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Chapter7DCMachines7/10/2024DCMachines1IntroductionTheDCmachinehasafieldonthestator,whichsetsupthemainair-gapflux,andanarmatureontherotor,whichhandlestheelectricalpowerThearmatureisconnectedtostationaryterminalsbybrushesridingontherotatingcommutatorThecommutator-brushcombinationactsasamechanicalconverterfromDCattheterminalstotime-varyingarmaturecurrentsthatsetupastationaryMMFThemagneticaxisofthearmatureis90electricaldegreesfromthemagneticaxisofthefield7/10/2024DCMachines2Schematicrepresentationofatwo-poledcmachinethedirectaxisisalignedwiththefieldaxisthearmatureaxisisperpendicular7/10/2024DCMachines3ElectromagnetictorqueTmechisduetointeractionofd-axisfluxperpoleFdandspacefundamentalarmaturemmfFa1

7/10/2024DCMachines4ThedevelopmentofthisequationusesseveralresultsfromChapter4,andevaluatedtheanglebetweenthedaxisandarmatureaxisas90electricaldegreesThetorqueisshowntobeproportionaltothefluxFdtimesthearmaturecurrentiaThegeneratedvoltageissumoftherectifiedvoltagesofthecoilsinseriesbetweenthebrushes7/10/2024DCMachines5Thisvoltageisalsocalledthespeedvoltage,andwithallquantitiesinSIunits7/10/2024DCMachines6Fourpossiblecircuitconnections:(a)separateexcitation,(b)series,(c)shunt,(d)compound.Ashuntfieldmustberatedforarmaturevoltageandaseriesfieldmustberatedforarmaturecurrent7/10/2024DCMachines7Generatorvolt-amperecharacteristics7/10/2024DCMachines8Motorspeed-torquecharacteristicsCommutatoraction7/10/2024DCMachines9Electric-circuitanalysis7/10/2024DCMachines10EffectofarmatureMMFArmatureMMFincreasesfluxunderhalfoffieldpolebutdecreasesitundertheotherhalfThishinderscommuatationanddecreasesnetflux(becauseofsaturationintheiron7/10/2024DCMachines11Steady-stategeneratorperformanceSelf-exictedshuntgeneratorwillhavevoltagebuild-upduetoresidualmagnetism,whichcausesasmallfieldcurrent,causingvoltagebuild-upThevoltagewillbuildupalongwiththefieldcurrentuntilsteady-stateoperatingpointisreachedattheintersectionofthemagnetizationcurveandthefieldresistanceline7/10/2024DCMachines127/10/2024DCMachines13FieldresistancelineV=RfIfMagnetizationcurveDCmotoranalysisExample7.9andpracticeproblem7.6illustratetheuseoftheequivalentcircuittoanalyzetheshuntmotorExample7.10andpracticeproblem7.7showtheanalysisofaDCpermanent-magnetmotorExample7.11isdiscussedonthenextfewslides,againforthepermanent-magnetmotor7/10/2024DCMachines14Example7.11GivenRa=1.03W,Vt=50V,Ia=1.25A,n=2100r/min.FindKm,theno-loadrotationallossandthepoweroutputwhenoperatingfroma48-Vsourceat1700r/min7/10/2024DCMachines15Equivalentcircuitforpermanent-magnetDCmotor.Fd=constant,soKm=Ka

Fd.7/10/2024DCMachines167/10/2024DCMachines17InterpoleandcompensatingwindingsInterpolewindings:woundonsmallpolesbetweenthemainfieldpolesusedforimprovingcommutationbynullingoutthearmaturefluxintheregionwherecoilsareundergoingcommutationCompensatingwindings:PlacedinthemainpolefacestocompensatefordemagetizingeffectofarmaturefluxThesewindingsareconnectedinserieswiththearmature7/10/2024DCMachines187/10/2024DCMachines19SchematicsectionofDCmachineshowinginterpolesandcompensatingwindingsSeriesuniversalmotorsAsmallseries-fieldcommutatormotorcanbedesignedwithlaminatedstatorandrotorirontobeusedineitherACorDCDCoperationisasanordinaryseriesmotor,andACoperationissimilarsincethetimevariationsinthearmaturecurrentcoincidewiththoseoftheflux(sincethearmatureandfieldareinseries)WithACoperation,thefieldandarmaturebothhavereactancevoltagedrops,affectingperformance7/10/2024DCMachines207/10/2024DCMachines21WithACoperation,saturationatthepeakswillreducethermsflux,furtheraffectingperformanceSummaryTheconventionalDCmachinewithacommutatorandbrushesisusefulasamotororageneratorTheprimarydisadvantagesarecomplexityofconstruct

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