MarineEngineeringEnglish《輪機英語》APracticalCourseforEngineersWorkingonBoardShips

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

4part1

NewWordsandExpressions5part2

TextAnalysisLesson27ACmotorsSupplyingalternatingcurrenttoacoilwhichisfreetorotateinamagneticfieldwillnotproduceamotoreffectsincethecurrentisconstantlychangingdirection.#給一個在磁場中能自由旋轉的線圈通入交流電是不會產生電動機效應的，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生一個旋轉的磁場用于感應電機或是鼠籠式電機。Lesson27ACmotorsTherotorhasaseriesofcopperconductorsalongitsaxiswhicharejoinedbyringsattheendstoformacage.#轉子有一系列的銅導體，沿著轉子軸線銅導體在其末端由環狀物所連接，形成一個籠子。Lesson27ACmotorsLesson27ACmotorsWhenthemotorisstartedtherotatingmagneticfieldinducesae.m.f.,

inthecageandthusacurrentflow.#當電動機啟動時就會在籠子中形成電動勢和電流。Lesson27ACmotorsThecurrent-carryingconductorinamagneticfieldproducesthemotoreffectwhichturnstherotor.#在磁場中的帶電導體就會產生轉動轉子的電動機效應。Lesson27ACmotorsThemotorspeedbuildsuptoavalue，justlessthanthespeedofrotationofthemagneticfield.#電動機的速度達到一個值，比旋轉磁場的速度小一點。Lesson27ACmotorsThemotorspeeddependsupontheEMF

inducedintherotorandthisdependsuponthedifferenceinspeedbetweentheconductorsandthemagneticfield.#電動機的轉速取決于轉子感應產生的電動勢，電動勢取決于導體和旋轉磁場速度的差異。Lesson27ACmotorsIftheloadisincreasedtherotorslowsdownslightly,causinganincreaseininducedEMF,andthusagreatertorquetodealwiththeincreasedload.如果電動機負荷增加，轉子轉速稍微降低，將導致產生的電動勢增加，因此有更大的轉矩以應對負荷的增加。Lesson27ACmotorsThemotorisalmostconstantspeedoverallvaluesofload.#在所有負荷下，電動機幾乎是恒定的轉速。Lesson27ACmotorsItwillstartagainstabouttwotimesfullloadtorquebutdrawsastartingcurrentof

aboutsixtimesthenormalfullloadcurrent.#啟動轉矩是全負荷轉矩的2倍，而啟動電流則是正常全負荷的6倍。Lesson27ACmotorsThestartingcurrentcanbereducedbyhavingadoublecagearrangementontherotor.#可以在轉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，在轉子中一個在另一個的下邊。Lesson27ACmotorsWhenstarting,theouterhighresistancecagecarriesalmostalltherotorcurrent.#當啟動時，外側高阻值的籠子承擔了幾乎所有的轉子電流。Lesson27ACmotorsAsthemotoracceleratesthelowresistanceinnerwindingtakesmoreandmoreofthecurrentuntilitcarriesthemajority.#電動機轉速逐漸增加，內側低阻值的繞組線圈承擔越來越多的電流，直到它承擔大部分。Lesson27ACmotorsAnumberofdifferentfixedspeedsarepossiblebypolechanging.#通過改變極數，(獲得)多個不同固定的轉速是可能的。Lesson27ACmotorsThespeedofaninductionmotorisproportionaltofrequencydividedbythenumbersofpairsofpoles.#感應電動機的轉速正比于頻率和極對數的比值。Lesson27ACmotorsIfthereforeaswitchisprovidedwhichcanalterthenumbersofpairsofpoles,thenvariousfixedspeedsarepossible.#因此如果一個開關能改變極對數，那么多個穩定轉速是可能（實現）的。Lesson27ACmotorsThenumberofpolesaffectsthestartingcharacteristicssuchthatthemorepolesthelessthestartingtorquetofullloadtorqueratio.#極對數會影響到啟動特性，以致于極對數越多，啟動轉矩和全負荷轉矩的比值就越低。Lesson27ACmotorsOnlytheinductiontypeofACmotorhasbeendescribedsinceitisalmostexclusivelyusedinmaritimework.#僅僅交流感應電動機被描述，因為它是船舶專用的。Lesson27ACmotorsSynchronousmotorsareanothertypewhichhasbeenusedforelectricalpropulsionsystemsbutnotauxiliarydrives.#同步電動機是另外一種型式，其用于電氣推進系統而不是輔助傳動。Lesson27ACmotorsAnumberofdifferentarrangementscanbeusedforstartinganinductionmotor.#很多不同裝置都可以用來啟動感應電動機。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載，由于啟動電流的熱效應，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.#啟動轉矩也降低到直接啟動時的三分之一。Lesson27ACmotorsArapidchangeovertodeltaisrequiredatabout75%offullloadspeedwhenthemotorwilldrawaboutthree-and-a-halftimesitsfullloadcurrent.#當電機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，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.#電動機靠抽頭之一被啟動，然后在大約75%轉速時快速轉換到全電壓。Lesson27ACmotorsThetappingchosenwilldependuponthestartingtorquerequiredwitha60%tappinggivingabout70%offullloadtorque.#抽頭的選擇取決于啟動轉矩，60%抽頭提供大約70%全負荷轉矩。Lesson27ACmotorsAsmallerpercentagetappingwillgiveasmallerstartingtorqueandvice-versa.#小的抽頭百分比只能提供一個小的啟動轉矩，反之亦然。Lesson27ACmotorsThestatorresistancestarterhasaresistanceinthestatorcircuit

whenthemotorisstarted.Anadjustabletimingdeviceoperatestoshortcircuitthisresistancewhenthemotorhasreachedaparticularspeed.#串電阻啟動是在電動機啟動時在啟動回路里串聯一個電阻。當達到特定轉速時，一個可調節的定時裝置動作短接這個電阻。Lesson27ACmotorsModernelectronictechniquesenableAC

inductionmotorstobeusedinspeed-controlsystems.#現代電子技術使交流感應電動機能夠用于速度控制系統。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定，首先進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備的振蕩器的電源。這些可能是晶閘管(功率高達1.5MW或以上)或晶體管(功率高達幾十千瓦)。Lesson27ACmotorsThehigh-poweroscillatoroutputiscontrolledinfrequencyandvoltagebyafeedbacksystem.Themotorspeedisvariedbychangingtheoscillatoroutputfrequency.#高功率振蕩器輸出由反饋系統控制頻率和電壓。通過改變振蕩器輸出頻率來改變電機速度。Lesson27ACmotorsThemotorcurrentnecessarytoobtainthedesiredtorque(atsmallanglesofslip)isnormallyobtainedbymaintainingthevoltagealmostproportionaltofrequency.獲得所需轉矩（在小滑失角下）所需的電機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.#在電動機的電路里會有適當的保護設備，比如缺相保護、過載保護和欠壓保護。Lesson27ACmotorsSinglephasingoccurswhenonephaseinathreephasecircuitbecomesopencircuited.#三相電路中有一相斷開時，缺相發生。Lesson27ACmotorsTheresultisexcessivecurrentsinallthewindingswith,inthecaseofadeltaconnectedstatorrunningatfullload,onewindingtakingthreetimesitsnormalloadcurrent.#如果連接定子的三角形運行在全負荷，結果是所有繞組中電流都過大，其中一相繞組承擔的電流大概是正常負荷電流的3倍，。Lesson27ACmotorsAmachinewhichisrunningwhensinglephasingoccurswillcontinuetorunbutwithanunbalanceddistributionofcurrent.#當運轉中的機器發生缺相時，它會在一個不平衡的電流帶動下繼續運轉。Lesson27ACmotorsAnoverloadprotectiondevicemaynottripifthemotorisrunningatlessthanfullload.#如果電動機負載低于全負載時，過載保護可能不脫扣。Lesson27ACmotorsOnemethodofsinglephasingprotectionutilizesatemperaturesensitivedevicewhichisolatesthemachinefromthesupplyatsomeparticularwindingtemperature.#還有一種斷相保護方法是利用一個感溫設備在線圈達到特定的溫度時把機器和供電設備給隔離。Lesson27ACmotorsOverloadprotectiondevicesarealsofittedandmaybeseparateorcombinedwiththesinglephaseprotectiondevice.#過載保護也被配帶，可能和缺相保護分開或結合起來。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.#欠壓或是失壓保護來保證電動機在供電設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