Generators.docx
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Generators.docx
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Generators
Page1GeneratorsandMotors
Fromreference1
1.Direct-currentgeneratorsimpressonthelineadirectorcontinuousemf,onethatisalwaysinthesamedirection.Commercialdcgeneratorshavecommutators,whichdistinguishthemfromacgenerators.ThefunctionofacommutatorandtheelementaryideasofgenerationofemfandcommutationarediscussedinDiv.1.Additionalinformationaboutcommutationasappliedtodcmotors,whichingeneralistruefordcgenerators,isgivenbelow.
2.Excitationofgeneratorfields.Togenerateanemf,conductorsmustcutamagneticfieldwhichincommercialmachinesmustberelativelystrong.Apermanentmagnetcanbeusedforproducingsuchafieldinageneratorofsmalloutput,suchasatelephonemagnetoorthemagnetoofaninsulationtester,butingeneratorsforlightandpowerthefieldisproducedbyelectromagnets,whichmaybeexcitedbythemachineitselforbeseparatelyexcitedfromanothersource.Self-excitedmachinesmaybeoftheseries,shunt,orcompoundtype,dependinguponthemannerofconnectingthefieldwindingtothearmature.Intheseriestypeofmachine,thefieldwinding(thewindingwhichproducesthemagneticfield)isconnectedinserieswiththearmaturewinding.Intheshunttype,thefieldwindingisconnectedinparallel,shunt,withthearmaturewinding.Compoundmachineshavetwofieldwindingsoneachpole.Oneofthesewindingsisconnectedinserieswiththearmaturewinding,andtheotherisconnectedinparallelorshuntwiththearmaturewinding.
3.Armaturewindingofdcmachinesmaybeofthelaporthewavetype.Thedifferenceinthetwotypesisinthemannerofconnectingthearmaturecoilstothecommutator.Acoilistheportionofthearmaturewindingbetweensuccessiveconnectionstothecommutator.Inthelaptypeofwinding(seeFig.7.1)thetwoendsofacoilareconnectedtoadjacentcommutatorsegments.Inthewavetypeofwinding(seeFig.7.2)thetwoendsofacoilareconnectedtocommutatorsegmentsthataredisplacedfromeachotherbyapproximately360electricaldegrees.Thetypeofarmaturewindingemployedaffectsthevoltageandcurrentcapacityofthemachinebuthasnoeffectuponthepowercapacity.Thisisduetothefactthatthenumberofparallelpathsbetweenarmatureterminalsisaffectedbythetypeofwinding.Forawavewoundmachinetherearealwaystwopathsinparallelinthearmaturewindingbetweenarmatureterminals.Foralap-woundmachinethereareasmanyparallelpathsinthearmaturewindingastherearepairsofpolesonthemachine.Forthesamenumberandsizeofarmatureconductors,amachinewhenwave-connectedwouldgenerateavoltagethatwouldequalthevoltagegeneratedwhenlap-connectedtimesthenumberofpairsofpoles.Butthecurrentcapacitywouldbedecreasedinthesameproportionthatthevoltagewasincreased.Thecurrentcapacityofamachinewhenwave-connectedisthereforeequaltothecapacitywhenlap-connecteddividedbythenumberofpairsofpoles.
4.Thevalueofthevoltagegeneratedbyadcmachinedependsuponthearmaturewinding,thespeed,andthefieldcurrent.Foragivenmachine,therefore,thevoltagegeneratedcanbecontrolledbyadjustingeitherthespeedorthefieldcurrent.Sincegeneratorsareusuallyoperatedataconstantspeed,thevoltagemustbecontrolledbyadjustingthefieldcurrent.
5.Separatelyexciteddcgeneratorsareusedforelectroplatingandforotherelectrolyticworkforwhichthepolarityofamachinemustnotbereversed.Self-excitedmachinesmaychangetheirpolarities.TheessentialdiagramsareshowninFig.7.3.Thefieldscanbeexcitedfromanydcconstant-potentialsource,suchasastoragebattery,orfromarectifierconnectedtoanacsupply.Thefieldmagnetscanbewoundforanyvoltagebecausetheyhavenoelectricconnectionwiththearmature.Withaconstantfieldexcitation,thevoltagewilldropslightlyfromnoloadtofullloadbecauseofarmaturedropandarmaturereaction.Separateexcitationisadvantageouswhenthevoltagegeneratedbythemachineisnotsuitableforfieldexcitation.Thisistrueforespeciallylow-orhigh-voltagemachines.
6.Series-woundgeneratorshavetheirarmaturewinding,fieldcoils,andexternalcircuitconnectedinserieswitheachothersothatthesamecurrentflowsthroughallpartsofthecircuit(seeFig.7.4).Ifaseriesgeneratorisoperatedatnoload(externalcircuitopen),therewillbenocurrentthroughthefieldcoils,andtheonlymagneticfluxpresent
inthemachinewillbethatduetotheresidualmagnetismwhichhasbeenretainedbythepolesfrompreviousoperation.Therefore,theno-loadvoltageofaseriesgeneratorwillbeonlyafewvoltsproducedbycuttingtheresidualflux.Iftheexternalcircuitisclosedandthecurrentincreased,thevoltagewillincreasewiththeincreaseincurrentuntilthemagneticcircuitbecomessaturated.Withanyfurtherincreasesofloadthevoltagewilldecrease.Seriesgeneratorshavebeenusedsometimesinstreet-railwayservice.Theyhavebeenconnectedinserieswithlongtrolleyfeederssupplyingsectionsofthesystemdistantfromthesupplypointinordertoboostthevoltage.However,powerrectifiershavereplaceddcgeneratorsformostinstallationsofthistype.
Keywords:
generator
Fromreference2
Sincetriphasedasynchronousgeneratorsaremainlyusedinconversionsystemsofaeolianenergyintoelectricenergy,theirfunctionalstabilityrepresentis
ofgreatimportance.Asafirststep,thefactorsthatradicallyaffectthefunctionalstabilityofthesegeneratorshavebeenestablished.Thus,itwasdecelatthepowerfulinfluenceofthecapacitorbank–thatprovidesthenecessaryreactivepowerforthe
magnetizationoftheferromagneticcore–overthefunctionalstabilityofthetriphasedasynchronousgeneratorwithshortcircuitrotor.Thefunctionalstabilityisgreatlyinfluencedbythechargecharacter(type)aswell.Theexperimentalworkemphasized–throughthefunctionalfeatures–thewaytheseparametersinfluencethestabilityareaoftheasynchronousgenerators.Asfarastriphasedasynchronousgeneratorswithcoiledrotorareconcerned,thecontrollableblindpowerwasanalyzedtheanalogybeingmadewiththesituationofthenecessarycontrollablegeneratingcapacityforofthetriphasedasynchronousgeneratorwithshortcircuitrotor.
Keywords:
triphasedasynchronousgenerator.
[1]D.M.Eggleston,F.S.Stoddard–Windturbineengineeringdesign,VanNostrandReinholdCompanyNewYork1986;
[2]V.Ilie,L.Almaşi,şa–Utilizareaenergieivântului,Ed.Tehnică,Bucureşti,1984;
[3]KovacsPal–Analizaregimurilortranzitoriialemaşinilorelectrice,Ed.Tehnică,Bucureşti1980;
[4]R.J.Harrington,F.M.M.Bassiouny–NewApproachtoDeterminatetheCriticalCapacitanceforSelf-ExcitedInductionGenerators,IEEETrans.OnEnergyConversion,vol.13,no.3,sept.1998,pp.244-250;
[5]Colliez,C.,Tounzi,A.,Piriou,F.–VectorControlofaAutonomousInductionGeneratorconnectedtoaPWMRectifier.EPE`97,Trondheim,Norvegia,vol.2,pp.711-716;
[6]Alan,I.,Lipo,A.T.–ControlofaPolyphaseInduction-Generator/Induction-MotorPowerConversionSystemCompletelyIsolatedfromtheUtility.IEEETrans.OnInd.App.,vol.30,no.3,may/june1994,pp.636-647
[7]FlorinIov–Stadiulactualînconversiaenergieieoliene(Referatnr.1–încadrulpregătiriitezeidedoctorat)martie1998;
[8]FlorinIov–Studiulansambluluiturbinăeoliană–generatorasincronautoexcitat(Referatnr.2–încadrulpregătiriitezeidedoctorat)iunie1999;
Page2ElectricalEnergyTransmission
Fromreference1
Growingpopulationsandindustrializingcountriescreatehugeneedsforelectricalenergy.Unfortunately,electricityisnotalwaysusedinthesameplacethatitisproduced,meaninglong-distancetransmissionlinesanddistributionsystemsarenecessary.Buttransmittingelectricityoverdistanceandvianetworksinvolvesenergyloss.
So,withgrowingdemandcomestheneedtominimizethislosstoachievetwomaingoals:
reduceresourceconsumptionwhiledeliveringmorepowertousers.Reducingconsumptioncanbedoneinatleasttwoways:
deliverelectricalenergymoreefficientlyandchangeconsumerhabits.
Transmissionanddistributionofelectricalenergyrequirecablesandpowertransformers,whichcreatethreetypesofenergyloss:
theJouleeffect,whereenergyislostasheatintheconductor(acopperwire,forexample);
magneticlosses,whereenergydissipatesintoamagneticfield;
thedielectriceffect,whereenergyisabsorbedintheinsulatingmaterial.
TheJouleeffectintransmissioncablesaccountsforlossesofabout2.5%whilethelossesintransformersrangebetween1%and2%(dependingonthetypeandratingsofthetransformer).So,savingjust1%ontheelectricalenergyproducedbyapowerplantof1000megawattsmeanstransmitting10MWmoretoconsumers,whichisfarfromnegligible:
withthesameenergywecansupply1000-2000morehomes.
Changingconsumerhabitsinvolvesawareness-raisingprogrammers,oftenundertakenbygovernmentsoractivistgroups.Simplethings,suchasturningofflightsinunoccupiedrooms,orswitchingoffthetelevisionatnight(notjustputtingitintostandbymode),orsettingtaskssuchaslaundryfornon-peakhoursarebutafewexamplesamongthemyriadofpossibilities.
Ontheenergyproductionside,buildingmoreefficienttransmissionanddistributionsystemsisanotherwaytogoaboutit.Highefficiencytransformers,superconductingtransformersandhightemperaturesuperconductorsarenewtechnologieswhichpromisemuchintermsofelectricalenergyefficiencyandatthesametime,newtechniquesarebeingstudied.Theseincludedirectcurrentandultrahighvo
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