輪機(jī)英語21課件

MarineEngineeringEnglish《輪機(jī)英語》APracticalCourseforEngineersWorkingonBoardShips

MarineElectricalSystemandAutomatizationLesson27Alternatingcurrentmotors第27課交流電動機(jī)LearningObjectives:1.Graspsomeusefulwordsandexpressionsinthewholetext.2.Afterstudyingthelesson,heorshewillhaveabetterunderstandingofConstruction&PrincipleofACMotor.3

4part1

NewWordsandExpressions5part2

TextAnalysisLesson27ACmotorsSupplyingalternatingcurrenttoacoilwhichisfreetorotateinamagneticfieldwillnotproduceamotoreffectsincethecurrentisconstantlychangingdirection.#給一個在磁場中能自由旋轉(zhuǎn)的線圈通入交流電是不會產(chǎn)生電動機(jī)效應(yīng)的，因為電流是在不斷改變方向的。Lesson27ACmotorsUseisthereforemadeinaninductionorsquirrelcagemotorofarotatingmagneticfieldproducedbythreeseparatelyphasedwindingsinthestator.#三相單獨的定子線圈會產(chǎn)生一個旋轉(zhuǎn)的磁場用于感應(yīng)電機(jī)或是鼠籠式電機(jī)。Lesson27ACmotorsTherotorhasaseriesofcopperconductorsalongitsaxiswhicharejoinedbyringsattheendstoformacage.#轉(zhuǎn)子有一系列的銅導(dǎo)體，沿著轉(zhuǎn)子軸線銅導(dǎo)體在其末端由環(huán)狀物所連接，形成一個籠子。Lesson27ACmotorsLesson27ACmotorsWhenthemotorisstartedtherotatingmagneticfieldinducesae.m.f.,

inthecageandthusacurrentflow.#當(dāng)電動機(jī)啟動時就會在籠子中形成電動勢和電流。Lesson27ACmotorsThecurrent-carryingconductorinamagneticfieldproducesthemotoreffectwhichturnstherotor.#在磁場中的帶電導(dǎo)體就會產(chǎn)生轉(zhuǎn)動轉(zhuǎn)子的電動機(jī)效應(yīng)。Lesson27ACmotorsThemotorspeedbuildsuptoavalue，justlessthanthespeedofrotationofthemagneticfield.#電動機(jī)的速度達(dá)到一個值，比旋轉(zhuǎn)磁場的速度小一點。Lesson27ACmotorsThemotorspeeddependsupontheEMF

inducedintherotorandthisdependsuponthedifferenceinspeedbetweentheconductorsandthemagneticfield.#電動機(jī)的轉(zhuǎn)速取決于轉(zhuǎn)子感應(yīng)產(chǎn)生的電動勢，電動勢取決于導(dǎo)體和旋轉(zhuǎn)磁場速度的差異。Lesson27ACmotorsIftheloadisincreasedtherotorslowsdownslightly,causinganincreaseininducedEMF,andthusagreatertorquetodealwiththeincreasedload.如果電動機(jī)負(fù)荷增加，轉(zhuǎn)子轉(zhuǎn)速稍微降低，將導(dǎo)致產(chǎn)生的電動勢增加，因此有更大的轉(zhuǎn)矩以應(yīng)對負(fù)荷的增加。Lesson27ACmotorsThemotorisalmostconstantspeedoverallvaluesofload.#在所有負(fù)荷下，電動機(jī)幾乎是恒定的轉(zhuǎn)速。Lesson27ACmotorsItwillstartagainstabouttwotimesfullloadtorquebutdrawsastartingcurrentof

aboutsixtimesthenormalfullloadcurrent.#啟動轉(zhuǎn)矩是全負(fù)荷轉(zhuǎn)矩的2倍，而啟動電流則是正常全負(fù)荷的6倍。Lesson27ACmotorsThestartingcurrentcanbereducedbyhavingadoublecagearrangementontherotor.#可以在轉(zhuǎn)子上接雙籠式裝置來降低啟動電流。Lesson27ACmotorsTwoseparatedcagesareprovided,onebelowtheotherintherotor.#兩個獨立的籠子被提供，在轉(zhuǎn)子中一個在另一個的下邊。Lesson27ACmotorsWhenstarting,theouterhighresistancecagecarriesalmostalltherotorcurrent.#當(dāng)啟動時，外側(cè)高阻值的籠子承擔(dān)了幾乎所有的轉(zhuǎn)子電流。Lesson27ACmotorsAsthemotoracceleratesthelowresistanceinnerwindingtakesmoreandmoreofthecurrentuntilitcarriesthemajority.#電動機(jī)轉(zhuǎn)速逐漸增加，內(nèi)側(cè)低阻值的繞組線圈承擔(dān)越來越多的電流，直到它承擔(dān)大部分。Lesson27ACmotorsAnumberofdifferentfixedspeedsarepossiblebypolechanging.#通過改變極數(shù)，(獲得)多個不同固定的轉(zhuǎn)速是可能的。Lesson27ACmotorsThespeedofaninductionmotorisproportionaltofrequencydividedbythenumbersofpairsofpoles.#感應(yīng)電動機(jī)的轉(zhuǎn)速正比于頻率和極對數(shù)的比值。Lesson27ACmotorsIfthereforeaswitchisprovidedwhichcanalterthenumbersofpairsofpoles,thenvariousfixedspeedsarepossible.#因此如果一個開關(guān)能改變極對數(shù)，那么多個穩(wěn)定轉(zhuǎn)速是可能（實現(xiàn)）的。Lesson27ACmotorsThenumberofpolesaffectsthestartingcharacteristicssuchthatthemorepolesthelessthestartingtorquetofullloadtorqueratio.#極對數(shù)會影響到啟動特性，以致于極對數(shù)越多，啟動轉(zhuǎn)矩和全負(fù)荷轉(zhuǎn)矩的比值就越低。Lesson27ACmotorsOnlytheinductiontypeofACmotorhasbeendescribedsinceitisalmostexclusivelyusedinmaritimework.#僅僅交流感應(yīng)電動機(jī)被描述，因為它是船舶專用的。Lesson27ACmotorsSynchronousmotorsareanothertypewhichhasbeenusedforelectricalpropulsionsystemsbutnotauxiliarydrives.#同步電動機(jī)是另外一種型式，其用于電氣推進(jìn)系統(tǒng)而不是輔助傳動。Lesson27ACmotorsAnumberofdifferentarrangementscanbeusedforstartinganinductionmotor.#很多不同裝置都可以用來啟動感應(yīng)電動機(jī)。Lesson27ACmotorsTheseincludedirecton-line,stardelta,autotransformerandstatorresistance.#包括直接啟動，星三角啟動，降壓啟動和串電阻啟動。Lesson27ACmotorsDirecton-linestartingisusualwherethedistributionsystemcanacceptthestartingcurrent.#直接啟動用于配電系統(tǒng)能承受啟動電流沖擊的場合。Lesson27ACmotorsWhereaslowmovinghighinertialoadisinvolvedthestartingtimemustbeconsideredbecauseoftheheatingeffectofthestartingcurrent.#如果涉及緩慢移動的高慣性負(fù)載，由于啟動電流的熱效應(yīng)，必須考慮啟動時間。Lesson27ACmotorsThestardeltastarterconnectsthestatorwindingsfirstinstarandwhenrunningchangesovertodelta.#星三角啟動是在啟動時將繞組接成星形而運行時又接回三角形。Lesson27ACmotorsThestarconnectionresultsinabouthalfofthelinevoltagebeingappliedtoeachphasewiththereforeareductioninstartingcurrent.#星型連接導(dǎo)致大約一半的線電壓作用在每一相上，因此啟動電流較小。Lesson27ACmotorsThestartingtorqueisalsoreducedtoaboutone-thirdofitsdirectonlinevalue.#啟動轉(zhuǎn)矩也降低到直接啟動時的三分之一。Lesson27ACmotorsArapidchangeovertodeltaisrequiredatabout75%offullloadspeedwhenthemotorwilldrawaboutthree-and-a-halftimesitsfullloadcurrent.#當(dāng)電機(jī)將消耗大約三倍半的滿載電流時，在約75%的滿載速度下需要快速轉(zhuǎn)換為三角形啟動。Lesson27ACmotorsTheautotransformerstarterisusedonlyforlargemotors.Itusestapingsfromatransformertoprovide,forexample,40%,60%and75%ofnormalvoltage.#降壓啟動僅用于大型的電動機(jī)，它用從變壓器來的抽頭提供，例如，40%、60%或是75%的正常電壓。Lesson27ACmotorsThemotorisstartedononeofthetapingsandthenquicklyswitchedtofullvoltageatabout75%fullspeed.#電動機(jī)靠抽頭之一被啟動，然后在大約75%轉(zhuǎn)速時快速轉(zhuǎn)換到全電壓。Lesson27ACmotorsThetappingchosenwilldependuponthestartingtorquerequiredwitha60%tappinggivingabout70%offullloadtorque.#抽頭的選擇取決于啟動轉(zhuǎn)矩，60%抽頭提供大約70%全負(fù)荷轉(zhuǎn)矩。Lesson27ACmotorsAsmallerpercentagetappingwillgiveasmallerstartingtorqueandvice-versa.#小的抽頭百分比只能提供一個小的啟動轉(zhuǎn)矩，反之亦然。Lesson27ACmotorsThestatorresistancestarterhasaresistanceinthestatorcircuit

whenthemotorisstarted.Anadjustabletimingdeviceoperatestoshortcircuitthisresistancewhenthemotorhasreachedaparticularspeed.#串電阻啟動是在電動機(jī)啟動時在啟動回路里串聯(lián)一個電阻。當(dāng)達(dá)到特定轉(zhuǎn)速時，一個可調(diào)節(jié)的定時裝置動作短接這個電阻。Lesson27ACmotorsModernelectronictechniquesenableAC

inductionmotorstobeusedinspeed-controlsystems.#現(xiàn)代電子技術(shù)使交流感應(yīng)電動機(jī)能夠用于速度控制系統(tǒng)。Lesson27ACmotorsTheship'ssupply,whichmaynotbeasstableinvoltageorfrequencyasthatashore,isfirstrectifiedtoprovideaDC

supply.#船舶電力供給，在電壓或頻率上可能不如岸上的電源穩(wěn)定，首先進(jìn)行整流以提供直流電源。Lesson27ACmotorsThisisthenusedasthepowersupplyofanoscillatorusinghigh-powerelectronicdevices.Thesemaybethyristors(forpowersupto1.5MWormore)ortransistors(forpowersuptoafewtensofkilowatts).#然后將其用作使用大功率電子設(shè)備的振蕩器的電源。這些可能是晶閘管(功率高達(dá)1.5MW或以上)或晶體管(功率高達(dá)幾十千瓦)。Lesson27ACmotorsThehigh-poweroscillatoroutputiscontrolledinfrequencyandvoltagebyafeedbacksystem.Themotorspeedisvariedbychangingtheoscillatoroutputfrequency.#高功率振蕩器輸出由反饋系統(tǒng)控制頻率和電壓。通過改變振蕩器輸出頻率來改變電機(jī)速度。Lesson27ACmotorsThemotorcurrentnecessarytoobtainthedesiredtorque(atsmallanglesofslip)isnormallyobtainedbymaintainingthevoltagealmostproportionaltofrequency.獲得所需轉(zhuǎn)矩（在小滑失角下）所需的電機(jī)電流通常是通過保持電壓幾乎與頻率成比例來獲得的。Lesson27ACmotorsCertainprotectivedevicesarefittedin

themotorcircuittoprotectagainstfaultssuchassinglephasing,overloadorundervoltage.#在電動機(jī)的電路里會有適當(dāng)?shù)谋Ｗo(hù)設(shè)備，比如缺相保護(hù)、過載保護(hù)和欠壓保護(hù)。Lesson27ACmotorsSinglephasingoccurswhenonephaseinathreephasecircuitbecomesopencircuited.#三相電路中有一相斷開時，缺相發(fā)生。Lesson27ACmotorsTheresultisexcessivecurrentsinallthewindingswith,inthecaseofadeltaconnectedstatorrunningatfullload,onewindingtakingthreetimesitsnormalloadcurrent.#如果連接定子的三角形運行在全負(fù)荷，結(jié)果是所有繞組中電流都過大，其中一相繞組承擔(dān)的電流大概是正常負(fù)荷電流的3倍，。Lesson27ACmotorsAmachinewhichisrunningwhensinglephasingoccurswillcontinuetorunbutwithanunbalanceddistributionofcurrent.#當(dāng)運轉(zhuǎn)中的機(jī)器發(fā)生缺相時，它會在一個不平衡的電流帶動下繼續(xù)運轉(zhuǎn)。Lesson27ACmotorsAnoverloadprotectiondevicemaynottripifthemotorisrunningatlessthanfullload.#如果電動機(jī)負(fù)載低于全負(fù)載時，過載保護(hù)可能不脫扣。Lesson27ACmotorsOnemethodofsinglephasingprotectionutilizesatemperaturesensitivedevicewhichisolatesthemachinefromthesupplyatsomeparticularwindingtemperature.#還有一種斷相保護(hù)方法是利用一個感溫設(shè)備在線圈達(dá)到特定的溫度時把機(jī)器和供電設(shè)備給隔離。Lesson27ACmotorsOverloadprotectiondevicesarealsofittedandmaybeseparateorcombinedwiththesinglephaseprotectiondevice.#過載保護(hù)也被配帶，可能和缺相保護(hù)分開或結(jié)合起來。Lesson27ACmotorsTheymusthaveatimedelayfittedsothatoperationdoesnotoccurduringthehighstartingcurrentperiod.#它們必須要有一個時間延遲以保證在高啟動電流期間不會動作。Lesson27ACmotorsAnundervoltageor“novolts”

protectivedeviceensuresthatthemotorisproperlystartedafterasupplyfailure.#欠壓或是失壓保護(hù)來保證電動機(jī)在供電設(shè)備故障后能正常啟動。part3

Exercise&ExaminationLowhorsepower,three-phase,inductionmotorscanbestartedwithfullvoltagebymeansof_____.A.compensatorstartersB.autotransformerstartersC.across-the-linestartersD.primary-resistorstartersWhenreadingelectricalmotorcontrollerdiagrams,ithelpstoknowthat_____.A.currentpathsinthecontrolcircuitaredrawnasheavylinesandinthepowercircuitaslighterlinesB.currentpathsinthepowercircuitaredrawnasheavylinesandincontrolcircuitaslighterlin