原文說(shuō)明原文說(shuō)明的內(nèi)容是：文章闡述了電機(jī)的工作原理、發(fā)展過(guò)程、以及伺服電機(jī)的工作控制原理。并且舉例說(shuō)明了伺服電機(jī)所適用的場(chǎng)合。題名ServomotorsElementsandApplications作者NEWMARKER來(lái)源佳工機(jī)電網(wǎng)HowDoesaMotorWork?Anelectricmotorconvertselectricityintomechanicalmotion.Electricmotorsareusedinhouseholdappliances,electricfans,remote-controlledtoys,andinthousandsofotherapplications.TheelectricmotorgrewoutofoneoftheearliestdiscoveriesinelectricscienceAragosrotations.In1824,FrancoisAragodiscoveredthatamagneticneedlesuspendedoveracopperdiskwouldrotatewhenthediscwasspun.Thenextyear,computerpioneerCharlesBabbageandastronomerJohnHerschelshowedthattheactioncouldbereversed:spinningamorepowerfulmagnetabovethecopperdiskwouldspinthecopperdisc.Then,in1831,MichaelFaradayconductedexperimentsthathelpedexplainwhythistookplace.Whilethislaidthegroundworkfortheelectricmotor,itwasanotherhalfcenturybeforeelectricmotorsweredoingusefulwork.Overthenextfewdecadesmanyinventorsmadeimproveddevicesforturningelectricityintomotion.OneofthesewasHippolytePixiis1832improvementcalledthecommutator,whichswitchedtheflowofcurrentbetweentwoormoresetsofstationaryelectromagnetstokeepamotorcontinuouslyrotating.ThomasDavenportwasthefirsttobuildanelectricmotorlargeenoughtobeusedinindustry,andhewasalsothefirsttoseekapatentonamotor.Soonelectricmotorswerebeingusedforsuchthingsastransportation.Moritz-HermannDeJacobiusedanelectricmotoronaboatontheNevaRiver,andCharlesG.Pageusedonetobuildasmalllocomotive.Aftertheappearanceofcommercialelectricpowersystemsinthe1880s,largerelectricmotorswerepossible.Edisonencouragedtheuseofelectricmotorsinindustrialapplicationsanddesignedseveralnewelectricmotorsforthatpurpose.Animportantchangecameinthelater1880sand1890s,whenelectricpowercompaniesbeganconsideringtheswitchtoalternatingcurrent.Alternatingcurrentwasperfectforthedistributionofelectricpoweroverlongdistances,anditworkedwellwiththeEdisonelectriclamp,butnopracticalACmotorexisteduntiltheworksofGalileoFerrarisinItalyandNikolaTeslaintheUnitedStates.TeslascontributionsarerememberedtodaymorethanFerrarisinpartbecauseTeslawassubsequentlyhiredbytheWestinghousecorporation,whichusedhispatentsalongwithmanyotherstobecomeoneofthemajorproducersofelectricequipment.WithasuitableACmotoravailable,ACpowertookoff.Itisstillinusetoday.ServomotorServomotorsareavailableasACorDCmotors.EarlyservomotorsweregenerallyDCmotorsbecausetheonlytypeofcontrolforlargecurrentswasthroughSCRsformanyyears.Astransistorsbecamecapableofcontrollinglargercurrentsandswitchingthelargecurrentsathigherfrequencies,theACservomotorbecameusedmoreoften.Earlyservomotorswerespecificallydesignedforservoamplifiers.Todayaclassofmotorsisdesignedforapplicationsthatmayuseaservoamplifieroravariable-frequencycontroller,whichmeansthatamotormaybeusedinaservosysteminoneapplication,andusedinavariable-frequencydriveinanotherapplication.Somecompaniesalsocallanyclosed-loopsystemthatdoesnotuseasteppermotoraservosystem,soitispossibleforasimpleACinductionmotorthatisconnectedtoavelocitycontrollertobecalledaservomotor.Somechangesthatmustbemadetoanymotorthatisdesignedasaservomotorincludestheabilitytooperateatarangeofspeedswithoutoverheating,theabilitytooperateatzerospeedandretainsufficienttorquetoholdaloadinposition,andtheabilitytooperateatverylowspeedsforlongperiodsoftimewithoutoverheating.Older-typemotorshavecoolingfansthatareconnecteddirectlytothemotorshaft.Whenthemotorrunsatslowspeed,thefandoesnotmoveenoughairtocoolthemotor.Newermotorshaveaseparatefanmountedsoitwillprovideoptimumcoolingair.ThisfanispoweredbyaconstantvoltagesourcesothatitwillturnatmaximumRPMatalltimesregardlessofthespeedoftheservomotor.Oneofthemostusabletypesofmotorsinservosystemsisthepermanentmagnet(PM)typemotor.ThevoltageforthefieldwindingofthepermanentmagnettypemotorcanbeACvoltageorDCvoltage.Thepermanentmagnet-typemotorissimilartootherPMtypemotorspresentedpreviously.Figure-1showsacutawaypictureofaPMmotorandFig.-2showsacutawaydiagramofaPMmotor.Fromthepictureanddiagramyoucanseethehousing,rotorandstatoralllookverysimilartotheprevioustypePMmotors.Themajordifferencewiththistypeofmotoristhatitmayhavegearredu