(电气工程与自动化专业英语)第6章DC Motor Drives课件

(电气工程与自动化专业英语)第6章DCMotorDrives(电气工程与自动化专业英语)第6章DCMotorDriv1DCMotorDrives

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Studyofthed.c.driveisvaluableforseveralreasons:thestructureandoperationofthed.c.drivearereflectedinalmostallotherdrives;thed.c.drivetendstoremaintheyardstickbywhichotherdrivesarejudged.Thefirstandmajorpartofthischapterisdevotedtothyristor-fed(可控硅供电)drives,afterwhichwewilllookbrieflyatcontrolarrangementsforDCdrives.DCMotorDrives

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TextAThyristorDCDrivers

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Formotorsuptoafewkilowattsthearmatureconvertercanbesuppliedfromeithersingle-phaseorthree-phasemains,butforlargermotorsthree-phaseisalwaysused.Aseparatethyristorordioderectifier(二极管整流器)isusedtosupplythefieldofthemotor:thepowerismuchlessthanthearmaturepower,sothesupplyisoftensingle-phase,asshowninFigure6.1.TextAThyristorDCDrivers

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TextAThyristorDCDrivers

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Themainpowercircuitconsistsofasix-thyristorbridgecircuit(桥式电路),whichrectifiestheincominga.c.supplytoproducead.c.supplytothemotorarmature.Theassemblyofthyristors,mountedonaheatsink(散热器),isusuallyreferredtoasthe‘stack’.Byalteringthefiringangleofthethyristorsthemeanvalueoftherectifiedvoltagecanbevaried,therebyallowingthemotorspeedtobecontrolled.TextAThyristorDCDrivers

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TextAThyristorDCDrivers

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Themainpowercircuitconsistsofasix-thyristorbridgecircuit(桥式电路),whichrectifiestheincominga.c.supplytoproducead.c.supplytothemotorarmature.Theassemblyofthyristors,mountedonaheatsink(散热器),isusuallyreferredtoasthe‘stack’.Byalteringthefiringangleofthethyristorsthemeanvalueoftherectifiedvoltagecanbevaried,therebyallowingthemotorspeedtobecontrolled.TextAThyristorDCDrivers

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TextAThyristorDCDrivers

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Lowpowercontrolcircuitsareusedtomonitortheprincipalvariables(主变量)ofinterest(usuallymotorcurrentandspeed),andtogenerateappropriatefiringpulsessothatthemotormaintainsconstantspeeddespitevariationsintheload.The‘speedreference’(Figure6.1)istypicallyananaloguevoltagevaryingfrom0to10V,andobtainedfromamanualspeed-settingpotentiometerorfromelsewhereintheplant.TextAThyristorDCDrivers

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TextAThyristorDCDrivers

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Thecombinationofpower,control,andprotectivecircuitsconstitutestheconverter.Standardmodularconvertersareavailableasoff-the-shelfitemsinsizesfrom0.5kWuptoseveralhundredkW,whilelargerdriveswillbetailoredtoindividualrequirements.Individualconvertersmaybemountedinenclosureswithisolators,fusesetc.,orgroupsofconvertersmaybemountedtogethertoformamulti-motordrive.TextAThyristorDCDrivers

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6.1MotoroperationwithconvertersupplyTextAThyristorDCDrivers1

6.2Motorcurrentwaveforms

82Outline3

6.3Discontinuouscurrent

6.4Converteroutputimpedance:overlap45

6.5Four-quadrantoperationandinversion6.1Motoroperationwithcon86.1Motoroperationwithconvertersupply9

Thebasicoperationoftherectifyingbridgehasbeendiscussed,andwenowturntothematterofhowthed.c.motorbehaveswhensuppliedwith‘d.c.’fromacontrolledrectifier.BasicIntroductionBynostretchofimaginationcouldthewaveformsofarmaturevoltage

bethoughtofasgoodd.c.,and

itwouldnotbeunreasonabletoquestionthewisdomoffeedingsuchan

unpleasantlookingwaveformtoad.c.motor.6.1Motoroperationwithconve96.1Motoroperationwithconvertersupply10

Firstly,thearmatureinductanceofthemotorcausesthewaveformofarmaturecurrenttobemuchsmootherthanthewaveformofarmaturevoltage,whichinturnmeansthatthetorquerippleismuchlessthanmighthavebeenfeared.BasicIntroduction

Infactitturnsoutthat

themotorworksalmostaswellasitwouldiffedwithpured.c.,fortwo

mainreasons.Secondly,theinertiaofthe

armatureissufficientlylargeforthespeedtoremainalmoststeady

despitethetorqueripple.6.1Motoroperationwithconve10

6.1MotoroperationwithconvertersupplyTextAThyristorDCDrivers1

6.2Motorcurrentwaveforms

112Outline3

6.3Discontinuouscurrent

6.4Converteroutputimpedance:overlap45

6.5Four-quadrantoperationandinversion6.1Motoroperationwithcon116.2Motorcurrentwaveforms12

1Forthesakeofsimplicitywewilllookatoperationfromasingle-phase(2-pulse)converter,butsimilarconclusionsapplytothe6-pulseconverter.Thevoltage(Va)appliedtothemotorarmatureistypicallyasshowninFigure6.2(a):itconsistsofrectified‘chunks’oftheincomingmainsvoltage,thepreciseshapeandaveragevalue(平均值)dependingonthefiringangle(触发角).armaturecurrent6.2Motorcurrentwaveforms12126.2Motorcurrentwaveforms13

1Thevoltagewaveformcanbeconsideredtoconsistofameand.c.level(Vdc),andasuperimposedpulsatingorripplecomponentwhichwecandenotelooselyasVac.Themeanvoltagecanbealteredbyvaryingthefiringangle,whichalsoincidentallyalterstheripple(i.e.Vac).armaturecurrent6.2Motorcurrentwaveforms13136.2Motorcurrentwaveforms14

2Theripplevoltage(纹波电压)causesaripplecurrenttoflowinthearmature,butbecauseofthearmatureinductance,theamplitudeoftheripplecurrentissmall.Inotherwords,thearmaturepresentsahighimpedancetoa.c.voltages.ThissmoothingeffectofthearmatureinductanceisshowninFigure6.2(b),Armaturevoltage6.2Motorcurrentwaveforms14146.2Motorcurrentwaveforms15

2Itcanbeseenthatthecurrentrippleisrelativelysmallincomparisonwiththecorrespondingvoltageripple.Theaveragevalueoftheripplecurrentisofcoursezero,soithasnoeffectontheaveragetorqueofthemotor.Thereisneverthelessavariationintorqueeveryhalf-cycleofthemains,butbecauseitisofsmallamplitudeandhighfrequencythevariationinspeed)willnotusuallybenoticeable.Armaturevoltage6.2Motorcurrentwaveforms15156.2Motorcurrentwaveforms16

3Thecurrentattheendofeachpulseisthesameasatthebeginning,soitfollowsthattheaveragevoltageacrossthearmatureinductance(L)iszero.Wecanthereforeequatetheaverageappliedvoltagetothesumofthebacke.m.f.(assumedpured.c.becauseweareignoringspeedfluctuations)andtheaveragevoltageacrossthearmatureresistance,toyield:BasicIntroductionItunderlinesthefactthatwecancontrolthemean

motorvoltage,andhencethespeed,simplybyvaryingtheconverter

delayangle.6.2Motorcurrentwaveforms16166.2Motorcurrentwaveforms17

3Thesmoothingeffectofthearmature(电枢)inductanceisimportantinachievingsuccessfulmotoroperation:thearmatureactsasalow-passfilter,blockingmostoftheripple,andleadingtoamoreorlessconstantarmaturecurrent.BasicIntroductionForthesmoothingtobeeffective,thearmaturetime-constantneedstobelongcomparedwiththepulseduration(halfacyclewitha2-pulsedrive,butonlyonesixthofacycleina6-pulsedrive).Thisconditionismetinall6-pulsedrives,andinmany2-pulseones.6.2Motorcurrentwaveforms17176.2Motorcurrentwaveforms18

3Theno-loadspeedisdeterminedbytheappliedvoltage(whichdependsonthefiringangleoftheconverter);thereisasmalldropinspeedwithloadandaswehavepreviouslynoted,theaveragecurrentisdeterminedbytheload.BasicIntroductionWeshouldnotethatthecurrentripple（纹波）remainsthesame–onlytheaveragecurrentchangeswithload.Thespeedisdeterminedbytheconverterfiringangle,whichrepresentsaverysatisfactorystatebecausewecancontrolthefiringanglebylow-powercontrolcircuitsandtherebyregulatethespeedofthedrive.6.2Motorcurrentwaveforms18186.2Motorcurrentwaveforms19

3BasicIntroductionThecurrentwaveformsinFigure6.2(b)arereferredtoas‘continuous’,becausethereisneveranytimeduringwhichthecurrentisnotflowing.This‘continuouscurrent’conditionisthenorminmostdrives,anditishighlydesirablebecauseitisonlyundercontinuouscurrentconditionsthattheaveragevoltagefromtheconverterisdeterminedsolelybythefiringangle,andisindependentoftheloadcurrent.6.2Motorcurrentwaveforms1919

6.1MotoroperationwithconvertersupplyTextAThyristorDCDrivers1

6.2Motorcurrentwaveforms

202Outline3

6.3Discontinuouscurrent

6.4Converteroutputimpedance:overlap45

6.5Four-quadrantoperationandinversion6.1Motoroperationwithcon206.3Discontinuouscurrent21

1WecanseefromFigure6.2thatastheloadtorqueisreduced,therewillcomeapointwheretheminimaofthecurrentrippletouchesthezero-currentline,i.e.thecurrentreachestheboundarybetweencontinuousanddiscontinuouscurrent.Theloadatwhichthisoccurswillalsodependonthearmatureinductance,becausethehighertheinductancethesmootherthecurrent(i.e.thelesstheripple).BasicIntroduction6.3Discontinuouscurrent211216.3Discontinuouscurrent22

2Discontinuouscurrentmodeisthereforemostlikelytobeencounteredinsmallmachineswithlowinductance(particularlywhenfedfromtwo-pulseconverters)andunderlight-loadorno-loadconditions.Discontinuouscurrent6.3Discontinuouscurrent222226.3Discontinuouscurrent23

2DiscontinuouscurrentTypicalarmaturevoltageandcurrentwaveformsinthediscontinuous

modeareshowninFigure6.3,thearmaturecurrentconsistingof

discretepulsesofcurrentthatoccuronlywhilethearmatureisconnected

tothesupply,withzerocurrentfortheperiod.6.3Discontinuouscurrent232236.3Discontinuouscurrent24

2Theshapeofthecurrentwaveformcanbeunderstoodbynotingthatwithresistanceneglected,equationcanberearrangedas:whichshowsthattherateofchangeofcurrent(thegradientofthe

lowergraphinFigure6.3)isdeterminedbytheinstantaneousdifference

betweentheappliedvoltageVandthemotionalE.Discontinuouscurrent6.3Discontinuouscurrent242246.3Discontinuouscurrent25

2Valuesof(V-E)areshownbytheverticalhatchingsinFigure6.3,fromwhichitcanbeseenthatifV>E,thecurrentisincreasing,whileifV<E,thecurrentisfalling.Thepeakcurrentisthusdeterminedbytheareaoftheupperorlowershadedareasoftheuppergraph.Discontinuouscurrent6.3Discontinuouscurrent252256.3Discontinuouscurrent26

2Itshouldbeclearbycomparingthesefiguresthatthearmaturevoltagewaveforms(solidlines)differbecause,inFigure6.3,thecurrentfallstozerobeforethenextfiringpulsearrivesandduringtheperiodshownasuthemotorfloatsfree,itsterminalvoltageduringthistimebeingsimplythemotionale.m.f.(E)(动生电动势).Discontinuouscurrent6.3Discontinuouscurrent262266.3Discontinuouscurrent27

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TosimplifyFigure6.3ithasbeenassumedthatthearmatureresistanceissmallandthatthecorrespondingvolt-drop(IaRa)

(电压降)canbeignored.Inthiscase,theaveragearmaturevoltage(Vdc)mustbeequaltothemotionale.m.f.,becausetherecanbenoaveragevoltageacrossthearmatureinductance(电枢电感)

whenthereisnonetchangeinthecurrentoveronepulse:thehatchedareas–representingthevolt-secondsintheinductor–arethereforeequal.Discontinuouscurrent6.3Discontinuouscurrent272276.3Discontinuouscurrent28

2ThemostimportantdifferencebetweenFigure6.2andFigure6.3isthattheaveragevoltageishigherwhenthecurrentisdiscontinuous,andhencethespeedcorrespondingtotheconditionsinFigure6.3ishigherthaninFigure6.2despitebothhavingthesamefiringangle.Discontinuouscurrent6.3Discontinuouscurrent282286.3Discontinuouscurrent29

2Andwhereasincontinuousmodealoadincreasecanbemetbyanincreasedarmaturecurrentwithoutaffectingthevoltage(andhencespeed),thesituationisverydifferentwhenthecurrentisdiscontinuous.Inthelattercase,theonlywaythattheaveragecurrentcanincreaseiswhenspeed(andhenceE)fallssothattheshadedareasinFigure6.3becomelarger.Fromtheuser’sviewpointthebehaviorofthemotor

indiscontinuousmodeismuchworsethaninthecontinuouscurrent

mode,becauseastheloadtorqueisincreased,thereisaseriousdropin

speed.Discontinuouscurrent6.3Discontinuouscurrent292296.3Discontinuouscurrent30

2Underverylightorno-loadconditions,thepulsesofcurrentbecomevirtuallynon-existent,theshadedareasinFigure6.3becomeverysmallandthemotorspeedreachesapointatwhichthebacke.m.f.isequaltothepeakofthesupplyvoltage.Discontinuouscurrent6.3Discontinuouscurrent302306.3Discontinuouscurrent31

3Itiseasytoseethatinherenttorque–speedcurveswithsuddendiscontinuitiesoftheformshowninFigure6.4areveryundesirable.Ifforexamplethefiringangleissettozeroandthemotorisfullyloaded,itsspeedwillsettleatpointA,itsaveragetorque–speedcurves6.3Discontinuouscurrent313316.3Discontinuouscurrent32

3Itiseasytoseethatinherenttorque–speedcurveswithsuddendiscontinuitiesoftheformshowninFigure6.4areveryundesirable.Ifforexamplethefiringangleissettozeroandthemotorisfullyloaded,itsspeedwillsettleatpointA,itsaveragetorque–speedcurves6.3Discontinuouscurrent323326.3Discontinuouscurrent33

3Armaturevoltageandcurrenthavingtheirfull(rated)values.Astheloadisreduced,currentremainingcontinuous,thereistheexpectedslightriseinspeed,untilpointBisreached.Thisisthepointatwhichthecurrentisabouttoenterthediscontinuousphase.Anyfurtherreductionintheloadtorquethenproducesawhollydisproportionate–nottosayfrightening–increaseinspeed,especiallyiftheloadisreducedtozerowhenthespeedreachespointC.torque–speedcurves6.3Discontinuouscurrent333336.3Discontinuouscurrent34

3Therearetwowaysbywhichwecanimprovetheseinherentlypoorcharacteristics.Firstly,wecanaddextrainductanceinserieswiththe

armaturetofurthersmooththecurrentwaveformandlessenthelikelihood

ofdiscontinuouscurrent.Theeffectofaddinginductanceisshown

bythedottedlinesinFigure6.4.Secondly,wecanswitch

fromasingle-phaseconvertertoa3-phaseconverterwhichproduces

smoothervoltageandcurrentwaveforms.torque–speedcurves6.3Discontinuouscurrent343346.3Discontinuouscurrent35

3Whentheconverterandmotorareincorporatedinaclosed-loopcontroltheusershouldbeunawareofanyshortcomingsintheinherentmotor/convertercharacteristicsbecausethecontrolsystemautomaticallyaltersthefiringangletoachievethetargetspeedatallloads.torque–speedcurves6.3Discontinuouscurrent35335

6.1MotoroperationwithconvertersupplyTextAThyristorDCDrivers1

6.2Motorcurrentwaveforms

362Outline3

6.3Discontinuouscurrent

6.4Converteroutputimpedance:overlap45

6.5Four-quadrantoperationandinversion6.1Motoroperationwithcon366.4Converteroutputimpedance:overlap37

Wehavetreatedtheconverterasanidealvoltagesource.Theoutputvoltagefromtheconverterasindependentofthecurrentdrawnbythemotor,anddependedonlyonthedelayangle1Inpracticethesupplyhasafiniteimpedance,andwemustthereforeexpectavolt-dropwhichdependsonthecurrentbeingdrawnbythemotor.Perhapssurprisingly,thesupplyimpedance(whichismainlyduetoinductiveleakagereactance(漏电抗)intransformers）manifestsitselfattheoutputstageoftheconverterasasupplyresistance,sothesupplyvolt-drop(orregulation)isdirectlyproportionaltothemotorarmaturecurrent.BasicIntroduction6.4Converteroutputimpedance376.4Converteroutputimpedance:overlap38

Overlap（重叠）:weshouldnotethattheeffectoftheinductivereactanceofthesupplyistodelaythetransfer(orcommutation)ofthecurrentbetweenthyristors;aphenomenonknownasoverlap.2Theconsequenceofoverlapisthatinsteadoftheoutputvoltagemakinganabruptjumpatthestartofeachpulse,thereisashortperiodwhentwothyristorsareconductingsimultaneously（同时地）.Overlap6.4Converteroutputimpedance386.4Converteroutputimpedance:overlap39

Duringthisintervaltheoutputvoltageisthemeanofthevoltagesoftheincomingandoutgoingvoltages,asshowntypicallyinFigure6.5.Itisimportantforuserstobeawarethatoverlapistobeexpected,asotherwisetheymaybealarmedthefirsttimetheyconnectanoscilloscopetothemotorterminals.2Overlap6.4Converteroutputimpedance396.4Converteroutputimpedance:overlap40

2Whenthedriveisconnectedtoa‘stiff’(i.e.lowimpedance)industrialsupplytheoverlapwillonlylastforperhapsafewmicroseconds,sothe‘notch’showninFigure6.5wouldbebarelyvisibleonanoscilloscopeOverlapFigure6.5:witha50or60Hz

supply,iftheoverlaplastsformorethansay1ms,theimplicationisthat

thesupplysystemimpedanceistoohighforthesizeofconverterin

question,orconversely,theconverteristoobigforthesupply.6.4Converteroutputimpedance406.4Converteroutputimpedance:overlap41

2Returningtothepracticalconsequencesofsupplyimpedance,wesimplyhavetoallowforthepresenceofanextra‘sourceresistance’inserieswiththeoutputvoltageoftheconverter.Thissourceresistanceisinserieswiththemotorarmatureresistance,andhencethemotortorque–speedcurvesforeachvalueofahaveasomewhatsteeperdroopthantheywouldifthesupplyimpedancewaszero.Overlap6.4Converteroutputimpedance41

6.1MotoroperationwithconvertersupplyTextAThyristorDCDrivers1

6.2Motorcurrentwaveforms

422Outline3

6.3Discontinuouscurrent

6.4Converteroutputimpedance:overlap45

6.5Four-quadrantoperationandinversion6.1Motoroperationwithcon426.5Four-quadrantoperationandinversion43

1Machinerunninginthepositivedirectionandactingasamotor.Thisisknownasone-quadrantoperation,byreferencetoquadrant（象限）1ofthecompletetorque–speedplane.Four-quadrantoperation6.5Four-quadrantoperationan436.5Four-quadrantoperationandinversion44

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Machineisinherentlyabidirectional(双向的)energyconverter.Four-quadrantoperation

Ifweapplyapositivevoltage

VgreaterthanE,acurrentflowsintothearmatureandthemachine

runsasamotor.IfwereduceVsothatitislessthanE,thecurrent,

torqueandpowerautomaticallyreversedirection,andthemachineacts

asagenerator,convertingmechanicalenergy(itsownkineticenergyin

thecaseofregenerativebraking)intoelectricalenergy.6.5Four-quadrantoperationan446.5Four-quadrantoperationandinversion45

1Four-quadrantoperation

Andifwewanttomotororgeneratewiththereversedirectionofrotation,allwehavetodoistoreversethepolarityofthearmaturesupply.Thed.c.machineisinherentlyafour-quadrantdevice,butneedsasupplywhichcanprovidepositiveornegativevoltage,andsimultaneouslyhandleeitherpositiveornegativecurrent.6.5Four-quadrantoperationan456.5Four-quadrantoperationandinversion46

2Thed.c.currentcanonlybepositive,but(provideditisafullycontrolledconverter)thed.c.outputvoltagecanbeeitherpositiveornegative.Thepowerflowcanthereforebepositive(rectification)ornegative(inversion).Four-quadrantinversionFornormalmotoringwheretheoutputvoltageispositive(andassuming

afullycontrolledconverter),thedelayangle(延迟角)willbeupto.6.5Four-quadrantoperationan466.5Four-quadrantoperationandinversion47

2Whenaisgreaterthan,however,theoutputvoltageisnegative,andisshowninFigure6.6.Asinglefullycontrolledconverterthereforehasthepotentialfortwo-quadrantoperation,thoughithastobeadmittedthatthiscapabilityisnoteasilyexploitedunlesswearepreparedtoemployreversingswitches(换向开关)inthearmatureorfieldcircuits.Four-quadrantinversion6.5Four-quadrantoperationan47

6.6CurrentcontrolTextBControlArrangementsforDCDrives6

6.7Torquecontrol

487Outline8

6.8Speedcontrol

6.9ArmaturevoltagefeedbackandIRcompensation96.6CurrentcontrolTextBCo48TextBControlArrangementsforDCDrives49Two-loopcontrol.；Thishasaninnerfeedbacklooptocontrolthecurrent(andhencetorque)andanouterlooptocontrolspeed.Whenpositioncontroliscalledfor,afurtherrouterpositionloopisadded.TextBControlArrangementsfo49TextBControlArrangementsforDCDrives50Thespeederorisamplifiedbythespeedcontroller(whichismoreaccuratelydescribedasaspeed-erroramplifier)andtheoutputservesasthereferenceorinputsignaltotheinnercontrolsystem.Theinnerfeedbackloopisacurrent-controlloop,sowhenthecurrentreferenceincreases,sodoesthemotorarmaturecurrent,therebyprovidingextratorqueandinitiatingacceleration.TextBControlArrangementsfo50

6.6CurrentcontrolTextBControlArrangementsforDCDrives6

6.7Torquecontrol

517Outline8

6.8Speedcontrol

6.9ArmaturevoltagefeedbackandIRcompensation96.6CurrentcontrolTextBCo516.6Currentcontrol52

1Theclosed-loopcurrentcontroller,orcurrentloop,isattheheartofthedrivesystemandisindicatedbytheshadedregioninFigure6.7.Thepurposeofthecurrentloop6.6Currentcontrol521Theclo526.6Currentcontrol53

1Thepurposeofthecurrentloopistomaketheactualmotorcurrentfollowthecurrentreferencesignal(Iref)showninFigure6.7.Itdoesthisbycomparingafeedbacksignalofactualmotorcurrentwiththecurrentreferencesignal,amplifyingthedifference(orcurrenterror),andusingtheresultingamplifiedcurrenterrorsignal(ananaloguevoltage)tocontrolthefiringanglea–andhencetheoutputvoltage–oftheconverter.Thecurrentfeedbacksignalisobtainedeitherfromad.c.currenttransformer(whichgivesanisolatedanaloguevoltageoutput),orfroma.ccurrenttransformer/rectifiers

inthemainssupplylines.Thepurposeofthecurrentloop6.6Currentcontrol531Thepur536.6Currentcontrol54

1Thejobofcomparingthereference(demand)andactualcurrentsignalsandamplifyingtheerrorsignaliscarriedoutbythecurrent-erroramplifier(误差电流放大器).Bygivingthecurrenterroramplifierahighgain,theactualmotorcurrentwillalwayscorrespondcloselytothecurrentreferencesignal,i.e.thecurrenterrorwillbesmall,regardlessofmotorspeed.Inotherwords,wecanexpecttheactualmotorcurrenttofollow

the‘currentreference’signalatalltimes,thearmaturevoltagebeing

automaticallyadjustedbythecontrollersothat,regardlessofthespeed

ofthemotor,thecurrenthasthecorrectvalue.Thepurposeofthecurrentloop6.6Currentcontrol541Thejob546.6Currentcontrol55

1Thecurrent-erroramplifier((误差电流放大器))tobeoftheproportionalplusintegral(PI)type(seebelow),inwhichcasetheactualanddemandedcurrentswillbeexactlyequalundersteady-stateconditions.Thepurposeofthecurrentloop6.6Currentcontrol551Thecur556.6Currentcontrol56

2Theimportanceofpreventingexcessiveconvertercurrentsfromflowinghasbeenemphasizedpreviously,andthecurrentcontrolloopprovidesthemeanstothisend.TheprotectionofthecurrentAslongasthecurrentcontrolloopfunctions

properly,themotorcurrentcanneverexceedthereferencevalue.Hence

bylimitingthemagnitudeofthecurrentreferencesignal(bymeansofa

clampingcircuit(钳位电路)),themotorcurrentcanneverexceedthespecified

value.6.6Currentcontrol562Theimp566.6Currentcontrol57

2ThisisshowninFigure6.8,whichrepresentsasmallportionofFigure6.7.Theprotectionofthecurrent6.6Currentcontrol572Thisis576.6Currentcontrol58

2Thecharacteristicsofthespeedcontrollerareshownintheshadedpanel,fromwhichwecanseethatforsmallerrorsinspeed,thecurrentreferenceincreasesinproportiontothespeed,therebyensuring‘linearsystem’behaviorwithasmoothapproachtothetargetspeedTheprotectionofthecurrentOncethespeederrorexceedsalimit,theoutputofthe

speed-erroramplifiersaturatesandthereisthusnofurtherincreasein

thecurrentreference.Byarrangingforthismaximumcurrentreference

tocorrespondtothefull(rated)currentofthesystemthereisno

possibilityofthecurrentinthemotorandconverterexceedingitsratedvalue,nomatterhowlargethespeederrorbecomes.6.6Currentcontrol582Thecha586.6Currentcontrol59

2TheprotectionofthecurrentThefirstthingweshouldaimatwhensettingupadriveisagoodcurrentloop.Inthiscontext,‘good’meansthatthesteady-statemotor

currentshouldcorrespondexactlywiththecurrentreference,andthe

transientresponsetostepchangesinthecurrentreferenceshouldbefast

andwelldamped.This‘electroniccurrentlimiting’isbyfarthemostimportantprotectivefeatureofanydrive.6.6Currentcontrol592Thepro596.6Currentcontrol60

2Onapointofjargon(术语),itshouldperhapsbementionedthatthecurrent-erroramplifierismoreoftenthannotcalledeitherthe‘currentcontroller’(asinFigure6.7)orthe‘currentamplifier’.Thefirstofthesetermsisquitesensible,butthesecondcanbeverymisleading:thereisafterallnoquestionofthemotorcurrentitselfbeingamplified.Theprotectionofthecurrent6.6Currentcontrol602Onapo60

6.6CurrentcontrolTextBControlArrangementsforDCDrives6

6.7Torquecontrol

617Outline8

6.8Speedcontrol

6.9ArmaturevoltagefeedbackandIRcompensation96.6CurrentcontrolTextBCo616.7Torquecontrol62

Forapplicationsrequiringthemotortooperatewithaspecifiedtorqueregardlessofspeed(e.g.inlinetensioning),wecandispensewiththeouter(