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Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-0PowerPointSlidestoaccompanyElectricMachineryFourthEditionStephenJ.ChapmanChapter7DCMachinesCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-1Introduction•PrincipleofDCmachinesADCmotorisadeviceforconvertingDCelectricalenergyintorotary(orlinear)mechanicalenergy.Thisprocesscanbereversed,asinaDCgenerator,toconvertmechanicaltoelectricalpower.TheworkingprincipleoftheDC(andACaswell)generatorisFaraday’sLaw,whichstatesthatemfandelectriccurrentifthecircuitisclosed,isproducedwhenaconductorcutsthroughmagneticforcelines.TheoppositeofthelawappliesfortheDC(andAC)motor.Motionisproducedwhenacurrentcarryingwireisputinamagneticfield.Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-2•InDCmachinesthecurrentineachwireofthearmatureisactuallyalternating,andhenceadeviceisrequiredtoconvertthealternatingcurrentgeneratedintheDCgeneratorbyelectromagneticinductionintodirectcurrent,oratthearmatureofaDCmotortoconverttheinputdirectcurrentintoalternatingcurrentatappropriatetimes,asillustratedinFig.1.•Fig.1(a):DCgenerator:inducedACemfisconvertedtoDCvoltage;(b)DCmotor:inputdirectcurrentisconvertedtoalternatingcurrentinthearmatureatappropriatetimestoproduceaunidirectionaltorque.Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-3Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-4Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-5OperationofCommutatorandBrushes•Whentheturnpassestheinterpolarregion•theturnisshort-circuited•thecurrentintheturnwillreverseCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-6DCMachineConstructionCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-7MajorComponents•Thestatorofthedcmachinehaspoles,whichareexcitedbydccurrenttoproducemagneticfields.•Intheneutralzone,inthemiddlebetweenthepoles,commutatingpolesareplacedtoreducesparkingofthecommutator.Thecommutatingpolesaresuppliedbydccurrent.•Compensatingwindingsaremountedonthemainpoles.Theseshort-circuitedwindingsdamprotoroscillations.Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-8•Thepolesaremountedonanironcorethatprovidesaclosedmagneticcircuit.•Themotorhousingsupportstheironcore,thebrushesandthebearings.•Therotorhasaring-shapedlaminatedironcorewithslots.•Coilswithseveralturnsareplacedintheslots.Thedistancebetweenthetwolegsofthecoilisabout180electricdegrees.•Thecoilsareconnectedinseriesthroughthecommutatorsegments.Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-9•Theendsofeachcoilareconnectedtoacommutatorsegment.•Thecommutatorconsistsofinsulatedcoppersegmentsmountedonaninsulatedtube.•Twobrushesarepressedtothecommutatortopermitcurrentflow.•Thebrushesareplacedintheneutralzone,wherethemagneticfieldisclosetozero,toreducearcing.Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-10CommutatorCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-11Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-12DCmotorstatorwithpolesvisible(4polemachine)Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-13RotorofaDCmotorCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-14CutawayviewofaDCmotorCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-15TheMagneticFieldofDCMachineClassificationofDCMachines•fieldandarmaturecircuitscanbeconnectedinvariousways•widevarietyofperformancecharacteristic•fieldpolecanbeexitedbyseriesandshuntwindings•serieswindinglargecurrent,shuntwindingsmallcurrent•ifbothareusedserieswindingontopofshuntwinding•alsopermanentmagnetcanbeused(equivalenttoa)Copyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-16•a-separatelyexcitedmachine•b-seriesmachine•c-shuntmachineCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-17•d-compoundmachineCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-18DCMachinesMagnetThickness•Themaximumdemagnetisingampsproducedbythearmatureisbasicallytheampsembracedbytheoutermostarmaturereactionfluxpath.Outermostarmaturereactionfluxpathnormallyembracesabout70to80%oftotalarmatureconductorsCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-19ArmatureReaction•no-load•armaturefield•fluxdensityincreasesinonehalfofthepoleanddecreasesintheotherhalfofthepole•saturation-reductionoffluxperpoleCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-20•saturationeffect-fluxperpoledecreases•thezerofluxdensityregionmovesfromtheq-axisCopyright©TheMcGraw-HillCompanies,Inc.Permissionrequiredforreproductionordisplay.7-21ArmatureReaction-CompensatingWinding•Arma