XITRON XT2640 - Performing Measurements of Pwm Generated Signals; Choice of Channels; Wiring Recommendations; Recommended 3-Phase, 3-Wire Wiring Method Selection

XT2640OperatingManual 13July2016 Page78of187
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16 PERFORMINGMEASUREMENTSOFPWMGENERATEDSIGNALS
Becauseofits’widebandwidthandspread‐spectrumsamplingmethodtheXT2640canmakehighaccuracymeasurementsofPWM
basedsignalshowevertheusershouldtakesomespecialprecautionswhenwiringandconfiguringtheXT2640specificallyforthese
measurements.Thissectiondetailsthoseprecautions.
ItisassumedinthissectionthatyouareconversantwiththegeneraloperatingmethodsusedintheXT2640asdescribedinsection
12.
Note:
InPWMapplicationsthevoltagesignalscontainsignificantsignalcontentattwoormorefrequencies,typically–
 The“carrierfrequency”.ThisissometimescalledthePWMswitchingfrequency,alsosometimescalledthePWM
Frequency.Insomesourcesthismaybeatafixedfrequency,whileinothersitmayvarytoreduceinterference,improve
efficiency,and/orimprovewaveformshapeofthemodulationsignalcontent.Inmostcasesthisisa‘square‐wave’voltage
whosepeaksarefixedbytheinternalDCvoltagefromwhichthesourceisgenerated(sometimescalledthe“busvoltage”)
andthedutycycleiscontinuouslyvariedtocontrolthemodulationsignalcomponent.Thecarrierfrequencycomponent
oftenthelargestcomponentofthevoltagesignals.
 The“modulationfrequency”.Thisisthefrequencyatwhichthecarrierfrequencydutycycleisvariedtocausesignal
contentatafrequencymuchlowerthanthecarrierfrequency,includingDCinsomecases.Inthecasewheretheloadisa
motorthisissometimescalledthedrivefrequencyandforasynchronousmotorthisfrequencysetsthemotorspeed.For
aninductiveload(e.g.amotor)thecurrentsignalgenerallyincludesverylittleofacarrierfrequencycomponent,being
primarilyonlyatthemodulationfrequency.
16.1 CHOICEOFCHANNELS
WhenconsideringwhichchannelstouseinPWMapplicationsitisimportanttoconsiderthetypeofchannel(the1
st
letterofthetwo
letterchanneloption).The2
nd
letterisonlyimportantwithregardstothecurrentlevelandthetypeoftransducerifoneisbeing
used.
AandSchanneltypesarenotrecommendedforhighaccuracyapplicationswherethecarrierfrequencyisabove80kHz(10kHzif
harmonicsaccuracyisneeded)andWchanneltypesaregenerallynotrecommendedabovea400kHzcarrierfrequency(50kHzif
harmonicsaccuracyisneeded).
InmanyPWMapplications(e.g.motordrives)thedutycycleofthewaveformscanrangefromafewpercentuptoalmost
100%.Thismeansthatforbestaccuracythesignalintegritymusttypicallybemaintaineduptoafrequencyof>10times
thecarrierfrequencyfornon‐harmonicmeasurements(e.g.RMSvoltage,RMScurrentandtotalpower)and>80timesthe
carrierfrequencyforharmonicmeasurements(e.g.individualharmonicsandTHD).Iftherangeofdutycyclesisknownto
belessthanthisandisnominallycenteredcloseto50%thenhighercarrierfrequenciescanbeaccommodatedwith
accuracy.
16.2 WIRINGRECOMMENDATIONS
Youshouldkeepallwiringbetweenthesource,theXT2640,andtheloadasshortaspossible.Ifusingthe3ø3w(3ch)methodyou
shouldensurethattheinter‐connectionbetweenthethreeV‐terminalsiskeptasshortaspossibleandisleftfloating.
GenerallyPWMgeneratedvoltagesignalshaveveryshortriseandfalltimesontheirswitchingedges.Ifthewiringislong
thenthesesignalscanhaveverysignificantovershootandringingcausedbytheinductanceofthewiring.Suchovershoots
andringingcancauseinterferencewhichmayaffectnearbyelectronicequipment,andinseverecasesmayaffectthe
measurementaccuracyandtheovershootvoltagesmaybehigherthantheXT2640isabletowithstand.
16.2.1 RECOMMENDED 3‐PHASE, 3‐ WIREWIRINGMETHODSELECTION
Thissectionisonlyapplicableifthesourceandloadare3‐phasewithnoneutralconnection(i.e.3‐phase,3‐wire).Forsinglephase,
2‐phase(2ø3w),and3‐phasewithaneutralconnection(3ø4w)applicationsusethenormallyapplicablewiringmethod.
Iftheaccuracyofharmonicsresultsisnotofsignificantimportancetoyoutheneither3ø3w(2ch)or3ø3w(3ch)wiringmethodscan
beused.
Iftheaccuracyofharmonicsresultsisofsignificantimportancetoyouthenitisrecommendedtousethe3ø3w(3ch)wiringmethod.
Generallytheaccuracyofnon‐harmonicresultsareunaffectedbythischoice.
IntheXT2640thereisverylittledifferencebetweenthesechoicesinregardstotheavailablemeasurementresults.Forboth
choicesthemajorityofinter‐phasenon‐harmonicandharmonicmeasurementsareavailableandinter‐phasevoltagescope
signalsareavailable.
Theindividualphasevoltageshavesignificantlylesshighfrequencycontentthantheinter‐phasevoltagesinmulti‐phase
PWMapplications.Atnearzerocrossingsoftheinter‐phasevoltagesignalthetwophasesignalshavenearlythesamePWM
dutycycle,sotheinter‐phasesignalconsistsofoneortwo(dependingontheswitchingmethod)veryslimpulseshavingfull
DCbusvoltageamplitudeandaverylowdutycycle(significantlylessthan1%).Thesepulseshaveverysignificantcontentat
highfrequencieswellabovethespecifiedfrequencyrangeofthepoweranalyzersoanyhighfrequencynon‐linearityor
samplingartifactswithinthepoweranalyzermaybecomesignificantandcausetheappearanceofharmonicsinthoseresults,
possiblyuptoafewpercentattypically3
rd
harmonic.
BecausetheXT2640isnotdesignedsolelyformainspowerapplicationsbuthasawidebandwidthextendinguptoseveral
MHz,thepresenceofsuchsignalswellabovethespecifiedbandwidthmayaffectitslightly.Byusingthe3‐channelmethod
thiseffectiseliminatedasextremelyslimpulsewidthswillonlyoccurattheveryextremesofoutputamplitudeineachphase
andgenerallyPWMswitchingsystemsaredesignedtonotachievesuchslimpulsessincethesealsohaveasignificantimpact
onthePWMdriveitself.