HowD.C.toA.C.InvertersWorkByStevenMcFadyenonApril22nd,2014

Traditionallygenerationofelectricityhasinvolvedrotatingmachinestoproducealternatingsinusoidalvoltageandcurrent(a.c.systems).Withthedevelopmentofpowerelectronicsduringthelastseveraldecades,semiconductordevicesarenowfrequentlyusedtoconvertdirectcurrent(d.c.)toa.c.topowerconventionalalternatingcurrentsystems.

Devicesusedtoconvertd.c.toa.c.arecalledinverters.Theseareusednumerousapplications,includingPVsystems,batterystoragesystems,tractiondrives,variablespeeddrivesandothers.Whiletheyaresaidtoconvertd.c.toa.c.,thisisasimplification,andaswewillsee,theoutputofanyinverterisbuiltupbypulsedd.c.voltages.

Ifyouarenotfamiliarwithsinusoidala.c.voltageandcurrent,youcanrefertothefollowingnotesforsomerevision:

AlternatingCurrentCircuitsGenerationofaSineWave

Contents[hide]1. TypeofInverter2. PulseWidthModulation3. HBridgeCircuit4. Puttingittogether

TypeofInverter

Convertingad.c.voltagetoasinewaveisnotastraightforwardprocess.Thegeneralapproachistochop(pulse)thed.c.voltagesothatitapproximatelyresemblesasinewave.Thiswaveformcanthenbefilteredtobringitclosertothatofasinewave.Thelevel(andassociatedcostliness)towhichthesetechniquesareapplieddeterminethefinalqualityofanysinewaveproduced.

Whenconsideringinverters,thequalityoftheiroutputisoftenclassifiedintogeneralcategories:

SquareWaveInverterModifiedSineWaveInverterPureSineWaveInverter

Comparisonofvariouswaveformtypes

Asquarewaveisverysimple,withthed.c.supplyswitchedbetweenpositiveandnegative.Dependingonthecircuitry,thesimplesquarewavecanbeadaptedtogiveamodifiedsinewaveasshown.ByutilisingPulseWidthModulation(seebelow)andfilteringtechniques,thewaveformcanberefineduntilitcloselyresemblethatofapuresinewave.

Thereisnoexactcutoffwhichdefinesapuresinewaveandvariousmanufacturerswillhavedifferingspecifications.Itisgenerallyarguedthatwhenthetotalharmonicdistortionofthevoltagewaveformislessthan3%,forallpracticalpurposesthiscanbeconsideredasatruesinewave.

Alotofequipmentwillworkwellonmodifiedsinewaveinverters,includingmotors,householdappliancesandotheritems.Sometypesofloadstheycanbeproblematicanddorequireapuresinewaveconverter.Awellknowexampleareloadsrequiringapuresinewavearedevicesthatincludecrystaloscillatorelectronictimingcircuitswhichrelyonazerocrossoverofthesinewaveforthefunctioning.

Puresinewaveinvertersaremorecomplexandcostmore.Itisbesttoselectthetypeofinvertertomatchtheapplicationforwhichitwillbeused.

PulseWidthModulation

MostinvertersuseatechniquecalledPulseWidthModulation(PWM)toturnthed.c.voltageonandoff.Thewidthofeachpulseisvaried,sothattheoverallelectricaleffectissimilartothatofasinewave.Thistechniqueisoftenappliedwhenpoweringa..cmotors.ForamoredetailedexplanationofhowPWMworks,pleaseseeour VariableFrequencyDrivenote ).

PulseWidthModulationthewidthofpulsesisvariedtosimulateasinewave

PulseWidthModulationthewidthofpulsesisvariedtosimulateasinewave

Itshouldbeunderstoodthattheoutputofthistypeofinverterisnotapuresinewave.Itisaseriesofd.c.pulses.Thiscanmakeitunsuitableforcertaintypesofequipment.

ThediagrambelowillustratesaPWMwaveformforastandardinverterwhereasingled.c.voltageiswithswitchedonoroftogeneratetherequiredoutput.Inthisinstance,moreinputd.c.voltagelevelsareusedtocreateanoutputwaveformwhichmorecloselyresemblesasinewave.Multileveltypeinventorsaremorecomplexandcostlytoproduce.

Multilevel(3levels)WaveForm

HarmonicswiththePWMwaveformsnotbeingsinusoidal,harmonicswillbegenerated.Formultilevelinverters,the

morelevelsemployed,theclosertheoutputwillapproximateasinewaveandtheharmoniccontentwillbelower.

Invertersfrequentlyemploytheuseoftransformers,capacitorsandinductorstofilterthePWMwaveformandreducetheharmoniccontent.

HBridgeCircuit

Atitssimplest,aninverterconsistsofwhatisknownasaHBridgearrangement.ThecircuitbelowillustratestheimplementationofasinglephaseHBridgecircuitusingInsulatedGateBipolarTransistors(IGBT).

HBridgeusingIGBT

Theoperationofthebridgeisstraightforward.TheIGBTactasaswitch(whenasignalisappliedtothegate,theyturnonandthenturnoffwhenthesignalisremoved).ByclosingQ1andQ4,apositived.c.supplyisappliedtotheload.Q2andQ3willresultinanegatived.c.supplyacrosstheload.ControlcircuitsareusedtogeneratethenecessarygatesignalstoproducetherequiredPWMwaveform.

Toavoidshortcircuits(closingbothQ1andQ2atthesametimeforexample)whenchangingpolarityitisnecessarytoturnoffonesetofIGBTbeforeturningonthenext.Duringthetransition,alltheIGBTonoff.Diodesprovideanecessarypathforinductivecurrentinordertolimitpotentialvoltagebuildupduringthetransitionperiod.

Thecapacitorprovidessmoothingtoevenoutanyvariationinthed.c.supply.

InverterBlockDiagram

ByutilisingsixIGBT,itispossibletousethebridgearrangementtosupplythreephaseloads.

HBridgearraignmentsarealsocommonlyimplementedusingBipolarJunctionTransistors((BJT)orMetalOxideSemiconductorFieldEffectTransistors(MOSFET)

Puttingittogether

Theblockdiagramillustratesthekeycomponentsofad.c.toa.c.inverter.

InputFiltertheinputfilterremovesanyrippleorfrequencydisturbancesonthed.c.supply,toprovideacleanvoltagetotheinvertercircuit.Inverterthisisthemainpowercircuit.Itisherethatthed.c.isconvertedintoamultilevelPWMwaveform.OutputFiltertheoutputfilterremovesthehighfrequencycomponentsofthePWMwave,toproduceanearsinusoidaloutput.

Note:ifyouarefamiliarwithFourierAnalysis,itwillbeseenthattheperiodicPWMwaveformconsistsofamainfundamentalfrequencycomponentandhigherorder(butlowermagnitude)harmoniccomponents.Itisthesehigherorderharmonicswhichtheoutputfilteriseliminating.

Invertersarecomplexdevices,buttheyareabletoconvertd.c.toa.c.forgeneralpowersupplyuse.Withadvancesinpowerelectronicsandmicroprocessors,theapplicationofinvertersinmanyfieldsisincreasing.Overall,invertersallowustotapintothesimplicityofd.c.systemsandutiliseequipmentdesignedthesetoworkinaconventionala.cenvironment.