IEEETRANSACTIONSONENERGYCONVERSION,VOL.19,NO.3,SEPTEMBER2004469DesignandImplementationofPLC-BasedMonitoringControlSystemforInductionMotorMariaG.Ioannides,SeniorMember,IEEEAbstractTheimplementationofamonitoringandcontrolsystemfortheinductionmotorbasedonprogrammablelogiccon-troller(PLC)technologyisdescribed.Also,theimplementationofthehardwareandsoftwareforspeedcontrolandprotectionwiththeresultsobtainedfromtestsoninductionmotorperformanceisprovided.ThePLCcorrelatestheoperationalparameterstothespeedrequestedbytheuserandmonitorsthesystemduringnormaloperationandundertripconditions.TestsoftheinductionmotorsystemdrivenbyinverterandcontrolledbyPLCproveahigheraccuracyinspeedregulationascomparedtoaconventional86102controlsystem.TheefficiencyofPLCcontrolisincreasedathighspeedsupto95%ofthesynchronousspeed.Thus,PLCprovesthemselvesasaveryversatileandeffectivetoolinindustrialcontrolofelectricdrives.IndexTermsComputer-controlledsystems,computerizedmonitoring,electricdrives,inductionmotors,motioncontrol,programmablelogiccontrollers(PLCs),variable-frequencydrives,voltagecontrol.I.INTRODUCTIONSINCEtechnologyformotioncontrolofelectricdrivesbe-cameavailable,theuseofprogrammablelogiccontrollers(PLCs)withpowerelectronicsinelectricmachinesapplicationshasbeenintroducedinthemanufacturingautomation1,2.Thisuseoffersadvantagessuchaslowervoltagedropwhenturnedonandtheabilitytocontrolmotorsandotherequip-mentwithavirtuallyunitypowerfactor3.ManyfactoriesusePLCsinautomationprocessestodiminishproductioncostandtoincreasequalityandreliability49.Otherapplica-tionsincludemachinetoolswithimprovedprecisioncomput-erizednumericalcontrol(CNC)duetotheuseofPLCs10.Toobtainaccurateindustrialelectricdrivesystems,itisnec-essarytousePLCsinterfacedwithpowerconverters,personalcomputers,andotherelectricequipment1113.Neverthe-less,thismakestheequipmentmoresophisticated,complex,andexpensive14,15.Fewpaperswerepublishedconcerningdcmachinescon-trolledbyPLCs.Theyreportboththeimplementationofthefuzzymethodforspeedcontrolofadcmotor/generatorsetusingaPLCtochangethearmaturevoltage16,andtheincorporationofanadaptivecontrollerbasedontheself-tuningregulatortechnologyintoanexistingindustrialPLC17.Also,othertypesofmachineswereinterfacedwithPLCs.Thereby,anindustrialPLCwasusedforcontrollingsteppermotorsManuscriptreceivedDecember2,2002.Paperno.TEC-00004-2002.ThisworkwassupportedbytheNationalTechnicalUniversityofAthens.TheauthoriswiththeFacultyofElectricalandComputerEngineeringoftheNationalTechnicalUniversityofAthens,Athens15773,Greece(e-mail:mioannidece.ntua.gr).DigitalObjectIdentifier10.1109/TEC.2003.822303inafive-axisrotorposition,directionandspeed,reducingthenumberofcircuitcomponents,loweringthecost,andenhancingreliability18.Forswitchedreluctancemotorsasapossiblealternativetoadjustablespeedacanddcdrives,asinglechiplogiccontrollerforcontrollingtorqueandspeedusesaPLCtoimplementthedigitallogiccoupledwithapowercontroller19.OtherreportedapplicationconcernsalinearinductionmotorforpassengerelevatorswithaPLCachievingthecontrolofthedrivesystemandthedataacquisition20.Tomonitorpowerqualityandidentifythedisturbancesthatdisruptproductionofanelectricplant,twoPLCswereusedtodeterminethesensitivityoftheequipment21.OnlyfewpaperswerepublishedinthefieldofinductionmotorswithPLCs.Apowerfactorcontrollerforathree-phaseinductionmotorutilizesPLCtoimprovethepowerfactorandtokeepitsvoltagetofrequencyratioconstantunderthewholecontrolconditions3.Thevectorcontrolintegratedcircuitusesacomplexprogrammablelogicdevice(CPLD)andintegerarithmeticforthevoltageorcurrentregulationofthree-phasepulse-widthmodulation(PWM)inverters22.Manyapplicationsofinductionmotorsrequirebesidesthemotorcontrolfunctionality,thehandlingofseveralspecificanaloganddigitalI/Osignals,homesignals,tripsignals,on/off/reversecommands.Insuchcases,acontrolunitin-volvingaPLCmustbeaddedtothesystemstructure.ThispaperpresentsaPLC-basedmonitoringandcontrolsystemforathree-phaseinductionmotor.Itdescribesthedesignandimplementationoftheconfiguredhardwareandsoftware.Thetestresultsobtainedoninductionmotorperformanceshowimprovedefficiencyandincreasedaccuracyinvariable-loadconstant-speed-controlledoperation.Thus,thePLCcorrelatesandcontrolstheoperationalparameterstothespeedsetpointrequestedbytheuserandmonitorstheinductionmotorsystemduringnormaloperationandundertripconditions.II.PLCASSYSTEMCONTROLLERAPLCisamicroprocessor-basedcontrolsystem,designedforautomationprocessesinindustrialenvironments.Itusesaprogrammablememoryfortheinternalstorageofuser-orien-tatedinstructionsforimplementingspecificfunctionssuchasarithmetic,counting,logic,sequencing,andtiming23,24.APLCcanbeprogrammedtosense,activate,andcontrolin-dustrialequipmentand,therefore,incorporatesanumberofI/Opoints,whichallowelectricalsignalstobeinterfaced.Inputde-vicesandoutputdevicesoftheprocessareconnectedtothePLCandthecontrolprogramisenteredintothePLCmemory(Fig.1).0885-8969/04$20.002004IEEE470IEEETRANSACTIONSONENERGYCONVERSION,VOL.19,NO.3,SEPTEMBER2004Fig.1.ControlactionofaPLC.Inourapplication,itcontrolsthroughanaloganddigitalin-putsandoutputsthevaryingload-constantspeedoperationofaninductionmotor.Also,thePLCcontinuouslymonitorstheinputsandactivatestheoutputsaccordingtothecontrolpro-gram.ThisPLCsystemisofmodulartypecomposedofspe-cifichardwarebuildingblocks(modules),whichplugdirectlyintoaproprietarybus:acentralprocessorunit(CPU),apowersupplyunit,input-outputmodulesI/O,andaprogramterminal.Suchamodularapproachhastheadvantagethattheinitialcon-figurationcanbeexpandedforotherfutureapplicationssuchasmultimachinesystemsorcomputerlinking.III.CONTROLSYSTEMOFINDUCTIONMOTORInFig.2,theblockdiagramoftheexperimentalsystemisillustrated.Thefollowingconfigurationscanbeobtainedfromthissetup.a)Aclosed-loopcontrolsystemforconstantspeedopera-tion,configuredwithspeedfeedbackandloadcurrentfeedback.Theinductionmotordrivesavariableload,isfedbyaninverter,andthePLCcontrolstheinverteroutput.b)Anopen-loopcontrolsystemforvariablespeedoperation.Theinductionmotordrivesavariableloadandisfedbyaninverterinconstantcontrolmode.ThePLCisinactivated.c)Thestandardvariablespeedoperation.Theinductionmotordrivesavariableloadandisfedbyaconstantvoltage-constantfrequencystandardthree-phasesupply.Theopen-loopconfigurationb)canbeobtainedfromtheclosed-loopconfigurationa)byremovingthespeedandloadfeedback.Ontheotherhand,operationc)resultsiftheentirecontrolsystemisbypassed.IV.HARDWAREDESCRIPTIONThecontrolsystemisimplementedandtestedforawoundrotorinductionmotor,havingthetechnicalspecificationsgiveninTableI.Theinductionmotordrivesadcgenerator,whichsup-pliesavariableload.Thethree-phasepowersupplyiscon-nectedtoathree-phasemainswitchandthentoathree-phasethermaloverloadrelay,whichprovidesprotectionagainstcur-rentoverloads.Therelayoutputisconnectedtotherectifier,whichrectifiesthethree-phasevoltageandgivesadcinputtotheinsulatedgatebipolartransistor(IGBT)inverter.Itstech-nicalspecifications25aresummarizedinTableII.TheIGBTFig.2.Electricaldiagramofexperimentalsystem.TABLEIINDUCTIONMOTORTECHNICALSPECIFICATIONSIOANNIDES:DESIGNANDIMPLEMENTATIONOFPLC-BASEDMONITORINGCONTROLSYSTEMFORINDUCTIONMOTOR471TABLEIIINVERTERTECHNICALSPECIFICATIONSinverterconvertsthedcvoltageinputtothree-phasevoltageoutput,whichissuppliedtothestatoroftheinductionmotor.Ontheotherhand,theinverterisinterfacedtothePLC-basedcontroller.ThiscontrollerisimplementedonaPLCmodularsystem5,2628.ThePLCarchitecturereferstoitsinternalhard-wareandsoftware.Asamicroprocessor-basedsystem,thePLCsystemhardwareisdesignedandbuiltupwiththefollowingmodules2937:centralprocessorunit(CPU)；discreteoutputmodule(DOM)；discreteinputmodule(DIM)；analogoutputsmodule(AOM)analoginputsmodule(AIM)powersupply.OtherdetailsofthePLCconfigurationareshowninTablesIIIandIV.Aspeedsensorisusedforthespeedfeedback,acurrentsensorisusedfortheloadcurrentfeedback,andasecondcurrentsensorisconnectedtothestatorcircuits32.Thus,thetwofeedbackloopsoftheclosed-loopsystemaresetupbyusingtheloadcurrentsensor,thespeedsensor,andtheAIM.Atachogenerator(permanentmagnetdcmotor)isusedforspeedsensing.Theinductionmachinedrivesitsshaftmechani-callyandanoutputvoltageisproduced,themagnitudeofwhichisproportionaltothespeedofrotation.Polaritydependsonthedirectionofrotation.Thevoltagesignalfromthetachogener-atormustmatchthespecifiedvoltagerangeoftheAIM(05Vdcand200-kinternalresistance).OtherPLCexternalcontrolcircuitsaredesignedusingalow-voltagesupplyof24Vdc.Forthemanualcontrol,theschemeisequippedwithstart,stop,andtrippushbuttons,aswellaswithaforwardandback-warddirectionselectorswitch.AsshowninFig.2,allofthedescribedcomponents:amainswitch,anautomaticthree-phaseswitch,anautomaticsinglephaseswitch,athree-phasethermaloverloadrelay,aloadautomaticswitch,signallamps(forward,backward,start,stop,trip),pushbuttons(start,stop,trip),ase-lectorswitch(fortheforward/backwarddirectionofrotation),aspeedselector,againselector,aswellasthePLCmodulesandtherectifier-inverterareinstalledinacontrolpanel.Thepro-gramisdownloadedintothePLCfromapersonalcomputerPCandanRS232serialinterface.V.SOFTWAREDESCRIPTIONPLCsprogrammingisbasedonthelogicdemandsofinputdevicesandtheprogramsimplementedarepredominantlylog-Fig.3.Flowchartofthemainprogram.TABLEIIIPLCCONFIGURATIONicalratherthannumericalcomputationalalgorithms.Mostoftheprogrammedoperationsworkonastraightforwardtwo-state“onoroff”basisandthesealternatepossibilitiescorrespondto“trueorfalse”(logicalform)and“1or0”(binaryform),respec-tively.Thus,PLCsofferaflexibleprogrammablealternativetoelectricalcircuitrelay-basedcontrolsystemsbuiltusinganalogdevices.Theprogrammingmethodusedistheladderdiagrammethod.ThePLCsystemprovidesadesignenvironmentintheformofsoftwaretoolsrunningonahostcomputerterminalwhichallowsladderdiagramstobedeveloped,verified,tested,anddiagnosed.First,thehigh-levelprogramiswritteninladderdiagrams,33,34.Then,theladderdiagramisconvertedintobinaryinstructioncodessothattheycanbestoredinrandom-accessmemory(RAM)orerasableprogrammableread-onlymemory(EPROM).EachsuccessiveinstructionisdecodedandexecutedbytheCPU.ThefunctionoftheCPUistocontroltheoperationofmemoryandI/Odevicesandtoprocessdataaccordingtotheprogram.EachinputandoutputconnectionpointonaPLChasanaddressusedtoidentifytheI/Obit.Themethodforthedirectrepresentationofdataassociatedwiththeinputs,outputs,andmemoryisbasedonthefactthatthePLCmemoryisorganizedintothreeregions:input472IEEETRANSACTIONSONENERGYCONVERSION,VOL.19,NO.3,SEPTEMBER2004TABLEIVPLCMODULESANDI/ODESIGNATIONimagememory(I),outputimagememory(Q),andinternalmemory(M).Anymemorylocationisreferenceddirectlyusing%I,%Q,and%M(TableIII).ThePLCprogramusesacyclicscaninthemainprogramloopsuchthatperiodicchecksaremadetotheinputvariables(Fig.3).Theprogramloopstartsbyscanningtheinputstothesystemandstoringtheirstatesinfixedmemorylocations(inputimagememoryI).Theladderprogramisthenexecutedrung-by-rung.Scanningtheprogramandsolvingthelogicofthevariousladderrungsdeterminetheoutputstates.Theupdatedoutputstatesarestoredinfixedmemorylocations(outputimagememoryQ).TheoutputvaluesheldinmemoryarethenusedtosetandresetthephysicaloutputsofthePLCsimultaneouslyattheendoftheprogramscan.ForthegivenPLC,thetimetakentocompleteonecycleorthescantimeis0,18ms/K(for1000steps)andwithamaximumprogramcapacityof1000steps.Thedevelopmentsystemcomprisesahostcomputer(PC)connectedviaanRS232porttothetargetPLC.Thehostcomputerprovidesthesoftwareenvironmenttoperformfileediting,storage,printing,andprogramoperationmonitoring.TheprocessofdevelopingtheprogramtorunonthePLCconsistsof:usinganeditortodrawthesourceladderprogram,convertingthesourceprogramtobinaryobjectcodewhichwillrunonthePLCsmicroprocessoranddownloadingtheobjectcodefromthePCtothePLCsystemviatheserialcommuni