Part 3 motor application considerations understanding power concepts rev 5 vincent wedelich ieee presentation

UnderstandingPowerConcepts
Part3
•Motors
–ACInductionMotors
–MotorEfficiencyandAssessment
–MotorControllers(VFDandHarmonics)
–ApplicationConsiderations
•Transformers
•Substations
•Switchgear
•Panels(LightingandPower)
•MCC
•DC/UPSSystems

Applicationconsiderations
•AClineharmonics
•WhileharmonicsinthePWMoutputcaneasilybefilteredbycarrier
frequencyrelatedfilterinductancetosupplynearsinusoidal
currentstothemotorload,
•theVFD'sdiodebridgerectifierconvertsAClinevoltagetoDC
voltageoutputbysuperimposingnonlinearhalfphasecurrent
pulsesthuscreatingharmoniccurrentdistortion,andhencevoltage
distortion,oftheAClineinput.
•WhentheVFDloadsarerelativelysmallincomparisontothelarge,
stiffpowersystemavailablefromtheelectricpowercompany,the
effectsofVFDharmonicdistortionoftheACgridcanoftenbe
withinacceptablelimits.

•AClineharmonics
•Furthermore,inlowvoltagenetworks,harmonicscaused
bysinglephaseequipmentsuchascomputersandTVsare
partiallycancelledbythreephasediodebridgeharmonics
becausetheir5thand7thharmonicsareincounterphase.
•However,whentheproportionofVFDandothernonlinear
loadcomparedtototalloadorofnonlinearloadcompared
tothestiffnessattheACpowersupply,orboth,isrelatively
largeenough,theloadcanhaveanegativeimpactonthe
ACpowerwaveformavailabletootherpowercompany
customersinthesamegrid.

•Twootherharmonicsmitigationtechniques
exploituseofpassiveoractivefilters
connectedtoacommonbuswithatleastone
VFDbranchloadonthebus.Passivefilters
involvethedesignofoneormorelowpassLC
filtertraps,eachtrapbeingtunedasrequired
toaharmonicfrequency(5th,7th,11th,13th,
...kq+/1,wherek=integer,q=pulsenumber
ofconverter).

•Whenthepowercompany'svoltagebecomes
distortedduetoharmonics,lossesinotherloads
suchasnormalfixedspeedACmotorsare
increased.Thismayleadtooverheatingand
shorteroperatinglife.
•Alsosubstationtransformersandcompensation
capacitorsareaffectednegatively.Inparticular,
capacitorscancauseresonanceconditionsthat
canunacceptablymagnifyharmoniclevels.

•Inordertolimitthevoltagedistortion,ownersofVFDload
mayberequiredtoinstallfilteringequipmenttoreduce
harmonicdistortionbelowacceptablelimits.
•Alternatively,theutilitymayadoptasolutionbyinstalling
filteringequipmentofitsownatsubstationsaffectedby
thelargeamountofVFDequipmentbeingused.
•Inhighpowerinstallationsharmonicdistortioncanbe
reducedbysupplyingmultipulserectifierbridgeVFDsfrom
transformerswithmultiplephaseshiftedwindings.

•Itisalsopossibletoreplacethestandarddiodebridge
rectifierwithabidirectionalIGBTswitchingdevice
bridgemirroringthestandardinverterwhichusesIGBT
switchingdeviceoutputtothemotor.
•Suchrectifiersarereferredtobyvariousdesignations
includingactiveinfeedconverter(AIC),activerectifier,
IGBTsupplyunit(ISU),activefrontend(AFE)orfour
quadrantoperation.

•Itisalsopossibletoreplacethestandarddiodebridgerectifierwith
abidirectionalIGBTswitchingdevicebridgemirroringthestandard
inverterwhichusesIGBTswitchingdeviceoutputtothemotor.
•Suchrectifiersarereferredtobyvariousdesignationsincluding
activeinfeedconverter(AIC),activerectifier,IGBTsupplyunit(ISU),
activefrontend(AFE)orfourquadrantoperation.
•WithPWMcontrolandsuitableinputreactor,AFE'sAClinecurrent
waveformcanbenearlysinusoidal.AFEinherentlyregenerates
energyinfourquadrantmodefromtheDCsidetotheACgrid.
•Thusnobrakingresistorisneededandtheefficiencyofthedriveis
improvedifthedriveisfrequentlyrequiredtobrakethemotor.

Longleadeffects
•Thecarrierfrequencypulsedoutputvoltageofa
PWMVFDcausesrapidrisetimesinthesepulses,
thetransmissionlineeffectsofwhichmustbe
considered.
•Sincethetransmissionlineimpedanceofthe
cableandmotoraredifferent,pulsestendto
reflectbackfromthemotorterminalsintothe
cable.

Longleadeffects
•Theresultingvoltagescanproduce
overvoltagesequaltotwicetheDCbusvoltageor
upto3.1timestheratedlinevoltageforlong
cableruns,puttinghighstressonthecableand
motorwindingsandeventualinsulationfailure.
•Notethatstandardsforthreephasemotorsrated
230Vorlessadequatelyprotectagainstsuch
longleadovervoltages.

Longleadeffects
•On460or575Vsystemsandinverterswith3rdgeneration0.1
microsecondrisetimeIGBTs,themaximumrecommendedcable
distancebetweenVFDandmotorisabout50mor150feet.
•Solutionstoovervoltagescausedbylongleadlengthsinclude
–minimizingcabledistance,
–loweringcarrierfrequency,
–installingdV/dtfilters,
–usinginverterdutyratedmotors(thatarerated600Vtowithstand
pulsetrainswithrisetimelessthanorequalto0.1microsecond,of
1,600Vpeakmagnitude),and
–installingLCRlowpasssinewavefilters.

Longleadeffects
•Regardingloweringofcarrierfrequency,notethataudiblenoiseis
noticeablyincreasedforcarrierfrequencieslessthanabout6kHzandis
mostnoticeableatabout3kHz.
•NotealsothatselectionofoptimumPWMcarrierfrequencyforACdrives
involvesbalancing
–noise,
–heat,
–motorinsulationstress,
–commonmodevoltageinducedmotorbearingcurrentdamage,
–smoothmotoroperation,andotherfactors.
FurtherharmonicsattenuationcanbeobtainedbyusinganLCRlowpasssine
wavefilterordV/dtfilter.

Motorbearingcurrents
•PWMdrivesareinherentlyassociatedwithhigh
frequencycommonmodevoltagesandcurrentswhich
maycausetroublewithmotorbearings.
•Whenthesehighfrequencyvoltagesfindapathto
earththroughabearing,transferofmetalorelectrical
dischargemachining(EDM)sparkingoccursbetween
thebearing'sballandthebearing'srace.
•OvertimeEDMbasedsparkingcauseserosioninthe
bearingracethatcanbeseenasaflutingpattern.

BearingCurrents
•BecausethevoltageoutputoftheVFDisa
resultofhighspeedswitching,thewaveform
morecloselyresemblesasquarewaveinstead
ofasinewave.
•Asaconsequence,animbalanceispresent
betweenthethreephasesthatareentering
themotorleads,referredtoascommonmode
voltage,

BearingCurrents
•AssoonasthemotorisstartedwithaVFD,the
commonmodevoltageimbalancesearchesforapath
toground,andaparasiticcouplingoftherotorand
statorwindingsdevelops.
•Oncetheoilfilminthedielectricbreaksdown,voltage
levelsincreaseonthemotorshaft,andthelowest
impedancepathforthecurrentimbalancebecomes
themotorbearingsthemselves.
•Voltagebeginstoarcacrossthebearingathigh
frequencies,leadingtoelectricaldischargemachining
(EDM).

•Inlargemotors,thestraycapacitanceofthe
windingsprovidespathsforhighfrequency
currentsthatpassthroughthemotorshaftends,
leadingtoacirculatingtypeofbearingcurrent.
•Poorgroundingofmotorstatorscanleadtoshaft
groundbearingcurrents.Smallmotorswith
poorlygroundeddrivenequipmentare
susceptibletohighfrequencybearingcurrents.

•Preventionofhighfrequencybearingcurrent
damageusesthreeapproaches:
–goodcablingandgroundingpractices,
–interruptionofbearingcurrents,and
–filteringordampingofcommonmodecurrents.

Goodcablingandgroundingpracticescanincludeuseofshielded,
symmetricalgeometrypowercabletosupplythemotor,
installationofshaftgroundingbrushes,andconductivebearing
grease.
Bearingcurrentscanbeinterruptedby:
–installationofinsulatedbearings
–andspeciallydesignedelectrostaticshieldedinductionmotors.
Filteringanddampinghighfrequencybearing,or,insteadofusing
standard2levelinverterdrives,usingeither3levelinverterdrives
ormatrixconverters.

•Sinceinverterfedmotorcables'high
frequencycurrentspikescaninterferewith
othercablinginfacilities,suchinverterfed
motorcablesshouldnotonlybeofshielded,
symmetricalgeometrydesignbutshouldalso
beroutedatleast50cmawayfromsignal
cables.

•Motorframesize—Motorbearingcurrentshave
adirectrelationshiptomotorsize.Specifically,
motorsabove450hphavebeenfoundtobethe
greatestcandidatesforbearingcurrents,dueto
thehigherlikelihoodofmagneticasymmetryin
theconstructionofthemotorwindings.While
largemotorswillexperiencesomeamountof
bearingcurrents,motorspoweredfromVFDs
introduceadditionalmagneticasymmetryfrom
commonmodevoltage.

•Roughhandling—
•Motorsareatanincreasedriskofshaftvoltageiftheywere
handledroughlyduringshipment.
•Oftentimes,damagecanoccurinthefactory,intransit,or
whenthemotorisbeingliftedontoitspedestal.
•Minormotordamageduringshipmentmaynotcause
noticeableproblemswithmotorperformance;however,it
increasesthelikelihoodofmagneticasymmetryleadingto
bearingcurrentdevelopment.

Part 3 motor application considerations understanding power concepts rev 5 vincent wedelich ieee presentation

MeasuringforBearingCurrents
•Aftertheambientnoisemeasurementhasbeencompleted,the
carbonbrushisplaceddirectlytothespinningmotorshaft.
•Severalmeasurementsshouldberecorded,witheffortstoensure
theentirewaveformcanbeviewedontheoscilloscope.
•Carefulobservationofthewaveformshouldrevealifasharpspike
involtageispresent,whichwouldbeindicativeofashaftvoltage
discharge.
•Voltageinexcessof0.5mV(peaktopeak)isoftentimescausefor
furtherinvestigation.However,inVFDs,thenominalvaluewill
almostalwaysbeinexcessof0.5mV.Theexamplesdiscussedbelow
indicatevariousexamplesofshaftvoltagetests.

MeasuringforBearingCurrents
•Theoscilloscopescreenshotshowninpresents
clearevidenceofshaftvoltage,withapeakto
peakvalueof74.3V.
•ThisiswellabovetheIEEE112allowablevalueof
0.5mV,aswellasthemanufacturer’sallowable
voltageof1Vto2VforVFDs.
•Apeakvoltageof74.3Vimposedupontheshaft
likelymeansthatthebearingwillnotsurvive
muchlongerunlessremedied.

CompareSimilarMotors
•Whentestingamotor,itisoftenusefultotestthe“B”
motortocomparevalues.
•Thescreenshotshowninisthe“B”motoroftheacrossthe
linemotorthatwasdroppedduringshipment.
•Clearly,thereisnoevidenceofshaftvoltageordamageto
thebearinginthiswaveform.
•Oftentimes,comparingtwosimilarmotorscanhelp
pinpointiftheproblemliesinthemotoritself,with
transientvoltagepowersupplies,orinafaultygrounding
system.

Dynamicbraking
•Torquegeneratedbythedrivecausesthe
inductionmotortorunatsynchronousspeedless
theslip.
•Ifloadinertiaenergyisgreaterthantheenergy
deliveredtothemotorshaft,motorspeed
decreasesasnegativetorqueisdevelopedinthe
motorandthemotoractsasagenerator,
convertingoutputshaftmechanicalpowerback
toelectricalenergy.

Dynamicbraking
•Thispowerisreturnedtothedrive'sDClinkelement(capacitoror
reactor).
•ADClinkconnectedelectronicpowerswitchorbrakingDC
chopper(eitherbuiltinorexternaltothedrive)transfersthisenergyto
externalresistorstodissipatetheenergyasheat.
•Coolingfansmaybeusedtopreventresistoroverheating.
•Dynamicbrakingwastesbrakingenergybytransformingittoheat.By
contrast,regenerativedrivesrecoverbrakingenergybyinjectingthis
energyontheACline.
•Thecapitalcostofregenerativedrivesishoweverrelativelyhigh.

•Dynamicbraking

•Regenerativedrives
•RegenerativeACdriveshavethecapacitytorecover
thebrakingenergyofaloadmovingfasterthanthe
designatedmotorspeed(anoverhaulingload)and
returnittothepowersystem.
•CurrentSource(Cycloconverter,Scherbius,matrix,CSI
andLCI)drivesinherentlyallowreturnofenergyfrom
theloadtotheline
•whilevoltagesourceinvertersrequireanadditional
convertertoreturnenergytothesupply.

Regenerativedrives
•RegenerationisonlyusefulinVFDswherethevalueofthe
recoveredenergyislargecomparedtotheextracostofa
regenerativesystem,andifthesystemrequiresfrequent
brakingandstarting.RegenerativeVFDsarewidelyused
wherespeedcontrolofoverhaulingloadsisrequired.
•Someexamples:
•Conveyorbeltdrivesformanufacturing,whichstopevery
fewminutes.Whilestopped,partsareassembledcorrectly;
oncethatisdone,thebeltmoveson.
•Acrane,wherethehoistmotorstopsandreverses
frequently,andbrakingisrequiredtoslowtheloadduring
lowering.

Part 3 motor application considerations understanding power concepts rev 5 vincent wedelich ieee presentation

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Part 3 motor application considerations understanding power concepts rev 5 vincent wedelich ieee presentation (20)

More from Vincent Wedelich, PE MBA (20)

Recently uploaded (20)

Part 3 motor application considerations understanding power concepts rev 5 vincent wedelich ieee presentation