轮机英语21课件

MarineEngineeringEnglish《轮机英语》APracticalCourseforEngineersWorkingonBoardShips

MarineElectricalSystemandAutomatizationLesson27Alternatingcurrentmotors第27课交流电动机LearningObjectives:1.Graspsomeusefulwordsandexpressionsinthewholetext.2.Afterstudyingthelesson,heorshewillhaveabetterunderstandingofConstruction&PrincipleofACMotor.3

4part1

NewWordsandExpressions5part2

TextAnalysisLesson27ACmotorsSupplyingalternatingcurrenttoacoilwhichisfreetorotateinamagneticfieldwillnotproduceamotoreffectsincethecurrentisconstantlychangingdirection.#给一个在磁场中能自由旋转的线圈通入交流电是不会产生电动机效应的，因为电流是在不断改变方向的。Lesson27ACmotorsUseisthereforemadeinaninductionorsquirrelcagemotorofarotatingmagneticfieldproducedbythreeseparatelyphasedwindingsinthestator.#三相单独的定子线圈会产生一个旋转的磁场用于感应电机或是鼠笼式电机。Lesson27ACmotorsTherotorhasaseriesofcopperconductorsalongitsaxiswhicharejoinedbyringsattheendstoformacage.#转子有一系列的铜导体，沿着转子轴线铜导体在其末端由环状物所连接，形成一个笼子。Lesson27ACmotorsLesson27ACmotorsWhenthemotorisstartedtherotatingmagneticfieldinducesae.m.f.,

inthecageandthusacurrentflow.#当电动机启动时就会在笼子中形成电动势和电流。Lesson27ACmotorsThecurrent-carryingconductorinamagneticfieldproducesthemotoreffectwhichturnstherotor.#在磁场中的带电导体就会产生转动转子的电动机效应。Lesson27ACmotorsThemotorspeedbuildsuptoavalue，justlessthanthespeedofrotationofthemagneticfield.#电动机的速度达到一个值，比旋转磁场的速度小一点。Lesson27ACmotorsThemotorspeeddependsupontheEMF

inducedintherotorandthisdependsuponthedifferenceinspeedbetweentheconductorsandthemagneticfield.#电动机的转速取决于转子感应产生的电动势，电动势取决于导体和旋转磁场速度的差异。Lesson27ACmotorsIftheloadisincreasedtherotorslowsdownslightly,causinganincreaseininducedEMF,andthusagreatertorquetodealwiththeincreasedload.如果电动机负荷增加，转子转速稍微降低，将导致产生的电动势增加，因此有更大的转矩以应对负荷的增加。Lesson27ACmotorsThemotorisalmostconstantspeedoverallvaluesofload.#在所有负荷下，电动机几乎是恒定的转速。Lesson27ACmotorsItwillstartagainstabouttwotimesfullloadtorquebutdrawsastartingcurrentof

aboutsixtimesthenormalfullloadcurrent.#启动转矩是全负荷转矩的2倍，而启动电流则是正常全负荷的6倍。Lesson27ACmotorsThestartingcurrentcanbereducedbyhavingadoublecagearrangementontherotor.#可以在转子上接双笼式装置来降低启动电流。Lesson27ACmotorsTwoseparatedcagesareprovided,onebelowtheotherintherotor.#两个独立的笼子被提供，在转子中一个在另一个的下边。Lesson27ACmotorsWhenstarting,theouterhighresistancecagecarriesalmostalltherotorcurrent.#当启动时，外侧高阻值的笼子承担了几乎所有的转子电流。Lesson27ACmotorsAsthemotoracceleratesthelowresistanceinnerwindingtakesmoreandmoreofthecurrentuntilitcarriesthemajority.#电动机转速逐渐增加，内侧低阻值的绕组线圈承担越来越多的电流，直到它承担大部分。Lesson27ACmotorsAnumberofdifferentfixedspeedsarepossiblebypolechanging.#通过改变极数，(获得)多个不同固定的转速是可能的。Lesson27ACmotorsThespeedofaninductionmotorisproportionaltofrequencydividedbythenumbersofpairsofpoles.#感应电动机的转速正比于频率和极对数的比值。Lesson27ACmotorsIfthereforeaswitchisprovidedwhichcanalterthenumbersofpairsofpoles,thenvariousfixedspeedsarepossible.#因此如果一个开关能改变极对数，那么多个稳定转速是可能（实现）的。Lesson27ACmotorsThenumberofpolesaffectsthestartingcharacteristicssuchthatthemorepolesthelessthestartingtorquetofullloadtorqueratio.#极对数会影响到启动特性，以致于极对数越多，启动转矩和全负荷转矩的比值就越低。Lesson27ACmotorsOnlytheinductiontypeofACmotorhasbeendescribedsinceitisalmostexclusivelyusedinmaritimework.#仅仅交流感应电动机被描述，因为它是船舶专用的。Lesson27ACmotorsSynchronousmotorsareanothertypewhichhasbeenusedforelectricalpropulsionsystemsbutnotauxiliarydrives.#同步电动机是另外一种型式，其用于电气推进系统而不是辅助传动。Lesson27ACmotorsAnumberofdifferentarrangementscanbeusedforstartinganinductionmotor.#很多不同装置都可以用来启动感应电动机。Lesson27ACmotorsTheseincludedirecton-line,stardelta,autotransformerandstatorresistance.#包括直接启动，星三角启动，降压启动和串电阻启动。Lesson27ACmotorsDirecton-linestartingisusualwherethedistributionsystemcanacceptthestartingcurrent.#直接启动用于配电系统能承受启动电流冲击的场合。Lesson27ACmotorsWhereaslowmovinghighinertialoadisinvolvedthestartingtimemustbeconsideredbecauseoftheheatingeffectofthestartingcurrent.#如果涉及缓慢移动的高惯性负载，由于启动电流的热效应，必须考虑启动时间。Lesson27ACmotorsThestardeltastarterconnectsthestatorwindingsfirstinstarandwhenrunningchangesovertodelta.#星三角启动是在启动时将绕组接成星形而运行时又接回三角形。Lesson27ACmotorsThestarconnectionresultsinabouthalfofthelinevoltagebeingappliedtoeachphasewiththereforeareductioninstartingcurrent.#星型连接导致大约一半的线电压作用在每一相上，因此启动电流较小。Lesson27ACmotorsThestartingtorqueisalsoreducedtoaboutone-thirdofitsdirectonlinevalue.#启动转矩也降低到直接启动时的三分之一。Lesson27ACmotorsArapidchangeovertodeltaisrequiredatabout75%offullloadspeedwhenthemotorwilldrawaboutthree-and-a-halftimesitsfullloadcurrent.#当电机将消耗大约三倍半的满载电流时，在约75%的满载速度下需要快速转换为三角形启动。Lesson27ACmotorsTheautotransformerstarterisusedonlyforlargemotors.Itusestapingsfromatransformertoprovide,forexample,40%,60%and75%ofnormalvoltage.#降压启动仅用于大型的电动机，它用从变压器来的抽头提供，例如，40%、60%或是75%的正常电压。Lesson27ACmotorsThemotorisstartedononeofthetapingsandthenquicklyswitchedtofullvoltageatabout75%fullspeed.#电动机靠抽头之一被启动，然后在大约75%转速时快速转换到全电压。Lesson27ACmotorsThetappingchosenwilldependuponthestartingtorquerequiredwitha60%tappinggivingabout70%offullloadtorque.#抽头的选择取决于启动转矩，60%抽头提供大约70%全负荷转矩。Lesson27ACmotorsAsmallerpercentagetappingwillgiveasmallerstartingtorqueandvice-versa.#小的抽头百分比只能提供一个小的启动转矩，反之亦然。Lesson27ACmotorsThestatorresistancestarterhasaresistanceinthestatorcircuit

whenthemotorisstarted.Anadjustabletimingdeviceoperatestoshortcircuitthisresistancewhenthemotorhasreachedaparticularspeed.#串电阻启动是在电动机启动时在启动回路里串联一个电阻。当达到特定转速时，一个可调节的定时装置动作短接这个电阻。Lesson27ACmotorsModernelectronictechniquesenableAC

inductionmotorstobeusedinspeed-controlsystems.#现代电子技术使交流感应电动机能够用于速度控制系统。Lesson27ACmotorsTheship'ssupply,whichmaynotbeasstableinvoltageorfrequencyasthatashore,isfirstrectifiedtoprovideaDC

supply.#船舶电力供给，在电压或频率上可能不如岸上的电源稳定，首先进行整流以提供直流电源。Lesson27ACmotorsThisisthenusedasthepowersupplyofanoscillatorusinghigh-powerelectronicdevices.Thesemaybethyristors(forpowersupto1.5MWormore)ortransistors(forpowersuptoafewtensofkilowatts).#然后将其用作使用大功率电子设备的振荡器的电源。这些可能是晶闸管(功率高达1.5MW或以上)或晶体管(功率高达几十千瓦)。Lesson27ACmotorsThehigh-poweroscillatoroutputiscontrolledinfrequencyandvoltagebyafeedbacksystem.Themotorspeedisvariedbychangingtheoscillatoroutputfrequency.#高功率振荡器输出由反馈系统控制频率和电压。通过改变振荡器输出频率来改变电机速度。Lesson27ACmotorsThemotorcurrentnecessarytoobtainthedesiredtorque(atsmallanglesofslip)isnormallyobtainedbymaintainingthevoltagealmostproportionaltofrequency.获得所需转矩（在小滑失角下）所需的电机电流通常是通过保持电压几乎与频率成比例来获得的。Lesson27ACmotorsCertainprotectivedevicesarefittedin

themotorcircuittoprotectagainstfaultssuchassinglephasing,overloadorundervoltage.#在电动机的电路里会有适当的保护设备，比如缺相保护、过载保护和欠压保护。Lesson27ACmotorsSinglephasingoccurswhenonephaseinathreephasecircuitbecomesopencircuited.#三相电路中有一相断开时，缺相发生。Lesson27ACmotorsTheresultisexcessivecurrentsinallthewindingswith,inthecaseofadeltaconnectedstatorrunningatfullload,onewindingtakingthreetimesitsnormalloadcurrent.#如果连接定子的三角形运行在全负荷，结果是所有绕组中电流都过大，其中一相绕组承担的电流大概是正常负荷电流的3倍，。Lesson27ACmotorsAmachinewhichisrunningwhensinglephasingoccurswillcontinuetorunbutwithanunbalanceddistributionofcurrent.#当运转中的机器发生缺相时，它会在一个不平衡的电流带动下继续运转。Lesson27ACmotorsAnoverloadprotectiondevicemaynottripifthemotorisrunningatlessthanfullload.#如果电动机负载低于全负载时，过载保护可能不脱扣。Lesson27ACmotorsOnemethodofsinglephasingprotectionutilizesatemperaturesensitivedevicewhichisolatesthemachinefromthesupplyatsomeparticularwindingtemperature.#还有一种断相保护方法是利用一个感温设备在线圈达到特定的温度时把机器和供电设备给隔离。Lesson27ACmotorsOverloadprotectiondevicesarealsofittedandmaybeseparateorcombinedwiththesinglephaseprotectiondevice.#过载保护也被配带，可能和缺相保护分开或结合起来。Lesson27ACmotorsTheymusthaveatimedelayfittedsothatoperationdoesnotoccurduringthehighstartingcurrentperiod.#它们必须要有一个时间延迟以保证在高启动电流期间不会动作。Lesson27ACmotorsAnundervoltageor“novolts”

protectivedeviceensuresthatthemotorisproperlystartedafterasupplyfailure.#欠压或是失压保护来保证电动机在供电设备故障后能正常启动。part3

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