高职电气自动化专业英语课件1

Chapter1ElectricandElectronicTechnologyFundamentalsUnit1ElectricElementsandComponentsTextNewWordsandExpressionsExercisesEndUnit1ElectricElementsandComponentsIntroductionElectricalelementsareconceptualabstractionsusedintheanalysisofelectricalnetworks.Anyelectricalnetworkcanbeanalyzedasmultiple,interconnectedelectricalelementsinaschematicdiagramorcircuitdiagram,eachofwhichaffectsthevoltageinthenetworkorcurrentthroughthenetwork.Theseidealelectricalelementsrepresentreal,physicalelectricalorelectroniccomponentsbuttheydonotexistphysicallyandtheyareassumedtohaveidealpropertiesaccordingtoalumpedelementmodel,whilecomponentsareobjectswithlessthanidealproperties,adegreeofuncertaintyintheirvaluesandsomedegreeofnonlinearity,eachofwhichmayrequireacombinationofmultipleelectricalelementsinordertoapproximateitsfunctions.Circuitanalysisusingelectricelementsisusefulforunderstandingmanypracticalelectricalnetworksusingcomponents.Byanalyzingthewayanetworkisaffectedbyitsindividualelementsitispossibletoestimatehowarealnetworkwillbehave.Unit1ElectricElementsandComponentsTheelementsThefourfundamentalcircuitvariablesarecurrent,I;voltage,V,charge,Q;andmagneticflux,Φ.Only5elements,producedbymanipulatingthesefourvariables,arerequiredtorepresentanycomponentornetworkinaLinearsystem.Twosources:·Currentsource,measuredinamperes-producesacurrentinawire.Anidealcurrentsourceisamathematicalmodel,whichrealdevicescanonlyapproachinperformance.Thecurrentthroughanidealcurrentsourceisindependentofthevoltageacrossit.Unit1ElectricElementsandComponents·Voltagesource,measuredinvolts-producesapotentialdifferenceacrosstwopoints.Anidealvoltagesourceisonlyexistsinmathematicalmodelsofcircuits.Similarly,thevoltageacrossanidealvoltagesourceisindependentofthecurrentthroughit.Threepassiveelements:·ResistanceR,measuredinohms-producesavoltageproportionaltothecurrentflowingthroughtheelement.Relatesvoltageandcurrentaccordingtotherelation·CapacitanceC,measuredinfarads-producesacurrentproportionaltotherateofchangeofvoltageacrosstheelement.RelateschargeandvoltageaccordingtotheUnit1ElectricElementsandComponents.iRurelationUnit1ElectricElementsandComponents,uCq).(dtduCdtdqi.·InductanceL,measuredinhenries-producesavoltageproportionaltotherateofchangeofcurrentthroughtheelement.Relatesfluxandcurrentaccordingtotherelation,iL).(dtdiLdtdu.TheComponentsAbatteryprovideselectromotiveforce(emf),orvoltage,inacircuit.Itcontainslayersofchemicalsthatcauseelectronstomoveinacertaindirection,fromitsnegativepoletoitspositivepole.Itismarkedwitharatingofhowmuchvoltagethereisacrossthetwopoles,anda(-)forthenegativepoleanda(+)forthepositivepole.Batteriesmayalsobemarkedwithanamperehour(Ah)rating,indicatingtotalchargecapacity.Anidealbatterycanthusberepresentedasavoltagesource.Inpractice,abatteryalsohasaninternalresistancethatisrepresentedasaresistanceinserieswiththevoltagesource.Unit1ElectricElementsandComponentsIfawireisusedtoconnectthetwopolesofabattery,electronsflowthroughthewirefromthenegativeendtothepositiveend.(Thewirewillalsogethotbecauseitisn'taperfectconductor,andthebatterywillquicklyexhaustallitspower.)Thusawirecanberepresentedasalow-valueresistor.Currentsourcesareoftenabsentfrombasicelectriccircuits,andaremorelikelytobefoundinelectroniccircuitscontainingsemiconductors.Forexample,onafirstdegreeofapproximation,abipolar(junction)transistormayberepresentedbyavariablecurrentsourcethatiscontrolledbytheinputvoltage.Unit1ElectricElementsandComponentsAresistorisacomponentwhosefunctionistoregulatethecurrentinthecircuit.Onecommonkindisalittlecylinderofgraphitewithmetalwirescomingoutofeitherend.Thesearepaintedwithcoloredstripesthatindicatetheresistance,inohms,andthetolerance,inpercent.Thissystemiscalledtheresistorcolorcode.Anotherkindofresistorisafilament,whichisacoilofmetalwirethatcanwithstandhightemperaturebuthasafiniteresistance.Whenacurrentispassedthroughafilament,itheatsupbecauseofthisresistance.Filamentsarecommonlyusedinlightbulbsandheaters.Theyaremarkedwiththevoltagethatshouldbeappliedtothem,andthepower,inwatts,thattheywillthengiveoffaslightandheat.Duetotheeffectofheating,afilament'sresistanceishigherwhenitishotthanwhenitiscold.Unit1ElectricElementsandComponentsAnelectricchargecanbestoredandthenquicklyreleasedbyacomponentcalledacapacitor.Acommontypeofcapacitorconsistsoftwopiecesofmetalfoil(orplates)withaninsulator(thedielectric)suchaswaxedpaperbetweenthem.Ifanelectricchargeisplacedontheplatesofacapacitor,itwillstaytherebecauseitcan'tcrosstheinsulatortotheotherplate.Ifawireisthenconnectedbetweenthetwoplates,thechargewillflowthroughthewiretobalancethechargesontheoppositeplates—thecapacitoristhensaidtobedischarged.Unit1ElectricElementsandComponentsUnit1ElectricElementsandComponentsSomecapacitorslooklikeacylinderorblobwithtwowirescomingoutoneend,andaremarkedtoindicatetheircapacitance(thechargethattheystorepervolt)inmicrofarads(μF),nanofarads(nF)orpicofarads(pF).Inductanceinacircuitisprovidedbycomponentscalledinductors,whicharealmostalwaysbuiltfromcoilsofwire.Largevaluesofinduct