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ElectricalGenerationGeneratorsTheAlternatorThesynchronousmachineconsistsessentiallyof(a)afieldsystemexcitedbydirectcurrentand(b)anarmature.Almostinvariablythearmatureisthestationarymemberandthefieldsystemtherotatingmember.Theinducede.m.f.inthearmaturewindingisamotionallyinducede.m.f.anditsmodeofproductionidenticalwiththatoftheD.C.machine.Theonlydifferenceisthatitisthemagneticfieldwhichmoveswhereasthearmatureconductorisstationary.

AswithaD.C.machinethee.m.f.inducedinanindividualarmaturecoiliaanalternatinge.m.f.andconsequentlybybringingthewindingouttofixedterminals,thee.m.f.betweenthesewillbealternatingalso.Thecompletefixedarmature,thatismagneticcoreandwindings,iscalledthestator,andtherotatingfieldsystemtherotor.Thegeneralconstructionalfeaturesofasalientpolealternatorareshownbelow

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Asthefieldsystemrotatesandcarriesitsfluxwithit,eachportionofthestatorcorewillexperiencereversalsofmagnetisation,andtherefore,asinadirectcurrentmachine,thecorehastobelaminated.Forventilationpurposes,aseriesofradialventilatingductsareprovided.Sincethefieldsystemrotates,itsexcitingwindinghastobefedbymeansoftwosliprings,butastheexcitationvoltageislowandthepowertakenbythefieldwindingsmall,thesepresentnodifficulties

Asalientpolehasonefieldcoilperpole,verylikeaD.C.machine.Fortheveryhighspeedsofturbinedrivenalternatorsitisnecessarytoadoptacylindricalconstructionfortherotorandinsuchacasethefieldwindinghastobehousedinanumberofslots.Asimplifiedformshowsthecrosssectionofafourpoleturboalternator,thedispostionoftherotorfieldofaturboalternatorrotormaybeashighas40,000ftperminor200metrespersec.Thestressesduetocentrifugalforceareexceedinglyhigh.Therotorsarethusmadefromsteelforging,orinsomecasesfromthicksteeldiscsboltedtogether.

Highspeedrotor

Theaxiallengthisnormallyconsiderablygreaterthanthediameter.Hastheadvantageofgreatstrengthandstiffness.Theexcitingcurrentiscarriedbybartypeconductorsinthegroupsofslotsshownbelow.Allcurrentsinonegroupareinthesamedirection,thoseonthenextgroupontheoppositedirection.Fluxproducedisdistributedoversurfaceapproximatelyaccordingtosinelaw.

Detailsofstationaryarmaturealternators

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Armaturestampingspressedoutofsheetsofspecialmagneticironorsteelalloy.Inthesmallersizesthestampingsarepressedoutincompeterings

Sectionthroughtopstatorofsalientpolemachine.Thearmaturecoreisbuiltupoflaminationswhichareheldtightlytogetherbyendclampingrings.Spacingstripsinsertedatintervalsleaveductsforcoolingairtopassthrough.Theairisdriventhroughbythefanactionoftherotorandescapesviatheaperturesinthecastironsupportingframe

Typesofarmatureslot.Thefilledslothasroundwiresbutitiscommontohaverectangularconductorstoeconomiseslotspace.

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SectionalsimplifieddiagramofaturboalternatorTherotoristurnedfromasteelforgingansslottedtocarrytheexcitingwindingstheslotsbeingarrangedasshownabove.Becauseofthehighrunningspeed,alternatorsforlargeoutputshaveaconsiderableaxiallengthcomparedwithrotordiameter.

LayoutsofA.C.generators

Conventionalexcitationscheme(Rotary)SeparatelyexcitedD.C.exciter(Outdated)

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Brushlessexcitationschemeusingshaftmounteddiodes(Rotary)Indirectselfexcitation(Error)

Comparisonofthevaluerequiredtocontrolwithafixedvalue.Whenthevariablediffersfromafixedreferencevaluean'error'existsandthefunctionofthecontrollingmediumistorestoreequilibriume.g.ifthevoltageoutputfallsonthebrushlessrotaryexcitedalternatorthea.v.r.controlstheexciterfieldtorestoreequilibrium.Moderncompoundscheme(static)Directselfexcited(Functional)

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Controlofthevoltagetoasetvalueisachievedbytheinherentcharacteristicsofthemachine.

Acompoundwoundd.c.generatorwithalevelcompoundcharacteristichasadditionalcurrentintheseriesfieldunderloadconditions.Intheselfexcitedcompoundalternatorthereisaconstantamountofexcitationrequiredfornoloadcondition.Additionalexcitationduetomorecurrentformthecurrenttransformersisobtainedinresponsetoextraexternaldemands

Recoverygraphsforfunctionaland'error'layouts

Shaftdrivengeneratingsystem

Methodsofdrive1. Beltorchaindriven2. Directcouplingengagingthepropellershaft3. Powertakenfromthemaingearbox.4. Powertakenfromthefreeendoftheengine

WithD.C.auxiliariespowercanbetakenbyeitherachainorbeltdrivefromthepropulsionsystemwithanA.V.R.maintainingconstantvoltage.

ForA.C.systemsmethodsusedincludetheuseofaD.C.generatorwithanD.C./A.C.converter,ordirectA.C.generation.Withthelattereitheraconstantspeeddriveisrequiredorafrequencyconverter.Witheithermethodtherevolutionsatwhichtheshaftalternatorcanbeusedislimited.InthiswaydirectdrivesystemswillgenerallybefittedinconjunctionwithaC.P.systemwhichmaintainsconstantenginespeedunderfullawayconditions.

AdvantagesSavingonfuelcosts,allowsefficientuseofheavyratherthangasoilreducedmaintenancecostsCapitalsavingonreducednumberofauxiliarysetsReducedspaceandweightReductioninnoise

DisadvantagesPoweravailableforpropulsionreducedCapitolcostofplantAuxiliariesrequiredformanoeuvring,althoughsomemediumspeedplantsarecapableofmanoeuvringwith

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shaftalternatorsandC.P.systemComplicatedconstantspeedorfrequencygearrequiredwithslowspeedengines

AirGapIftheairgaparoundarotorisnotuniformthemotormaynotstartincertainpositions.Becausetherotorisnotcentred,probablyduetowornbearings,thereisanoutofbalancemagneticpull.Radialplayinbetweentheshaftandthehousingshouldbedetectedbyhandandbearingweardetectedbyfeelergaugebetweentherotorandthestator,orarmatureandfieldpolesmaybemeasuredatthreeorfourfairlyequidistantpointsaroundthemachine.Ifpossibleonemeasurementshouldbemadeatthebottomofthemachineandanotherinlinewiththedrive.Comparewithpreviousrecordstocheckwear.Atminimumairgap.Clearanceofthebearingsshouldberenewedtoavoidthepossibilityoftherotorrubbingonthestator.Onsmallmachinestwofeelersonoppositesidesoftherotorshouldbeusedtoavoiderrorcausedbyrotormovementfromnormalpositionwhenonlyonefeelergaugeisused.InsynchronousmotorsandD.C.motorssparkingmayoccuriftheradialairgapsbetweenthearmatureandthefieldpolesareunequal.Ifnecessaryrenewbearingsoraddorremovesoftironshimsfromunderthepoleshoes.Unequalfieldstrengthhasasimilareffectofsparkingatthebrushes.Thismightbeduetoshortcircuitorearthfaultonthefieldcoils,orashortcircuitontheshuntandfieldcoils.Anincreaseofairgapgivesanincreasein'reluctance'.InasalientpoleA.C.generatorthisfactmaybeusedtoproduceasinusoidalfluxdensitycurvebygraduallyincreasingthelengthiftheairgaptowardsthepoletips.

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Intheinductionmotortheairgapshouldbeassmallaspossibleifthemotoristoactwithahighpowerfactor.Anincreaseinairgapincreasesthereactanceofthemotorandlowersitspowerfactor.Smallmotorsareaccuratelymachinedandcentringoftherotorisveryimportantsoballorrollerbearingsarefitted.

AirgapMotorsize0.25mm1kW0.75mm10kW2.0mm100kW

ParalleloperationofgeneratorsD.C.generatorsForcompoundwoundD.C.generatorsitisusuallysufficienttoensurethatthevoltagesoftheincominggeneratoristhesameasthebusbarvoltage.Theequalisingconnectionjoiningthejunctionsbetweenthearmaturesandtheirseriesfieldsisincorporatedinthecircuitbreakerinsuchawaythattheequalisingconnectionisautomaticallyclosedbeforeandopensafter,themaincontacts.ByadjustmentoftheshuntfieldregulatortheloadsharingmaybecontrolledA.C.alternators

ToparallelalternatorsthefollowingconditionsarerequiredSamevoltagecheckedwiththevoltmeterSamefrequencycheckedwiththefrequencymeterandsynchroscopeSamephaseanglecheckedwithsynchroscopeSamephaserotationcheckedwithrotationmeter.Onlyimportantwhen

connectingshoresupply,oraftermaintenanceonswitchgearoralternator.

LoadSharingOfAlternatorsInParallelAlternatorsinparallelmustalwaysrunatthesamespeed.Afteramachinehasbeenparalleledandisrequiredtotakeupitsshareoftheload,thiswillnotbeachievedbyadjustingthefieldexcitationcurrent.Althoughtheincreaseine.m.f.willcauseacurrenttoflowinthebusbars,andthiswillshowonthemachinesammeters,thisisareactivecurrentthatlagsthee.m.f.by90oandproducesareactive(kVAr)butnotkW.Itsonlyeffectistoaltertheoperatingpowerfactorofthealternator.Morepowermaybeobtainedatthebusbarsfromtheincomingalternatoronlybysupplyingmorepowertoitsprimemover.Thisincreaseofsteamorfuelsupplyisachievedbyalteringthegovernorsettingeitherelectricallyormanually.AfteradjustingthegovernortheincomingmachinetakesupitsdesiredamountofthekWloadingandthisisrecordedonthemachineswattmeter.However,ifthekWloadingissharedequallybetweentwomachinesitmaybefoundthattheLoadCurrentoftheincomingmachineismoreorlessthantheothermachine.Thisisfuetotheincomingmachinehavingadifferentpowerfactor.Thismaybecorrectedbyadjustingtheexcitationoftheincomingalternator.

ThusafterparallelinganalternatorAdjustprimemovergovernoruntilkWloadingiscorrectAdjustfieldexcitationcurrentuntilcurrentsharingiscorrect.

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Ifthealternatorshavesimilarloadcharacteristics,onceadjusted,theloadwillcontinuetobeshared.Iftheloadcharacteristicsofalternatorsvary,thekWloadingandloadcurrentsharingmayrequirereadjustingunderdifferentloadconditions.LoadsharingofalternatorsNo1onload

No1onload,No2synchronisedandtaking100kW

No1andNo2sharingloadafteradjustinggovernorsettings,excitationadjustedtopreventexcessivevoltdropinNo2

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No1andNo2sharingloadwithbalancedpowerfactorsbyadjustingexcitation

TheeffectsofalteringTorqueandExcitationonsinglephasealternatorplantandbyextrapolationa3phasecircuit

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Beforeparalleling,byvaryingRb,adjusttheexcitationcurrentintherotorfieldof'B'untilVa=Vb.Wheninphaseandatthesamefrequencysynchronisingmaytakeplace.IftherewasnoexternalloadonthebusbarsthetorqueontheprimemoversofAandBisonlythatrequiredbyitsownalternatorandRaandRbareadjustedsothatEaandEbareequal.

RelativetothebusbarsEaandEbareactinginthesamedirectionwitheachothermakingthetopbarpositivewithrespecttothebottombar.Varyingthedrivingtorque

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Ifthedrivingtorqueof'B'isreduced(lessfuelsupplied)therotorfallsbackbyananglesayp.f.(b)givingaresultante.m.f.ofEzintheclosedcircuit.Thee.m.f.EzcirculatesacurrentIwhichlagsbehindEzbyanglep.f.(a).ThiscirculatingcurrentIismoreorlessinphasewithEaandinoppositiontoEb.ThismeansthatAisgeneratingpowertomotorBandthiswillcompensateforanylossofpowerintheprimemoverofB.OncethepowerincreaseinAequalsthepowerlossofBbalanceisrestoredandAandBcontinuetoruninsynchronism.Thereforethepowerissharedbyadjustingthetorque(fuelinput.)SlightlossofpowerinBistakenupbyanincreaseinpowerfromA.Theterminalvoltagewillnotvaryandthespeedandfrequencywillstaythesameordroponlyveryslightly.

LargelossofpowerinBwithalargecirculatingcurrentfromAtoBthealternatorAwilltrytodriveBasasynchronousmotor.Theamountoffullloadpowerrequiredtodriveanalternatorasamotorisonly2to3%foraturbineand10to12%fordieselengine.AsthecirculatingcurrentflowsfromAtoBthereversepowertriponBwilloperateafterabout3to5seconds.AlltheloadnowfallsonAwhichwillprobablycausetheoverloadtriptooperateand'blackout'.Varyingexcitation

ConsiderAandBareexertingthetorquerequiredbyitsalternatorandthegeneratede.m.f.EaandEbareequal.Thereisnocirculatingcurrent.ByreducingRbtheexcitationcurrentinthefieldofBcanbeincreasedandEbwillincrease.Ezistheresultantdifference(EbEa)whichwillgiveacirculatingcurrentIthroughthesynchronousimpedancesofthetwoalternators.Asthemachinesaresimilartheimpedancedropineachwillbe1/2EzsotheterminalvoltageV1=EbEz=Ea+Ez

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ThereforeincreasingtheexcitationcurrentwillincreasetheterminalvoltageAsp.f.(a)isalmost90othePowercirculatingfromBtoAisverysmallEzICos[p.f.(a)]approxequalsZero(Cos90o=Zero)EffectofreducingExcitationByincreasingRbthereductionofthefieldexcitationcurrentofBwillreducetheterminalvoltageEa>EbterminalVoltageV=EaEz=Eb+EzThecirculatingcurrentIfromAtoBwillhavealarge'Wattless'component.MachineAnowhasmoreofthelaggingreactivecurrentanditspowerfactorisreduced.ToolargeareductioninexcitationcurrentinBwithsubsequentincreaseinloadcurrentinAcouldcausethecurrentoverloadtripofAtooperate.ThiscouldbefollowedbythelowvoltageortheoverloadtripofBoperatingcausingablackout.Voltageregulation

Thegraphdemonstratesthatexcitationmustbeincreased(generally)withincreasingloadtomaintainterminalvoltage

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Theworsethepowerfactortheworsetheterminalvoltagechangeduringloadchange.Voltageregulation=DVwhenloadremoved/FullloadterminalvoltageAt1.0p.f.=AC/OAAt0.8p.f=AD/OAThereforelowerp.f.=greatervoltageregulation

LimitingvoltagedipandresponsetimeunderimpactloadingTheeffectofalargeloadsuddenlyswitchedontoasmallpowerinstallationsuchasashipsplantwillbeaninstantaneousdipinthegeneratorvoltage.Thiseffect,duetothetransientreactanceonstarting,cannotbeobviatedeitherinaselfregulatedmachine,orinaconventionalgeneratorwithA.V.R.Thesluggishresponseoftheexcitationsystemslimitsthespeedofvoltagerecovery.Inaselfexcitedgeneratorthedipislessandtherecoverytimegreatlyimproved.(say0.3sagainst0.7s)Inordertomaintainconstantvoltage,undervaryingconditions,excitationmustbevaried.Variationofvoltageatconstantexcitation

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Variationofexcitationatconstantvoltage