freqency drive.docx
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freqency drive.docx
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freqencydrive
Variable-frequencydrive
Smallvariablefrequencydrive
VFDopenchassis
Avariable-frequencydrive(VFD)isasystemforcontrollingtherotationalspeedofanalternatingcurrent(AC)electricmotorbycontrollingthefrequencyoftheelectricalpowersuppliedtothemotor.[1][2][3]Avariablefrequencydriveisaspecifictypeofadjustable-speeddrive.Variable-frequencydrivesarealsoknownasadjustable-frequencydrives(AFD),variable-speeddrives(VSD),ACdrives,microdrivesorinverterdrives.
Variable-frequencydrivesareusedinawidenumberofapplicationstocontrolpumps,fans,hoists,conveyors,andothermachinery.
Contents
[hide]
∙1Benefits
o1.1Energysavings
o1.2Startingtorquecontrol
∙2VFDtypesandratings
o2.1Generictopologies
o2.2Controlplatforms
o2.3Loadtorqueandpowercharacteristics
o2.4Availablepowerratings
o2.5Drivesbymachines&detailedtopologies
∙3Systemdescriptionandoperation
o3.1Motor
o3.2Controller
o3.3Operatorinterface
o3.4Driveoperation
∙4Applicationconsiderations
o4.1Powerlineharmonics
o4.2Longleadeffects
o4.3Motorbearingcurrents
o4.4Dynamicbraking
o4.5Regenerativevariable-frequencydrives
∙5Seealso
∙6References
[edit]Benefits
[edit]Energysavings
ACmotor-drivenapplicationsthatdonotrequirefullspeedcansaveenergybycontrollingthemotorwithavariablespeeddrive.Energycostsavingwithvariabletorquecanbesignificant,oftenpayingforthecostofVFDwithinamatterofmonths.Invariabletorqueapplicationssuchasfansandblowers,thetorquerequiredvariesroughlywiththesquareofthespeed,andthehorsepowerrequiredvariesroughlywiththecubeofthespeed,resultinginalargereductionofhorsepowerforevenasmallreductioninspeed.Themotorwillconsumeonly25%asmuchpowerat63%speedthanitwillat100%speed.ThisisreferredtoastheAffinityLaws,whichdefinetherelationshipsbetweenspeed,flow,torque,andhorsepower.[4]
IntheUnitedStates,anestimated60-65%ofelectricalenergyisusedtosupplymotors,75%ofwhicharevariabletorquefan,pumpandcompressorloads.[5]Eighteenpercentoftheenergyusedinthe40millionmotorsintheU.S.couldbesavedbyefficientenergyimprovementtechnologiessuchasVFDs[6]Onlyabout3%ofallACmotorsareprovidedwithACdrives[7]
AnenergyconsumptionbreakdownoftheglobalpopulationofACmotorinstallationsisasshowninthefollowingtable:
Globalpopulationofmotors,2009[8]
Small
GeneralPurpose-Medium-Size
Large
Power
10Wto750W
750Wto375kW
375kWto100MW
Phase,voltage
1-ph.,<240V
3-ph.,200Vto1kV
3-ph.,1kVto20kV
%totalmotorenergy
9%
68%
23%
Totalstock
2billion
230million
0.6million
[edit]Startingtorquecontrol
Across-the-linesingle-speedstartersstartmotorsabruptly,subjectingthemotortoahighstartingtorqueandtocurrentsurgesthatareupto8timesthefull-loadcurrent.Variablespeeddrivesinsteadgraduallyrampthemotoruptooperatingspeedtolessenmechanicalandelectricalstress,reducingmaintenanceandrepaircosts,andextendingthelifeofthemotorandthedrivenequipment.
Reduced-voltagestartingmethodsalsoaccelerateamotorgradually,butVFdrivescanbeprogrammedtorampupthemotormuchmoregraduallyandsmoothly,andcanoperatethemotoratlessthanfullspeedtodecreasewearandtear.Variablespeeddrivescanalsorunamotorinspecializedpatternstofurtherminimizemechanicalandelectricalstress.Forexample,anS-curvepatterncanbeappliedtoaconveyorapplicationforsmootherdecel/accelcontrol,whichreducesthebacklashthatcanoccurwhenaconveyorisacceleratingordecelerating.
PerformancefactorstendingtofavoruseofDC,overAC,drivesincludesuchrequirementsascontinuousoperationatlowspeed,4-quadrantoperationwithregeneration,frequentaccelerationanddecelerationroutines,andneedformotortobeprotectedforhazardousarea.[9]ThefollowingtablecomparesACandDCdrivesaccordingtocertainkeyparameterrs:
[10][11][12]
Criteria
BrushTypeDC
ACV/HzControl
ACOpen-LoopVector
ACClosed-LoopFluxVector
Typicalspeedregulation(%)
0.01
1
0.5
0.01
Typicalspeedrangeatconstanttorque
0-100
10-100
3-100
0-100
Min.speedat100%torque(%ofbase)
Standstill
8%
2%
Standstill
Multiple-motoroperationrecommended
No
Yes
No
No
Faultprotection(Fusedonlyorinherenttodrive)
Fusedonly
Inherent
Inherent
Inherent
Maintenance
(Brushes)
Low
Low
Low
Feedbackdevice
Tachometerorencoder
N/A
N/A
Encoder
[edit]VFDtypesandratings
[edit]Generictopologies
Variablefrequencydrivescanbeclassifiedaccordingtothefollowinggenerictopologies:
[13][14]
∙Voltage-sourceinverter(VSI)drives:
InaVSIdrive,theDCoutputofthediode-bridgeconverterstoresenergyinthecapacitorbustosupplystiffvoltageinputtotheinverter.
∙Current-sourceinverter(CSI)drives:
InaCSIdrive,theDCoutputoftheSCR-bridgeconverterstoresenergyinseries-reactorconnectiontosupplystiffcurrentinputtotheinverter.
∙Variable-voltageinverter(VVI):
VVIdrivesareaspecialCSIcaseandarealsoreferredtoassix-steporpulse-amplitudemodulation(PAM)[15]orsquare-waveinverter(withD.C.chopper)drives.[16]InaVVIdrive,theDCoutputoftheSCR-bridgeconverterissmoothedviacapacitorbusandseries-reactorconnectiontosupplyviaDarlingtonPairorIGBTinverterquasi-sinusoidal,six-stepvoltageinputtoaninductionmotor.
∙Loadcommutatedinverter(LCI)drives:
InaLCIdrive,alsoaspecialCSIcase,theDCoutputoftheSCR-bridgeconverterstoresenergyviaDClinkinductorcircuittosupplystiffquasi-sinusoidalsix-stepcurrentinputtoasecondSCR-bridge'sinverterandanover-excitedsynchronousmachine.
∙Cycloconverterormatrixdrives:
Acycloconverteroperatesasa3-phasecurrentsourceinanti-parallelsix-pulseconfigurationwithnoDCrectification,eachcycloconverterphaseactingselectivelytoconvertfixedlinefrequencyACvoltagetoanalternatingvoltageatavariableloadfrequency.
∙Doublyfedsliprecoverysystems:
AdoublyfedsliprecoverysystemfeedsrectifiedslippowertoasmoothingreactortosupplypowertotheACsupplynetworkviaaninverter,thespeedofthemotorbeingcontrolledbyadjustingtheDCcurrent.
[edit]Controlplatforms
Mostdrivesuseoneormoreofthefollowingcontrolplatforms:
[13][17]
∙Pulsewidthmodulation(PWM)scalarorVolts/Hertz(V/Hz)control
∙PWMfield-orientedcontrol(FOC)orvectorcontrol
∙Directtorquecontrol(DTC).
[edit]Loadtorqueandpowercharacteristics
Variablefrequencydrivesarealsocategorizedbythefollowingloadtorqueandpowercharacteristics:
∙Variabletorque,suchasincentrifugalfan,pumpandblowerapplications
∙Constanttorque,suchasinconveyoranddisplacementpumpapplications
∙Constantpower,suchasinmachinetoolandtractionapplications.
[edit]Availablepowerratings
VFDsareavailablewithvoltageandcurrentratingscoveringawiderangeofsingle-phaseandmulti-phaseACmotors.Lowvoltage(LV)drivesaredesignedtooperateatoutputvoltagesequaltoorlessthan690 V.Whilemotor-applicationLVdrivesareavailableinratingsofuptotheorderof5or6MW,[18]economicconsiderationstypicallyfavormediumvoltage(MV)driveswithmuchlowerpowerratings.DifferentMVdrivetopologies(seeTable2)areconfiguredinaccordancewiththevoltage/current-combinationratingsusedindifferentdrivecontrollers'switchingdevices[19]suchthatanygivenvoltageratingisgreaterthanorequaltoonetothefollowingstandardnominalmotorvoltageratings:
generallyeither2.3/4.16 kV(60 Hz)or3.3/6.6 kV(50 Hz),withonethyristormanufacturerratedforupto12 kVswitching.InsomeapplicationsastepuptransformerisplacedbetweenaLVdriveandaMVmotorload.MVdrivesaretypicallyratedformotorapplicationsgreaterthanbetweenabout375 kW(500 hp)and750 kW(1000 hp).MVdriveshavehistoricallyrequiredconsiderablymoreapplicationdesigneffortthanrequiredforLVdriveapplications.[20][21]ThepowerratingofMVdrivescanreach100 MW,arangeofdifferentdrivetopologiesbeinginvolvedfordifferentrating,performance,powerqualityandreliabilityrequirements.[22][23][24]
[edit]Drivesbymachines&detailedtopologies
ItislastlyusefultorelateVFDsintermsofthefollowingtwoclassifications:
∙IntermsofvariousACmachinesasshowninTable1below[25][26]
∙IntermsofvariousdetailedlowandmediumvoltagetopologiesshowninTable2below.[27][24][23][14][13][28][29]
[show]Table1:
Drivesbymachines
Machines
Induction^^^
Cagemotor
CSI(VVIorPWM),cycloconverter,matrix,VSI
WRIM
Electro-mechanical
DoublyfedWRIM
Slipenergyrecovery(Kramer/Scherbius)
Synchronous^^^
WFSM
CSI(LCI),cycloconverter,VSI
PM
Axialordisk
Radial
Interior
Surface
TrapezoidalBLDM,SinusoidalPMSM
VSI
SyRM
VSI
VRM^^^
Switchedreluctancemotor,Steppermotor
VSI
[show]Table2:
DrivesbydetailedL&MVtopologies(T1toT13)
Topologies(T1toT13)
LVTopologies
IGBT^
T1-Multi-pulsematrix
PrimarilyYaskawa
T2-2-levelVSIPWM
GreatmajorityofLVmakes
T3-3-levelVSIPWM
PrimarilyYaskawa
SCR+IGBT
T4-VVI(six-steporPAM)drive
Forexample,Baker-Hughes
MVVSITopologies
T5-3-levelNPCdrive
GCT^
PrimarilyABB,GE
IGBT^
PrimarilyEaton,GE
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