelaycoil,butonlyafterthecoilhasbeencontinuouslypoweredforthespecifiedamountoftime.Inotherwords,thedirectionofthecontact'smotion(eithertocloseortoopen)isidenticaltoaregularNOcontact,butthereisadelayinclosingdirection.Becausethedelayoccursinthedirectionofcoilenergization,thistypeofcontactisalternativelyknownasanormally-open,on-delay:Thefollowingisatimingdiagramofthisrelaycontact'soperation(Figure3-1-4):下一頁返回上一頁Lesson1ElectromechanicalRelaysNextwehavethenormally-open,timed-open(NOTO)contact.LiketheNOTCcontact,thistypeofcontactisnormallyopenwhenthecoilisunpowered(de-energized),andclosedbytheapplicationofpowertotherelaycoil.However,unliketheNOTCcontact,thetimingactionoccursuponde-energizationofthecoilratherthanuponenergization.Becausethedelayoccursinthedirectionofcoilde-energization,thistypeofcontactisalternativelyknownasanormally-open,off-delay:Thefollowingisatimingdiagramofthisrelaycontact'soperation(Figure3-1-5):下一頁返回上一頁Lesson1ElectromechanicalRelaysNextwehavethenormally-closed,timed-open(NCTO)contact.Thistypeofcontactisnormallyclosedwhenthecoilisunpowered(de-energized).Thecontactisopenedwiththeapplicationofpowertotherelaycoil,butonlyafterthecoilhasbeencontinuouslypoweredforthespecifiedamountoftime.Inotherwords,thedirectionofthecontact'smotion(eithertocloseortoopen)isidenticaltoaregularNCcontact,butthereisadelayintheopeningdirection.Becausethedelayoccursinthedirectionofcoilenergization,thistypeofcontactisalternativelyknownasanormally-closed,on-delay:Thefollowingisatimingdiagramofthisrelaycontact'soperation(Figure3-1-6):下一頁返回上一頁Lesson1ElectromechanicalRelaysFinallywehavethenormally-closed,timed-closed(NCTC)contact.LiketheNCTOcontact,thistypeofcontactisnormallyclosedwhenthecoilisunpowered(de-energized),andopenedbytheapplicationofpowertotherelaycoil.However,unliketheNCTOcontact,thetimingactionoccursuponde-energizationofthecoilratherthanuponenergization.Becausethedelayoccursinthedirectionofcoilde-energization,thistypeofcontactisalternativelyknownasanormally-closed,offdelay:Thefollowingisatimingdiagramofthisrelaycontact'soperation(Figure3-1-7)返回上一頁Lesson2ContactorsWhenarelayisusedtoswitchalargeamountofelectricalpowerthroughitscontacts,itisdesignatedbyaspecialname:contactor.Contactorstypicallyhavemultiplecontacts,andthosecontactsareusually(butnotalways)normally-open,sothatpowertotheloadisshutoffwhenthecoilisde-energized.①Perhapsthemostcommonindustrialuseforcontactorsisthecontrolofelectricmotors.Thetopthreecontactsswitchtherespectivephasesoftheincoming3-phaseACpower(Figure3-2-1),typicallyatleast480Voltsformotors1horsepowerorgreater.下一頁返回Lesson2ContactorsThelowestcontactisan"auxiliary”contactwhichhasacurrentratingmuchlowerthanthatofthelargemotorpowercontacts,butisactuatedbythesamearmatureasthepowercontacts.②Theauxiliarycontactisoftenusedinarelaylogiccircuit,orforsomeotherpartofthemotorcontrolscheme,typicallyswitching120VoltACpowerinsteadofthemotorVoltage.Onecontactormayhaveseveralauxiliarycontacts,eithernormally-openornormally-closed,ifrequired.Thethree"opposed-question-mark"shapeddevicesinserieswitheachphasegoingtothemotorarecalledoverloadheaters.下一頁返回上一頁Lesson2ContactorsEach"heater"elementisalow-resistancestripofmetalintendedtoheatupasthemotordrawscurrent.Ifthetemperatureofanyoftheseheaterelementsreachesacriticalpoint(equivalenttoamoderateoverloadingofthemotor),anormally-closedswitchcontact(notshowninthediagram)willspringopen,andthenshutoffpowertothemotor.③Shownhereisacontactorforathree-phaseelectricmotor,installedonapanelaspartofanelectricalcontrolsystematamunicipalwatertreatmentplant(Figure3-2-2)下一頁返回上一頁Lesson2ContactorsThree-phase,480VoltACpowercomesintothethreenormally-opencontactsattherightofthecontactorViascrewterminalslabeled“L1”,“L2”,and“L3”(The“L2”terminalishiddenbehindasquare-shaped“snubber”circuitconnectedacrossthecontactor‘scoilterminals).PowertothemotorexitstheoverloadheaterassemblyatthebottomofthisdeviceViascrewterminalslabeled“T1”,“T2”,and“T3”。下一頁返回上一頁Lesson2ContactorsNewWordsandExpressions1.contactora.接觸器2.motor['mauta]a.電動機3.respective[ris'pektiv]adj.分別的4.3-phase三相的5.auxiliaryadj.輔助的，補助的6.scheme[ski:m]n.配置下一頁返回上一頁Lesson2Contactors7.screw[skru:]n.螺絲釘，螺旋8.low-resistanceadj.低電阻的9.critical[‘kritikal]adj.臨界的10.equivalent[i'kwivalant]adj.等價的11.moderateadj.緩和的12.springopen彈開13.shutoff關(guān)掉，切斷下一頁返回上一頁Lesson2ContactorsNotes①Contactorstypicallyhavemultiplecontacts,andthosecontactsareusually(butnotalways)normally-open,sothatpowertotheloadisshutoffwhenthecoilisde-energized.句中sothat引導目的狀語從句時，表示“以便;為了”。譯文:接觸器通常有許多個觸點，那些觸點通常(但不總是)是常開的，這樣，當線圈沒有電流的時候負載就從電源上被斷開了。②Thelowestcontactisan"auxiliary"contactwhichhasacurrentratingmuchlowerthanthatofthelargemotorpowercontacts,butisactuatedbythesamearmatureasthepowercontacts.下一頁返回上一頁Lesson2Contactors句中thanthatof…其中的that指代前而的currentrating。譯文:(圖中)最下面的觸點是一個輔助觸點,它允許通過的電流要比那些大功率觸點小得多,但輔助觸點和主觸點都是由同一個街鐵驅(qū)動的。③Ifthetemperatureofanyoftheseheaterelementsreachesacriticalpoint(equivalenttoamoderateoverloadingofthemotor),anormally-closedswitchcontact(notshowninthediagram)willspringopen,andthenshutoffpowertothemotor.譯文:如果這些發(fā)熱元件中的任意一個達到了臨界值(也就是說電動機已經(jīng)過載了)，那么一個常閉觸點就要彈開(圖中未顯示)，隨即切斷電動機的電源。下一頁返回上一頁Lesson2ContactorsOverloadHeatersThethree"opposed-question-mark"shapeddevicesinserieswitheachphasegoingtothemotorarecalledoverloadheaters.Each"heater"elementisalow-resistancestripofmetalintendedtoheatupasthemotordrawscurrent.Ifthetemperatureofanyoftheseheaterelementsreachesacriticalpoint(equivalenttoamoderateoverloadingofthemotor),anormally-closedswitchcontact(notshowninthediagram)willspringopen.Thisnormally-closedcontactisusuallyconnectedinserieswiththerelaycoil,sothatwhenitopenstherelaywillautomaticallyde-energize,therebyshuttingoffpowertothemotor.下一頁返回上一頁Lesson2ContactorsWewillseemoreofthisoverloadprotectionwiringinthenextchapter.Overloadheatersareintendedtoprovideovercurrentprotectionforlargeelectricmotors,unlikecircuitbreakersandRiseswhichservetheprimarypurposeofprovidingovercurrentprotectionforpowerconductors.Overloadheateraredesignedtothermallymimictheheatingcharacteristicoftheparticularelectricmotortobeprotected.Allmotorshavethermalcharacteristics,includingtheamountofheatenergygeneratedbyresistivedissipation(I2R),thethermaltransfercharacteristicsofheat"conducted"tothecoolingmediumthroughthemetalframeofthemotor,thephysicalmassandspecificheatofthematerialsconstitutingthemotor,etc.下一頁返回上一頁Lesson2ContactorsThesecharacteristicsaremimickedbytheoverloadheateronaminiaturescale:whenthemotorheatsuptowarditscriticaltemperature,sowilltheheatertowarditscriticaltemperature,ideallyatthesamerateandapproachcurve.Thus,theoverloadcontact,insensingheatertemperaturewithathermo-mechanicalmechanism,willsenseananalogueoftherealmotor.Iftheoverloadcontacttripsduetoexcessiveheatertemperature,itwillbeanindicationthattherealmotorhasreacheditscriticaltemperature(or,wouldhavedonesoinashortwhile).Aftertripping,theheatersaresupposedtocooldownatthesamerateandapproachcurveastherealmotor,下一頁返回上一頁Lesson2Contactorssothattheyindicateanaccurateproportionofthemotor'sthermalcondition,andwillnotallowpowertobere-applieduntilthemotoristrulyreadyforstart-upagain.Theoverloadheaterunitsthemselvesareblack,square-shapedblockswiththelabel"W34",indicatingaparticularthermalresponseforacertainhorsepowerandtemperatureratingofelectricmotor.Awhitepushbuttonlocatedbetweenthe"T1"and"T2"lineheatersservesasawaytomanuallyre-setthenormally-closedswitchcontactbacktoitsnormalstateafterhavingbeentrippedbyexcessiveheatertemperature.下一頁返回上一頁Lesson2ContactorsHowdoyouknowifthemotorisconsumingpowerwhenthecontactorcoilisenergizedandthearmaturehasbeenpulledin?Ifthemotor‘swindingsareburntopen,youcouldbesendingvoltagetothemotorthroughthecontactorcontacts,butstillhavezerocurrent,andthusnomotionfromthemotorshaft.Ifaclamp-onammeterisn‘tavailabletomeasurelinecurrent,youcantakeyourmultimeterandmeasuremillivoltageacrosseachheaterelement:ifthecurrentiszero,theVoltageacrosstheheaterwillbezero(unlesstheheaterelementitselfisopen,inwhichcasetheVoltageacrossitwillbelarge);下一頁返回上一頁Lesson2Contactorsifthereiscurrentgoingtothemotorthroughthatphaseofthecontactor,youwillreadadefinitemillivoltageacrossthatheater(SeeFigure3-2-3):Thisisanespeciallyusefultricktousefortroubleshooting3-phaseACmotors,toseeifonephasewindingisburntopenordisconnected,whichwillresultinarapidlydestructiveconditionknownas"single-phasing."Ifoneofthelinescarryingpowertothemotorisopen,itwillnothaveanycurrentthroughit(asindicatedbya0.00mVreadingacrossitsheater),althoughtheothertwolineswill(asindicatedbysmallamountsofvoltagedroppedacrosstherespectiveheaters).返回上一頁Lesson3HowtoWireaMotorStarterAmotorstarterisacombinationofdevicesusedtostart,run,andstopaninductionmotorbasedoncommandsfromanoperatororacontroller.InNorthAmerica,aninductionmotorwilltypicallyoperateat230Vor460V,3-phase,60Hzandhasacontrolvoltageof115VACor24VDC.①SeveralothercombinationsarepossibleinNorthAmericaandothercountries,andareeasilyderivedfromthemethodsshowninthisdocument.Themotorstartermusthaveatleasttwocomponentstooperate:acontactortoopenorclosetheflowofenergytothemotor,andanoverloadrelaytoprotectthemotoragainstthermaloverload.下一頁返回Lesson3HowtoWireaMotorStarterOtherdevicesfordisconnectingandshort-circuitprotectionmaybeneeded,typicallyacircuitbreakerorfuses.Short-circuitprotectionwillnotbeshownintheexamplesthatfollow.Thecontactorisa3-poleelectromechanicalswitchwhosecontactsareclosedbyapplyingvoltagetoacoil.Whenthecoilisenergized,thecontactsareclosed,andremainclosed,untilthecoilisde-energized.Thecontactorisspecificallydesignedformotorcontrol,butcanbeusedforotherpurposessuchasresistiveandlightingloads.Sinceamotorhasinductance,thebreakingofthecurrentismoredifficultsothecontactorhasbothahorsepowerandcurrentratingthatneedstobeadheredto.②下一頁返回上一頁Lesson3HowtoWireaMotorStarterTheoverloadrelayisadevicethathasthreecurrentsensingelementsandprotectsthemotorfromanovercurrent.Eachphasegoingfromthecontactortothemotorpassesthroughanoverloadrelaycurrent-sensingelement.Theoverloadrelayhasaselectablecurrentsettingbasedonthefullloadampratingofthemotor.Iftheoverloadcurrentexceedsthesettingoftherelayforasufficientlengthoftime,asetofcontactsopenstoprotectthemotorfromdamage.③ThisarticleshowshowtowireVariousmotorsusingtheFUJIseriesofcontactorssoldbyAutomationDirect.Otherbrandsofcontactorsmaybewiredthesameorsimilarly.Consultthemanufacturer'swiringdiagramsforotherbrandsofcontactors.下一頁返回上一頁Lesson3HowtoWireaMotorStarterTherearefourbasicwiringcombinations:1)Full-Voltagenon-reversing3-phasemotors;2)Full-Voltagereversing3-phasemotors;3)Single-phasemotors;4)Wyedeltaopentransition3-phasemotors.Youmustsupplyadisconnectswitch,propersizedwire,enclosures,terminalblocksandanyotherdevicesneededtocompleteyourcircuit.WARNING!Usetheinstructionssuppliedforeachspecificdevice.Failuretodosomayresultinelectricalshockordamage.Thefollowingcomponentswillbeused(Figure3-3-1):下一頁返回上一頁Lesson3HowtoWireaMotorStarterFull-voltagenon-reversing3-phasemotorsThefollowingdiagramdepicts3-phasenon-reversingmotorcontrolwith24VDCcontrolvoltageandmanualoperation.Wewilluseacontactor,anauxiliarycontactblock,anoverloadrelay,anormallyopenstartpushbutton,anormallyclosedstoppushbutton,andapowersupplywithafuse.ThestartandstopcircuitscanalsobecontrolledusingPLCinputsandoutputs(Figure3-3-2).下一頁返回上一頁Lesson3HowtoWireaMotorStarterFull-voltagereversing3-phasemotorsThisdiagramisfor3-phasereversingmotorcontrolwith24VDCcontrolvoltage.Itusestwocontactors,twoauxiliarycontactblocks,anoverloadrelay,amechanicalinterlock,twonormallyopenstartpushbuttons,anormallyclosedstoppushbutton,andapowersupplywithafuse.Theforward,reverse,andstopcircuitscanalsobecontrolledusingPLCinputsandoutputs(Figure3-3-3).下一頁返回上一頁Lesson3HowtoWireaMotorStarterFull-voltagesingle-phasemotorsThisdiagramisforsingle-phasemotorcontrol.Itusesacontactor,anoverloadrelay,oneauxiliarycontactblock,anormallyopenstartpushbutton,anormallyclosedstoppushbutton,andapowersupplywithafuse.ThestartandstopcircuitscanalsobecontrolledusingPLCinputsandoutputs(Figure3-3-4).下一頁返回上一頁Lesson3HowtoWireaMotorStarterWyedeltaopentransition3-phasemotorsThefollowingdiagramisshownfor3-phasemotorcontrolofadelta-starconnection.Itusesthreecontactors,anoverloadrelay,oneauxiliarycontactblock,anormallyopenstartpushbutton,anormallyclosedstoppushbutton,anondelaytimerof0-20secondsandapowersupplywithafuse.Thestart,stop,andtimingcircuitscanalsobecontrolledusingPLCinputsandoutputs(Figure3-3-5).下一頁返回上一頁Lesson3HowtoWireaMotorStarterNewWordsandEbination

n.結(jié)合2.documentn.文件3.component[kam‘paunant]n.元件4.disconnect[,diska'nekt]vt.分開，拆開5.short-circuit

v.使短路6.3-pole三極的7.horsepowern.馬力(功率單位，1馬力=0.735千瓦)下一頁返回上一頁Lesson3HowtoWireaMotorStarter8.adhere[ad'hia]v.堅持9.selectable[‘selikteibl]adj.可選擇的10.sufficientadj.充分的11.manufacturern.廠商12.wyedelta

n.星形/三角形連接法13.transitionn.轉(zhuǎn)變，轉(zhuǎn)換14.enclosuresn.圍欄15.protect...from…防止……遭受……下一頁返回上一頁Lesson3HowtoWireaMotorStarterNotes①InNorthAmerica,aninductionmotorwilltypicallyoperateat230Vor460V,3-phase,60HzandhasacontrolVoltageof115VACor24VDC.句中andhasa…其主語為之前的“aninductionmotor“。譯文:在北美，感應(yīng)電機工作環(huán)境通常為230V或460V，60Hz的三相交流電，它的控制電壓為115V的交流電或者24V的直流電。②Sinceamotorhasinductance,thebreakingofthecurrentismoredifficultsothecontactorhasbothahorsepowerandcurrentratingthatneedstobeadheredto.句中Since引導的為表原因的狀語從句。下一頁返回上一頁Lesson3HowtoWireaMotorStarter譯文:因為電機有電感，所以要分斷電流也就比較困難，因此(電路中的)接觸器的功耗和額定電流都比較大以至于能承受分斷的電流。③Iftheoverloadcurrentexceedsthesettingoftherelayforasufficientlengthoftime,asetofcontactsopenstoprotectthemotorfromdamage.句中toprotectthemotorfromdamage，意為使電機免受傷害。for引導表示時間的狀語從句，說明過載電流需要有時間上的延續(xù)。譯文:如果過載電流超過了繼電器的設(shè)定值一段時間后，(繼電器中的)一組觸點將斷開，使電機免受過載電流的傷害。下一頁返回上一頁Lesson3HowtoWireaMotorStarterSolid-stateRelaysAsVersatileaselectromechanicalrelayscanbe,theydosuffermanylimitations.Theycanbeexpensivetobuild,havealimitedcontactcyclelife,takeupalotofroom,andswitchslowly,comparedtomodernsemiconductordevices.Theselimitationsareespeciallytrueforlargepowercontactorrelays.Toaddresstheselimitations,manyrelaymanufacturersoffer"solid-state"relays,whichuseanSCR,TRIAC,ortransistoroutputinsteadofmechanicalcontactstoswitchthecontrolledpower.Theoutputdevice(SCR,TRIAC,ortransistor)isoptically-coupledtoanLEDlightsourceinsidetherelay.下一頁返回上一頁Lesson3HowtoWireaMotorStarterTherelayisturnedonbyenergizingthisLED,usuallywithlow-VoltageDCpower.Thisopticalisolationbetweeninputtooutputrivalsthebestthatelectromechanicalrelayscanoffer(Figure3-3-6).Beingsolid-statedevices,therearenomovingpartstowearout,andtheyareabletoswitchonandoffmuchfasterthananymechanicalrelayarmaturecanmove.Thereisnosparkingbetweencontacts,andnoproblemswithcontactcorrosion.However,solid-staterelaysarestilltooexpensivetobuildinveryhighcurrentratings,andsoelectromechanicalcontactorscontinuetodominatethatapplicationinindustrytoday.下一頁返回上一頁Lesson3HowtoWireaMotorStarterOnesignificantadvantageofasolid-stateSCRorTRIACrelayoveranelectromechanicaldeviceisitsnaturaltendencytoopentheACcircuitonlyatapointofzeroloadcurrent.BecauseSCR‘sandTRIAC‘sarethyristors,theirinherent

hysteresismaintainscircuitcontinuityaftertheLEDisde-energizeduntiltheACcurrentfallsbelowathresholdvalue(theholdingcurrent).Inpracticaltermswhatthismeansisthecircuitwillneverbeinterruptedinthemiddleofasinewavepeak.Suchuntimelyinterruptionsinacircuitcontainingsubstantialinductancewouldnormallyproducelargevoltagespikesduetothesuddenmagneticfieldcollapsearoundtheinductance.下一頁返回上一頁Lesson3HowtoWireaMotorStarterThiswillnothappeninacircuitbrokenbyanSCRorTRIAC.Thisfeatureiscalledzero-crossoverswitching.Onedisadvantageofsolidstaterelaysistheirtendencytofail"shorted"ontheiroutputs,whileelectromechanicalrelaycontactstendtofail"open".Ineithercase,itispossibleforarelaytofailintheothermode,butthesearethemostcommonfailures.Becausea"fail-open"stateisgenerallyconsideredsaferthana"fail-closed"state,electromechanicalrelaysarestillfavoredovertheirsolid-statecounterpartsinmanyapplications.返回上一頁Lesson4HardwareofaPLCSystemAprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures①(Figure3-4-1).PLCsareusedinmanyindustriesandmachines,suchaspackagingandsemiconductormachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetoVibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-Volatilememory.下一頁返回Lesson4HardwareofaPLCSystemAPLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.PLCsspa