電氣工程及其自動化專業(yè)英語第三章課件

Chapter3PowerElectronicTechnologyChapter31Section3DC-ACConvertersTextNewWordsandExpressionsExercisesEndTransitionofpartofspeechSection3DC-ACConvertersT2Section3DC-ACConvertersIntroductionDC-ACconvertersareconventionallycalledtheinverters.Suchconvertersareverypopularinthebattery-operatedpowersystemssuchastheuninterruptiblepowersupplies(UPS)forhospitals,andACmotordrives.Lowpowerlevelinvertersareusuallysingle-phasetypeandmediumandhighpowerinvertersarethree-phasetype.WewilllookintotheperformanceofDC-ACconvertersofseveralusefultopologies.Thefocuswillbeontheinverteroperationforlow-frequencyoutputSection3DC-ACConvertersI3Section3DC-ACConvertersvoltagessuchas50～60Hzor400Hzinaircraftsystems.AnalysesofDC-ACconvertercircuitsatlowandhighfrequenciesareverysimilarexceptthatathighfrequenciestheparasiticcapacitancesandinductances,chargestorage,andheatlocalizingproblemsinswitchingdevicesmustbetakenintoaccount.ControlledinversionTheDCtoACinvertersystemisshowninFig.3-5(a).InputisfromaDCsourceandtheoutputisdesiredtobeasinusoidalvoltageorcurrentwithSection3DC-ACConvertersv4Section3DC-ACConverterszeroDCcomponent,single-phaseorthree-phasesignal.TheloadisaR-L-Cload,anACvoltageoranACcurrentsink.Controlparametermaybe,asshallbeseenlater,anangle,apulsewidth,avoltageoracurrentsignal.Theconverterconsistsofswitches,reactivecomponentssuchasL,

C,transformersandresistances.Switchesincludingthediodesareassumedtobeidealandunidirectional,thatis,theyhavezeroon-statelosses,zerooff-statelosses,zeroswitchinglossesandunrestrictedvoltageorcurrentcarryingSection3DC-ACConvertersz5Section3DC-ACConverterscapabilities.Switchingdevicesinapracticaldesignexamplemustbeselectedwiththesedesiredcharacteristicsinmind.Inductors,capacitorsandtransformersareassumedtobeideal.Fig.3-5(a)ADC-ACinvertersystem;(b)operationalquadrantsontheUo-IoplaneSection3DC-ACConvertersc6Section3DC-ACConvertersElectricalloadscanbeofthreetypes:impedanceload,ACvoltagesinkloadorACcurrentsinkload.Examplesofimpedanceloadareelectromagneticacrelays,electromagneticpickupandliftequipments.Theloadcomprisesofalargevalueinductor.Examplesofacvoltagesinkaretheconstantspeeddrivesforinductionandsynchronousmotors.ExamplesofACcurrentsinkareconstanttorquedrivesfortheinductionandsynchronousmotors.Section3DC-ACConverters7Section3DC-ACConvertersGenerallyspeaking,theinputcurrentisnon-sinusoidalifthesourceisaDCvoltagesourceandviceversa.Theoutputvoltageandtheoutputcurrentontheotherhand,canbenon-sinusoidalsimultaneously.Hence,inputandoutputlow-passfiltersaregenerallyemployedtoreducehighfrequencycomponentsofthenon-sinusoidalsignalsoneitherside.Aseriesinductorisacurrentsmootherandaparallelcapacitorisavoltagesmoother.Section3DC-ACConverters8Section3DC-ACConvertersOutputcurrentgenerallydiffersinphasewiththeoutputvoltage.Hence,inaswitchingperiod,thepolaritiesofcurrentandvoltagevaryoverallfourquadrants,seeFig.3-6.Powerflowsfromtheconvertertotheloadinthefirstandthirdquadrantsandinthereversedirectioninthesecondandfourthquadrants.Aunidirectionalswitchintheinverters,therefore,hasaninverseconnecteddiodeinparalleltoallowbidirectionalcurrentflow.Section3DC-ACConverters9Section3DC-ACConvertersFig.3-6OutputcurrentandoutputvoltagewaveforminaninverterSection3DC-ACConvertersF10Section3DC-ACConvertersInvertersderivedfromvoltagesource(VSI)areideallysuitedforsupplyingpowertoaresistiveandinductiveloadandtheapplicationsthatrequireconstanttorqueoperationofACmotors.CurrentSourceInverters(CSI)areideallysuitedforsupplyingpowertolargelycapacitiveloadsandACvoltagebuses.Inverterscanbefurtherclassifiedonthebasisofthemodeofcontroloftheoutputvariable:voltageorcurrent.Invertercontrolinpracticeconcernsthreeaspects:fundamentalfrequency,amplitude,harmonicprofile.Section3DC-ACConverters11Section3DC-ACConvertersFundamentalfrequencyiseitherequaltotheswitchingfrequencyoftheinverter(square-waveandPWMinverters)oritssimplefraction(1/3,1/5,andsoon)suchasinMultiplePWMorsinePWM

inverters.Powerelectronicapplicationsrequiretheamplitudecontrolofoutputvoltageoroutputcurrent,calledVoltage-ModecontrolandCurrent-Modecontrolrespectively.AnexampleofVoltage-Modecontrolistheadjustablespeeddrivesofinductionandsynchronousmotors.AnexampleofCurrent-ModeSection3DC-ACConverters12Section3DC-ACConverterscontrolisaconstanttorquedriveforinductionmotors.OutputamplitudeinbothmodescanbecontrolledbyvaryingtheDCsourcevoltageorbyvaryingthepulse-width(pulse-widthmodulation,PWM).TheinvertersbasedonthecontrolofDCsourcevoltagearealsotermedDC-LinkInverters.TheseinvertersuseanACtocontrolledDCconverterontheinputside.Theinverterissquare-wavecontrolled.ThisarrangementisexpensivebutexhibitsafasterresponsetimethanPWMinverters.Thecontrolcircuitryisalsosimpler.Section3DC-ACConvertersc13Section3DC-ACConvertersHarmonicprofilingisanextrafeatureaddedovertheamplitudecontrol.Theobjectiveofharmonicprofilingistobolstertheamplitudeofthefundamentalcomponentoftheoutputvoltageandtheeliminationorreductionofhighorderharmonics.Oneofthetechniquesinvolvesthenotchingoraddingsteppulsestoabasepulse.Theothertechniqueinvolvesmultiplepulsespercycleofthefundamental,eachpulse-widthmodulatedtoachievedesiredwave-shapingoftheoutputwaveform(Sine-PWM).Section3DC-ACConverters14Section3DC-ACConvertersInvertertopologiesTheDC-ACinvertercanbethoughtofasathree-positionswitchasshowninFig.3-7(a).Theloadisconnectedtothepoleoftheswitchandthefirsttwopositionsareconnectedtodcvoltageordccurrentsourcesofoppositepolarities.Thethirdpositionisconnectedtothezeroterminalofthesource.Thepoleoftheswitchstaysinthefirstandsecondpositionsforequalamountsoftimeinaswitchingperiod.Theoutputwaveformforzeroamountoftimeonthethirdpositionisasquarewave.Theoutputwaveformfornonzerotimeispulsewave.Section3DC-ACConvertersI15Section3DC-ACConvertersFig.3-7(a)TheDC-ACinverterimplementedbya3-positionswitch;(b)the3-positionswitchsynthesizedfromtwo2-positionswitches;(c)full-bridgeinvertertopologySection3DC-ACConverters16Section3DC-ACConvertersThethree-positionswitchmaybesynthesizedbytwounidirectionalelectronicswitchesasshowninFig.3-7(b).ThethirdpositionoftheswitchissimulatedbyturningbothswitchesoffinVSItopologiesandoninCSItopologies.Thisisthebasicbridgeinvertertopology.Thetwodiodesprovidethepathforcurrentflowfromtheloadtothesourceasthismayberequiredinaninverteroperationinvolvingmorethanonequadrant.Invertersupplyingloadtoan

R-LloadoperatesinallfourquadrantsbecausetheoutputvoltageandSection3DC-ACConverters17Section3DC-ACConverterscurrentwaveforms,forsomesegmentsinaswitchingperiod,haveopposingpolarities.AMOSFEThasabuilt-inbodydiodethatservesasthepathforthecurrentfromthesourcetothedrain.AnIGBT,however,requiresanexternaldiode.The3-half-bridgeinverterismadeofthreehalf-bridgeinvertersconnectedinparalleleachswitched120apart.Thistopologyiscapableofhandlinghighpowerwithminimumcomponentstresses.Thehalf-bridgetopologyofFig.3-7(b)requirestwoseparatevoltagesourcesUS/2.Fig.3-7(c)showsafull-bridgetopologythatrequiresasinglevoltagesource.ReturnSection3DC-ACConvertersc18NewWordsandExpressionsinverter n.逆變器uninterruptiblepowersupplies(UPS)不間斷電源single-phase a.單相的three-phase a.三相的parameter n.參數unidirectional a.單極性的，單向的electromagnetic a.電磁的relay

v.中繼，轉播；n.繼電器NewWordsandExpressionsinver19NewWordsandExpressionssynchronousmotor 同步電動機torque n.轉矩non-sinusoidal a.非正弦的low-pass 低通smoother n.濾波器，整平器polarity n.極性bidirectionala.雙向的harmonics n.諧波NewWordsandExpressionssynch20NewWordsandExpressionshighorderharmonics 高次諧波induction n.感應square-wave 方波responsetime 響應時間notching n.;a.階梯式，下凹的，切口，開槽steppulse階躍脈沖ReturnNewWordsandExpressionshigh21TransitionofPartofSpeech三、轉換為漢語形容詞1、副詞轉換成形容詞(1)Roboticsissocloselyassociatedwithcyberneticsthatitissometimesmistakenlyconsideredtobesynonymous.機器人技術與控制論的聯系如此密切，以致兩者有時被錯誤地認為是同一回事。(2)Miniaturegasdetectorischieflyfeaturedbysmallsize,lightweight,completefunctionsandlongcontinuousworkingtime.袖珍式瓦斯檢測器的主要特點是體積小、重量輕、功能齊全、連續(xù)工作時間長。TransitionofPartofSpeech三、22TransitionofPartofSpeech(3)Gasesandliquidsare

perfectly

elastic.氣體和液體都是完全的彈性體。TransitionofPartofSpeech(323TransitionofPartofSpeech2、名詞轉換成漢語形容詞(1)ThenuclearpowersystemdesignedinChinaisofgreatprecision.中國設計的核動力系統(tǒng)十分精確。(2)Electroniccomputersandmicroprocessorsareofgreatimportancetous.電子計算機和微處理器對我們來說十分重要。(3)Singlecrystalsofhighperfectionareanabsolutenecessity

forthefabricationofintegratedcircuits.高度完整的單晶對于制造集成電路來說是絕對必要的。TransitionofPartofSpeech2、24TransitionofPartofSpeech四、轉換成漢語副詞

1、形容詞轉換成漢語副詞(1)Theenginehasgivenaconstantlygood

performance.這臺發(fā)動機一直運轉良好。(2)Thesameprinciplesoflowinternalresistancealsoapplytomilliammeters.低內阻的原理同樣適用于毫安表。(3)Agraphgivesavisualrepresentationoftherelationship.圖表可以直觀地顯示要說明的關系。TransitionofPartofSpeech四、25TransitionofPartofSpeech2、其他詞轉換成副詞(1)Rapidevaporationattheheating-surface

tendstomakethesteamwet.加熱面上的迅速蒸發(fā)，往往使蒸汽的濕度加大。(2)Theaddeddevicewillensureaccessibilityforpartloadingandunloading.增添這種裝置將保證工件裝卸方便。

(3)Weshalldeveloptheelectricpowerindustryinaway.我們將大規(guī)模發(fā)展電力工業(yè)。ReturnTransitionofPartofSpeech2、26ExercisesI.Choosethebestanswerintotheblank

1.Highpowerinvertersareusually

.A.single-phasetype B.two-phasetypeC.three-phasetype D.DC-DCconverters2.Theoutputofainverterisdesiredtobea

.A.dcvoltage B.sinusoidalvoltageC.dccurrent D.time-varyingvoltageExercisesI.Choosethebesta27ExercisesI.Choosethebestanswerintotheblank

3.Theconstantspeeddriveforsynchronousmotorsistheexampleof

.A.impedanceload B.acvoltagesinkloadC.ACcurrentsinkload D.DCload4.Voltagesourceinvertersaretheoreticallysuitedforsupplyingpowerto

.A.capacitiveloads B.ACvoltagebusesC.inductiveloads D.dcmotorsExercisesI.Choosethebesta28ExercisesI.Choosethebestanswerintotheblank

5.OutputamplitudeinVoltage-ModecontrolandCurrent-Modecontrolmodescanbecon-trolledbyvaryingtheDCsourcevoltageorbyvaryingthe

.A.dcsourcecurrent B.pulse-widthC.pulse-amplitude D.harmonicExercisesI.Choosethebesta29ExercisesII.Answerthefollowingquestionsaccordingtothetext

1.WhatparametersmaybeusedascontrolparametersintheDC-ACinvertersystem?2.WhatdoestheDC-ACinverterconsistof?3.Whatcharacteristicsdotheidealswitcheswiththediodehave?4.Whatfunctionsdolow-passfiltershave?5.Whataspectsareconcernedforinvertercontrolinpracticalapplications?ExercisesII.Answerthefollo30ExercisesIII.TranslatethefollowingintoChinese

1.Reducingtheharmoniccontentoftheoutputvoltageorcurrentisoneoftheimportanttasksofthedesigners.Harmonicsappearastheundesirablenoise.Harmonicsreducethepowerfactor.Theymayexcitemechanicalresonanceatdetrimentalfrequenciesorgenerateacousticalnoiseinacmotordrivesystems.Theripplecontent(harmonics)iseasilyreducedbypost-filteringaftertheconverter.Thepostconverterlow-passfiltermaybedesignedf