International Research Journal of Engineering and Technology (IRJET)e-ISSN: 2395 -0056 Volume: 02 Issue: 04 | July-2015 www.irjet.net p-ISSN: 2395-0072 2015, IRJET.NET- All Rights Reserved Page 1336 Steady state Direct Torque Control of induction motor by using Anti-windup PI controller Saidas M.G1 , Hima.T2 1 2Student, Electrical And Electronics Department, Adi Sankara Institute Of Engineering And Technology, Kerala, India ---------------------------------------------------------------------***---------------------------------------------------------------------Abstract-Inthisworkalow-costthree-phase inductionmotor(IM)underdirecttorquecontrol(DTC) usingtheinformationobtainedfromonlyoneshunt resistor(inserieswithlowsideswitchesina conventionalthree-phaseinverter).Theaimisto developedhighperformanceinductionmotordriveby obtaining the speed from the induction motor .From the obtainedspeedtorqueiscontrolled.Thepresented algorithmisrobustandverysimple.Atheoretical conceptisdeveloped,themodifiedlookuptableis presented.Problemslikewindupphenomenonappears andresultsinperformancedegradationwhenthe Proportional-Integral(PI)controlleroutputis saturated.AnewantiwindupPICONTROLLERis proposedtoimprovecontrolperformanceofvariable speedmotordrives.Thesteadystatevalueofthe integral state is predicted while the PI controller output issaturated.Thusthetorqueripplesarereduced.In thisworkcomparisonofthepicontrollerandtheanti-windup controller is performed.. Key Words: Induction motor, Direct torque control, pi controller, anti-windup pi controller etc 1. Introduction Thebasicconceptofdirecttorquecontrol(DTC)of induction-motordrivesistocontrolbothstatorflux-linkageandelectromagnetictorqueofthemachine simultaneouslybyusingaswitchingvectorlook-uptable. TheDTCstructureissimplecomparedtoavectorcontrol algorithm,becauseitdoesnotrequirecoordinate transformationandvoltagemodulationblock,andthus, can be implemented relatively easily. In addition it has fast dynamicperformance.WhileDTCiswidelyusedbecause oftheseadvantages,ithasdisadvantagessuchashigh ripplesintorqueandpossibleproblemsduringstartor lowspeedoperationandduringchangesintorque command.Toovercometheseshortcomings,various approachesforfluxandtorqueripplereduction.Mostof themareconcernedwithimprovementofthefluxand torqueestimatorandcombinedoperationofDTCwitha spacevector-modulation(SVM)techniqueshortcomings, variousapproachesforfluxandtorqueripplereduction. Most of them are concerned with improvement of theflux and torque estimator and combined operation of DTC with a space vector-modulation (SVM) technique. Ananti-windupcompensatorconsistsofanominal(most oftenlinear)controllerappendedwithanti-windup compensation.Animportantpropertyofanti-windup compensationisthatitleavestheloopunaffectedaslong assaturationdoesnotoccur.Consequently,thecontrol actionprovidedbytheanti-windupcompensatoris identicaltothatofthenominalcontroller,aslongasthe controlsignalsoperatewithinthesaturationlimits.The designcanbesplitintotwopartswherethefirstpart concernsthelinearcontrollerandthesecondtheanti-windupmodificationAnti-windupwasoriginallyusedfor preventingtheintegratorstateinPIDcontrollersfrom growing large and cause overshoots and limit cycles.Wheneveralinearcontrollerhasbeen designedundertheassumptionthatitsoutputwillaffect theplantinputdirectlyandunaltered,then,anyinput nonlinearity,suchasrate-and/oramplitudesaturation, causingdeviationbetweenthecontrolleroutputandthe plant input, almost always degrades the performance, and stability of the closed loop system may be put at risk Anti-windup compensation is the simplest and most commonly usedmodificationofalinearcontroller,aimingat retainingstabilityandmostoftheperformanceinsucha system. 1.1 Direct torque control In the direct torque control (DTC) of induction motor, flux linkage, and electromagnetic torque are controlled directly by the selection of a switching vector from a look-up table. However,theselectedvectorisnotalwaysthebestone International Research Journal of Engineering and Technology (IRJET)e-ISSN: 2395 -0056 Volume: 02 Issue: 04 | July-2015 www.irjet.net p-ISSN: 2395-0072 2015, IRJET.NET- All Rights Reserved Page 1337 sinceonlythesectorisconsideredwherethefluxlinkage space vector lies without considering its accurate location. TheDTCisoneofthemostexcellentdirectcontrol strategiesofstatorfluxandtorqueripplesofIMD.The conventionalDTCmethodhasbeenoptimizedbyusingPI controller in the speed regulating loop of IMD. Fig-1:Direct torque control ThemostpopularvariationofDTCofinductionmotor drives is the one that is based on space vector modulation (SVM), which normally referred to as DTC-SVM .The major difference between DTC-hysteresis-based induction motor and DTC-SVM is the way the stator voltage is generated. In DTC-hysteresis-basedinductionmotor,theappliedstator voltagedependsonvoltagevectors,whichareselected fromalookuptable.Theselectionsarebasedonthe requirementofthetorqueandfluxdemandsobtained fromthehysteresiscomparators.Ontheotherhand,in DTC-SVM,astatorvoltagereferenceiscalculatedor generatedwithinasamplingperiod,whichisthen synthesizedusingthespacevectormodulator.Thestator voltagereferencevectoriscalculatedbasedonthe requirement of torque and flux demands. 2.1 Integral WindupIntegral windup, also known as integrator windup or reset windup ,refers to the situation in a PID feedback controller wherealargechangeinsetpointoccurs(sayapositive change)andtheintegraltermsaccumulatesasignificant errorduringtherise(windup),thusovershootingand continuingtoincreaseasthisaccumulatederroris unwound(osetbyerrorsintheotherdirection).The specific problem is the excess over shooting .problems are Integral windup particularly occurs as a limitation ofphysicalsystems,comparedwithidealsystems,dueto theidealoutputbeingphysicallyimpossible(process saturation:theoutputoftheprocessbeinglimitedatthe toporbottomof itsscale,makingtheerrorconstant).For examplethepositionofavalvecannotbeanymoreopen thanfullyopenandalsocannotbeclosedanymorethan fullyclosed.Inthiscaseintegralwindupcanactually involvetheintegratorbeingturnedoforperiodsoftime until the response falls back into an acceptable range. This usuallyoccurswhenthecontroller'soutputcannolonger a ect the controlled variable, or if the controller is part of a selectionschemeanditisselectedright.Integralwindup wasmoreofaprobleminanalogcontrollers.Within modernDistributedControlSystemsandProgrammable LogicControllers,itismucheasiertopreventintegral windupbyeitherlimitingthecontrolleroutput,orby using external reset feedback, which is a means of feeding backtheselectedoutputtotheintegralcircuitofall controllers in the selection scheme so that a closed loop is maintained. 2.2 Anti-WindupAnanti-windupcompensatorconsistsofanominal (mostoftenlinear)controllerappendedwithanti-windup compensation.Animportantpropertyofanti-windup compensationisthatitleavestheloopunaffectedaslong assaturationdoesnotoccur.Consequently,thecontrol actionprovidedbytheanti-windupcompensatoris identicaltothatofthenominalcontroller,aslongasthe controlsignalsoperatewithinthesaturationlimits.The designcanbesplitintotwopartswheretherestpart concernsthelinearcontrollerandthesecondtheanti-windup modification . Anti-windupwasoriginallyusedfor preventing the integrator state in PID controllers from growinglargeandcauseovershootsandlimitcycles .Whenever a linear controller has been designed under theassumptionthatitsoutputwillaffecttheplant inputdirectlyandunaltered,then,anyinput nonlinearity,suchasrate-and/oramplitude saturation,causingdeviationbetweenthecontroller output and the plant input, almost always degrades the performance,andstabilityoftheclosedloopsystem maybeputatrisk.Anti-windupcompensationisthe simplestandmostcommonlyusedmodificationofa linear controller, aiming at retaining stability and most of the performance in such a system. International Research Journal of Engineering and Technology (IRJET)e-ISSN: 2395 -0056 Volume: 02 Issue: 04 | July-2015 www.irjet.net p-ISSN: 2395-0072 2015, IRJET.NET- All Rights Reserved Page 1338 Fig -2: Anti-windup 2.2.1 Anti-WindupPID, PIControllersIn Direct Torque Control Thewindupphenomenonappearsandresultsin performancedegradationwhentheproportional-integral-derivative (PID) controller output is saturated. Integral windup is analyzedon the PI plane, and anew anti-windupPIDcontrollerisproposedtoimprove controlperformanceofvariable-speedmotordrives andisexperimentallyappliedtothespeedcontrolofa vectorcontrolledinductionmotordrivenbyapulse width-modulatedvoltagesourceinverter.Thesteady-statevalueoftheintegralstateispredictedwhilethe PIDcontrolleroutputissaturated,andthisvalueis utilized as an initial value of the integral state when the PIDcontrollerbeginstooperateinalinearrange. Simulationandexperimentsresultinmoresimilar speedresponsesagainstloadconditionsandstep referencechangeoverconventionalanti-windup schemes.Controlperformance,suchasovershootand settlingtime,isverysimilartothatdeterminedbyPID gain in the linear range. Hence it becomes necessary to employ anti-windup strategy to prevent the controller from going into deep saturationandtocheckwinduporrolloverof controlleroutput.Anti-windupschemesforPI controllerareawellresearchedtopicthoughnewer andinnovativemethodsarestillcomingup.Several suchschemesforPIDcontrollersutilizinglimitedor conditionalintegrationmethodandtrackinganti-windup method. 2.2.2Anti-windup PI control Transfer function of a PI controller is expressed as GPI (s) =Kp+Ki/sHereKpistheproportionalconstantandKiis theintegralconstant.Theoutputfromcontrollerycanbe expressed as y = Kpe + Ki *(e*e)dt(1) Here e is the input error. Fig-3:Anti-Windup PI Control 2.3 Mathematical Modelling Of Induction MotorElectrical system equations: vs = Rsis + 1=w0(dfs=dt) + wkM(pi=2)fs(2) vr = Rrir + 1=w0(dfr=dt) + (wkwm)M(pi=2)fr (3) where the variables i,v,and f are 2-dimensional space vectors; for instance is = [idsiqs] (4) wk is the speed of the reference frame, wm the rotor speed, M(pi/2)represents a 90 degree space rotator namely M(pi/2) = [0 -1;1 0](5) Theflux linkage current relations are: fds = Lsids + Lmidr(6) fdr = Lmids + Lridr(Daxis)(7) Ls = Lm + Lsl; Lr = Lm + Lrl (8) fqs = Lsiqs + Lmiqr (9) fqr = Lmiqs + Lriqr(Qaxis)(10) Mechanical system equations: Te = 2H(dwm=dt) + Bmwm + Tl (11) where Te=fs(cross)is= M(pi/2) fs (dot) iss = fds iqs-fqsids= fs x iss = M(pi/2) fs =ir fr=Lm(ir is)=Lm/Lrr(fr iss) = 1/Lm0 (fr x fs) Lm0 =Lmc/(1-c)c=1 - Lm2/(Ls Lr). 2.4 Resultsand Discussions As the result observed from simulation by using the PI controller in the DTC settling timeand the torque ripples are increased. When Anti-windup PI controller is used in DTC settling time is achieved quickly and the torque ripples are reduced at a time period .3s. International Research Journal of Engineering and Technology (IRJET)e-ISSN: 2395 -0056 Volume: 02 Issue: 04 | July-2015 www.irjet.net p-ISSN: 2395-0072 2015, IRJET.NET- All Rights Reserved Page 1339 Fig -4: current with PI control Fig-5:Current with Anti-windup PI control Fig-6: Torque with Anti-windup PI control Fig-7: Speed with Anti-windup PI and PI control 3. CONCLUSIONS ThemainreasonforpopularityofDTCisduetoits simplestructure,particularlywhencomparedwitheld-orientedcontrol(FOC)scheme,whichwasintroduceda decadeearlier.ThemainadvantagesofferedbyDTCare decoupledcontroloftorqueandstatorfluxExcellent torquedynamicswithminimalresponsetime.Inherent motion-sensorlesscontrolmethodsincethemotorspeed isnotrequiredtoachievethetorquecontrol.Absenceof coordinate transform (required in FOC) Absence of voltage modulatoraswellasothercontrollersItisclearfromthe DTCthatdecoupledcontrolofstatorfluxandtorqueis possible From the overall it is clear that the DTC using the pihastorqueripplesbyusingtheanti-winduppi controller scheme the torque ripples can be reduced. ACKNOWLEDGEMENT FirstofallIthankLordAlmightyforthedivinegrace bestowed on me to complete this project successfully. Iwouldliketoexpressmysinceregratitudetoour institution,AdiSankaraInstituteofEngineeringand Technology, Kalady. I also thank our honorable Principal Dr.N. Hariharan for providing us with all the facilities for making the project a success. IamgratefultotheHeadoftheDepartment Mrs.Krishnakumari.T for her constant encouragement and support. Iwouldliketoextendmygratitudetomyguides Mrs.Hima.TandMrsGomathy.S(AssistantProfessorsof EEEdept)fortheirinspiringassistance,supportand timelyadviceandforherenterprisingattitudethatmade my project successful. AlsoIwouldliketothankourPGPPSri.A.K.Divakara Menonforgivingusinnovativesuggestionsandassisting us in times of needs. Thisacknowledgementwillstandincompleteifmy parents,friendsandclassmatesarenotthankedwhose constantencouragement and timely criticism helped usto agreatextent.Theywereinstrumentalinkeepingmy spirithighandtheirassociationwithmewillbealways remembered. 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[6]Anti-windupSchemesforProportionalIntegraland ProportionalResonantControllerAnirbanGhoshal and Vinod John Department of Electrical Engineering, IndianInstituteofScience,BangaloreEmail: anirban@ee.iisc.ernet.in,vjohn@ee.iisc.ernet.inNA-TIONAL POWER ELECTRONIC CONFERENCE 2010 . [7]C. Lascu, I. Boldea, and F. Blaabjerg, A modified direct torquecontrolforinductionmotorsensorlessdrive IEEETrans.Ind.Appl,vol.36,no.1,pp.122130, Jan./Feb. 2000.[8]AnOptimizedSwitchingStrategyforQuickDynamic TorqueControlinDTC-Hysteresis-BasedInduction MachinesAuzaniJidin,StudentMember,IEEE,Nik RumziNikIdris,SeniorMember,IEEE,AbdulHalim MohamedYatim,SeniorMember,IEEE,ToleSutikno, StudentMember,IEEE,andMalikE.Elbuluk,Senior Member,IEEE.IEEETRANSACTIONSONINDUSTRIAL ELECTRONICS, VOL. 58, NO. 8, AUGUST 2011[9]Anti-WindupPIDControllerWithIntegralState PredictorforVariable-SpeedMotorDrivesHwiBeom Shin,Member,IEEE,andJongGyuParkIEEE TRANSACTIONSONINDUSTRIALELECTRONICS,VOL. 59, NO. 3, MARCH 2012. 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Electron., vol. 58, no. 2, pp. 629646, Feb. 2011. [15] B.Bahrani, S.Kenzelmann, and A.Rufer, Multivariable-PI-basedd-qcurrentcontrolofvoltagesource converterswithsuperioraxesdecouplingcapability, IEEETrans.Ind.Electron.,vol.58,no.7,pp. 30163026, Jul. 2011. BIOGRAPHIES SaidasM.Gwasbornon11th January1991.Hereceivedhis Bachelor Of Technology Degree In ElectricalAndElectronicsfrom CmsCollegeOfEngineering CoimbatorecunderAnna University2013.Heiscurrently pursuing Master Of Technology In PowerElectronicsAndPower SystemAtAdiSankaraInstitute OfEngineeringAndTechnology Kalady,Ernakulam.Hiscurrent research interests include electric drivescontrolespeciallyindirect torque control of induction motor usingSlidingModeAndAnti-Windup PI Control. Hima.Twasbornon12th may1986.ShereceivedB-Tech DegreeInElectricaland ElectronicsfromAdiSankara InstituteofEngineeringAnd TechnologyKalady,Ernakulam.She receivedM-TechDegreeIn GuidanceAndNavigational ControlfromCETin2010.Heis currentlyworkingasAsst professor in Adi Sankara Institute OfEngineeringAndTechnology Kalady,Ernakulam.Hiscurrent researchinterestsincludecontrol system,electric drives.