专业外语教学资料专业英语第6单元docx.docx
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专业外语教学资料专业英语第6单元docx.docx
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专业外语教学资料专业英语第6单元docx
6DCMotorAndinductionMotor
6.1TYPESOFMOTORS
ThetypesofcommerciallyavailableDCmotorsbasicallyfallintofourcategories:
(Dpermanent-magnetDCmotor,②series-wound(串励)DCmotors,(§)shunt-wound(并励)DCmotors,and©compound-wound(复励)DCmotors.Eachofthesemotorshasdifferentcharacteristicsduetoitsbasiccircuitarrangementandphysicalproperties.
6.1.1Permanent-magnetDCmotors
Thepennanent-magnetDCmotor,showninFig.6.1,isconstructedinthesamemannerasitsDCgeneratorcounterpart.Thepermanent-magnetmotorisusedforlow-torqueapplications.Whenthistypeofmotorisused,theDCpowersupplyisconnecteddirectlytotheamiatureconductorsthroughthebrush/commutatorassembly.Themagneticfieldisproducedbypennanentmagnetsmountedonthestator.Therotorofpermanentmagnetisawoundarmature.
Thistypeofmotorordinarilyuseseitheralnico(乍吕镰钻合金)orceramic(陶瓷)permanentmagnetsratherthanfieldcoils.Thealnicomagnetsareusedwithhigh-horsepowerapplications.Ceramicmagnetsareusedforlow-horsepowershow-speedmotors.Ceramicmagnetsarehighlyresistanttodemagnetization,yettheyarerelativelylowinmagnetic-fluxlevel.Themagnetsareusuallymountedinthemotorfameandthenmagnetizedpriortotheinsertionofthearmature.
Thepermanent-magnetmotorhasseveraladvantagesoverconventionaltypeofDCmotors.Oneadvantageisreducedoperationalcost.Thespeedcharacteristicsofthepermanent-magnetmotoraresimilartothoseoftheshunt-woundDCmotor.Thedirectionofrotationsofapermanent-magnetmotorcanbereversedbyreversingthetwopowerlines.
6.1.2Series-woundDCmotors
ThemannerinwhichthearmatureandfieldcircuitsofaDCmotorsareconnecteddeterminesitsbasiccharacteristics.EachofthetypeofDCmotorsaresimilarinconstructiontothetypeofDCgeneratorthatcorrespondstoit.Theonlydifference,inmostcases,isthatthegeneratoractsasavoltagesourcewhilethemotorfunctionsasa
mechano-巴powerconversiondevice.
Theseries-woundmofor"showninFig.62has(hearmafureandfieldcircuitsconnes-edinaseriesarrangemenLThereisonlyonepathforcurrent:
foHowfrom(heDCvolfagesourceTherefore"thefieldiswoundofrelativelyfewfums(同)oflargediameferwire"givingfhefieldalowresistanceChangesinloadappliedfofhemoforshaftcauseschangesinfhecuirenfthroughfhefield.Iffhemechanic巴loadincreases"fhecurrent巴soincreases-TheincreasedcurrentcreafesastrongermagneticfiledThespeedofaseriesmoforvariesfromveryfasfafnoloadfoveryslowafheavyloads.Sincelargecurrentsmayflowthrough
fhelow’resisfancefiled"fheseriesmoforproducesahighdorqueoufpuhSeriesmotorsareusedwhereheavyloadsmust:
bemovedandspeedregulationisnofimportanhAfypic巴applicationisforautomobilesmarter
mofors-
(a)piaoa.aldiagram
sschematicdiagram
Fig・6・2Series-woundDCm2.or
6.1.3Shunt-woundDCmotors
Shunt-woundDCmotorsaremorecommonlyusedthananyothertypeofDCmotor.AsshowninFig.6.3,theshunt-woundDCmotorhasfieldcoilsconnectedinparallelwithitsarmature.ThistypeofDCmotorhasfieldcoilswhicharewoundofmanyturnsofsmall-diameterwireandhavearelativelyhighresistance.Sincethefieldisahigh-resistantparallelpartofthecircuitoftheshuntmotor,asmallamountofcurrentflowsthroughthefield.Astrongelectromagneticfieldisproducedduetothemanyturnsofwirethatfromthefieldwindings.
Alargemajority(about95%)ofthecurrentdrawn(汲取)bytheshuntmotorflowsinthearmaturecircuit.Sincethefieldcurrenthaslittleeffectonthestrengthofthefield,motorspeedisnotaffectedappreciably(略微)byvariationsinloadcurrent.TherelationshipofthecurrentthatflowthroughaDCshuntisasfollows:
Qla+If
Whereh—totalcurrentdrawnfromthepowersource;
Ia—armaturecurrent;
If—fieldcurrent.
Thefieldcurrentmaybevariedbyplacingavariableresistanceinserieswiththefieldwindings.Sincethecurrentinthefieldcircuitislow,alow-wattagerheostat(变阻器)maybeusedtovarythespeedofthemotorduetothevariationinfieldresistance(励磁回路阻抗).Asfield
resistanceincreases,fieldcurrentwilldecrease.Adecreaseinfieldcurrentreducesthestrengthoftheelectromagneticfield.Whenthefieldfluxisdecreased,thearmaturewillrotatefaster,duetoreducedmagnetic-fieldinteraction.ThusthespeedofaDCshuntmotormaybeeasilyvariedbyusingafieldrheostat.
Theshunt-woundDCmotorhasverygoodspeedregulation.Thespeeddoesdecreaseslightlywhentheloadincreasesduetotheincreaseinvoltagedropacrossthearmature.Duetoitsgoodspeedregulationcharacteristicanditseaseofspeedcontrol,theDCshuntmotoriscommonlyusedforindustrialapplications.Manytypesofvariable-speed
machinetoolsaredrivenbyDCshuntmotors.
(a)Pictorialdiagram(b)Schematicdiagram
Fig.6.3Shunt-woundDCmotor
6.1.4Compound-woundDCmotors
Thecompound-woundDCmotorsshowninFig.6.4,hastwosetsoffieldwindings,oneinseriesthearmatureandoneinparallel.Thismotorcombinesthedesirablecharacteristicsoftheseries-andshunt-woundmotors.Therearetwomethodsofconnectingcompoundmotors:
cumulativeanddifferential.Acumulativecompound(积复励)DCmotorhasseriesandshuntfieldsthataideachother.Differentialcompound(差复励)DCmotorshaveseriesandshuntfieldsthatopposeeachother.Therearealsotwowaysinwhichtheserieswindingsareplacedinthecircuit.Onemethodiscalledashortshunt(短复励)(seeFig.6.4),inwhichtheshuntfieldisplacedacross(跨接)thearmature.Thelong-shunt(长复励)methodhastheshuntfieldwindingplacedacrossboththearmatureandtheseriesfield(seeFig.6.4).
Compoundmotorshavehightorquesimilartoaseries-wouldmotor,togetherwithgoodspeedregulationsimilartoashuntmotor.Therefore,whengoodtorqueandgoodspeedregulationareneeded,thecompound-wounddemotorcanbeused.Amajordisadvantageofacompound-woundmotorisitsexpense.
Hich-resistance^himrn^i/4
(a)Pictorialdiagram
LongshuntShortshunt
(b)Schematicdiagram
Fig.6.4Compound-woundDCmotor
6.2DCMOTORANALYSIS
ADCmotorisaDCgeneratorwiththepowerflowreversed.IntheDCmotorelectricalenergyisconvertedtomechanicalfrom.Onethebasisofforegoingdiscussion,therearethreetypesofDCmotors:
theshuntmotor,thecumulativelycompoundedmotor,andtheseriesmotor.Thecompoundmotorisprefixedwiththewordcumulativeinordertostressthattheconnectionstotheseriesfieldwindingaresuchastoensurethattheseriesfieldfluxaidstheshunt-fieldflux.Theseriesmotor,unliketheseriesgenerator,findswideapplication,especiallyfortraction-typeloads.Hencedueattentionisgiventothismachineinthetreatmentthatfollows.
TheperformanceoftheDCmotoroperatinginanyoneofitsthreemodescanconveniently(常规)bedescribedintermsofanequivalentcircuit(等效电路),asetofperformanceequations(特性方程),a
power-flowdiagram(功率流程图).Andthemagnetizationcurve.The
equivalentcircuitisdepictedinFig.6.5.Itisworthwhiletonotethatnowthearmatureinducedvoltageistreatedasareaction(反作用)orcounteremf(反电动势).Byimposingsomeconstraints(约束),weobtainthe
correctequivalentcircuitforthedesiredmodeofoperation.Forexample,foraseriesmotortheappropriate
equivalent
circuitFig.6.5EquivalentcircuitoftheDCmotor
resultsuponremovingRffromthecircuitryofFig.6.5.
Thesetofequationsneededtocomputetheperformanceislistedbelow.
Ea=Ke①n
(6-1)
T=K/Da
(6-2)
Ut=Ea+Ia(Ra+Rs)
(6-3)
I厶Mia
(6-4)
Notethatthelasttwoequationsaremodifiedtoaccountfor(说明,解释)thefactthatforthemotorUtistheappliedorsourcevoltageandassuchmustbeequaltothesumofthevoltagedrops.Similarlythelinecurrentisequaltothesumratherthanthedifferenceofthearmaturecurrentandfieldcurrents.
Thepower-flowdiagramisillustratedinFig.6.6.TheelectricalpowerinputUtI£originatingfromthelinesuppliesthefieldpowerneededtoestablishthefluxfiledaswellasthearmature-circuitcopperlossneededtomaintaintheflowofla.Thiscurrentflowingthroughthearmatureconductorsimbedded(嵌入)inthefluxfieldcausestorquetobedeveloped.Thelawofconservation(守恒定律)ofT(Dm,wherecomisthesteady-stateoperatingspeed.Removaloftherotationallosses(损耗)fromthedevelopedmechanicalpoweryieldsthemechanicaloutputpower.
TheDCmotorisoftencalledupon(被迫)todothereallytoughjobsinindustrybecauseofitshighdegreeofflexibilityandeaseofcontrol.Thesefeaturescannoteasilybematchedbyotherelectromechanicalenergy-conversiondevices.TheDCmotoroffersawiderangeofcontrolofspeedandtorqueaswellasexcellentacceleration(力口速度)anddeceleration(减速).Forexample,bytheinsertionofanappropriatearmature-circuitresistance,ratedtorquecanbeobtainedatstartingwithnomorethanrattedcurrentflowing.Also,byspecialdesignoftheshunt-fieldwinding,speedadjustmentsoverarangeof4:
1arereadilyobtainable.Ifthisisthencombinedwitharmature-voltagecontrol,therangeofspeedadjustmentspeedsto6:
1.InsomeelectroniccontroldevicesthatareusedtoprovidetheDCenergytothefieldandarmaturecircuits,aspeedrangeof40:
lispossible.Thesizeofthemotorbeingcontrolled,however,islimited.
fig.6.6Power-flowdiagramoftheDCmotor
6.3DCMOTORSPEED-TROOUECHARACTERISTICS
HowdosetheDCmotorreacttotheapplicationofashaftload(轴荷)?
WhatisthemechanismbywhichtheDCmotoradaptsitselftosupplytotheloadthepoweritdemands?
Theanswerstothesequ
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