ee 2145230 chapter 8 electric tthavatc/ch 8 electric 2145230 chapter 8 electric motor ......

Download EE 2145230 Chapter 8 Electric  tthavatc/Ch 8 Electric   2145230 Chapter 8 Electric Motor ... generation of a rotating magnetic field. ... EE 2145230 Aircraft Electricity and Electronics

Post on 08-Mar-2018

216 views

Category:

Documents

4 download

Embed Size (px)

TRANSCRIPT

  • 1

    EE2145230Chapter8ElectricMotor

    8.1MotorTheory

    Anelectricmotorisadevicethatchangeselectricalenergytomechanicalenergy.

    Electrical Energy

    Mechanical Energy

    Motor

    Generator

    MotorscanbeclassifiedasDCandACmotors.

    Electromagnetism:Wheneveranelectriccurrent flows inaconductoramagnetic field is setup in thespacesurroundingtheconductor.Thefieldspreadsoutaroundtheconductor inconcentriccircleswiththegreatestdensityofmagneticfluxnearesttotheconductor.

    Ifweplaceacurrentcarryingconductorinamagneticfield,theconductorhasaforceexertedonit.Iftheconductorisfreetomovethisforcewillproducemotion.

  • 2

    8.2DCMotor

    DCmotorsaredescribedinpartbythetypeoffieldwindingandarmaturewindingconnection.Therearefourtypes:separatelyexcited,series,shuntandcompoundDCmotor.

    TheprincipleofDCmotor: ( )F I l B= F Bli= tT K Ia=

    Repulsionoflikemagneticpolesandattractionofunlikemagneticpoles.

    CharacteristicsofDCmotors

    Therearetwomainwindings:

    1.Fieldwinding(Stator):Thiswindingprovidesmagneticfluxandisstatic.

    2.Armaturewinding(Rotor):CurrentcarryingwiresinB(fromStator)ForceTorque.

  • 3

    Theactionofthecommutatorcontinuallyswitchestheinputcurrenttonewsectionsofthearmaturewindingsothatthetopofthearmatureisalwaysanorthpole;hencethearmaturecontinuestorotateinanefforttoalignitselfwiththefieldpoles.

    CounterorBackemf:Asanarmaturerotatesandcutsthroughthemagneticfluxfromthefieldcircuit,aninducedvoltageoccurs.Thisisbecauseanelectromotiveforce(emf)willbeinducedacrosstheendsofaconductorwhenthereisrelativemotionbetweenitandamagneticfield.

    ( )e l v B= tE K N=Kb =

    KVLonarmaturecircuit: b t a aE V I R=

    SeparatelyexcitedDCmotor:Fieldcircuitandarmaturecircuitareseparated.

  • 4

    ShuntDCmotor: L a fI I I= +

    Lowstartingtorque

    Thespeedisalmostconstantforallreasonableloads.

    Loads:electricfansandpumps

    SeriesDCmotor: T A A A F AV E R I R I= + +

    Highstartingtorque

    Ifaseriesmotorisnotconnectedmechanicallytoaload,thespeedofthemotorwillcontinuetoincreaseforaslongasthecounteremfissubstantiallybelowtheappliedemf.Thespeedmayincreasefarabovethenormaloperatingspeedofthemotor,andthismayresultinthearmatureflyingapartbecauseofthecentrifugalforcedevelopedbytherapidrotation.Aseriesmotorshouldalwaysbeconnectedmechanicallytoaloadtopreventitfromrunningaway.

    Loads:enginestarter,landinggear

  • 5

    DCmotorefficiency:Losses100% 100% 100%

    Lossesout out in

    in out in

    P P PP P P

    = = = +

    Fourmajorlossesare

    1.Copperlosses(Culosses):lossesfromfieldandarmaturewindings

    2.Coreloss:HysteresisandEddycurrentlosses

    3.Mechanicallosses:Frictionandwindagelosses

    4.Strayloadlosses:1%ofoutputpower

    Notethatcoreandmechanicallossesarecalledrotationallosses.

    Speedcontrol

    1. Forseparatelyexcitedmotors,speedcanbecontrolledbyvaryingthefieldcurrent,varyingthearmaturesourcevoltageorbyinsertingadditionalresistanceinserieswiththearmature.

    2. Forshuntconnectedmotors,speedcanbecontrolledbyvaryingthefieldcurrentorbyinsertingadditionalresistanceinserieswiththearmature.

    3. Forseriesconnectedmotors,itstorqueisalmostinverselyproportionaltothesquareofitsspeed.Thistypeofmotorissuitableforstartingheavyloads.Itcanreachdangerousspeedsiftheloadistotallyremoved

    8.3ACMotor

    BecausethespeedofanACmotorisdeterminedbythefrequencyoftheACsupplythatisappliedit,ACmotors arewell suited to constant speed applications.Theprincipleof allACmotors isbasedon thegenerationofarotatingmagneticfield.Itisthisrotatingfieldthatcausesthemotorsrotortoturn.

    ACmotorscanbegroupedinto1)Synchronousmotorsand2)Inductionmotors

    Ifthreewindingsareplacedroundastatorframe,andthreephaseACisappliedtothewindings,themagneticfieldsgeneratedineachofthethreewindingswillcombineintoamagneticfieldthatrotates.Atanygiveninstance,thesefieldscombinetogetherinordertoproducearesultantfieldthatwhichactsontherotor.Therotorturnsbecausethemagneticfieldrotates.

  • 6

    Synchronousmotor:ACisappliedtothestatorandDCisappliedtotherotor.

    IftherotorwindingisenergizedwithDC,itwillactlikeabarmagnetanditwillrotateinsympathywiththe rotating field.The speedof rotationof themagnetic fielddependson the frequencyof the threephase AC supply and, provided that the supply frequency remains constant, the rotorwill turn at aconstantspeed.Furthermore,thespeedofrotationwillremainconstantregardlessoftheloadapplied.

    Thesynchronousmotorissocalledbecauseitsrotorissynchronizedwiththerotatingfieldsetupbythestator.ItsconstructionisessentiallythesameasthatofasimpleACgenerator(alternator).

    Synchronousmotorsarenotselfstartingandmustbebroughtuptonearsynchronousspeedbeforetheycancontinuerotatingby themselves.When themotorspeedreachesapproximately97%ofnameplateRPM,theDCfieldcurrentisappliedtotherotorproducingPullinTorqueandtherotorwillpullinstepandsynchronizewiththerotatingfluxfieldinthestator.ThemotorwillrunatsynchronousspeedandproduceSynchronousTorque.

    TherequirementtohaveanexternalDCvoltagesourceaswellastheACfieldexcitationmakesthistypeofmotorsomewhatunattractive!

  • 7

    Advantagesofsynchronousmotors

    1)Precisespeedregulationmakesthesynchronousmotoranidealchoiceforcertainindustrialprocessesandasaprimemoverforgenerators.

    2)Synchronousmotorshave speed / torque characteristicswhichare ideally suited fordirectdriveoflargehorsepower,lowrpmloadssuchasreciprocatingcompressors.

    3)Synchronousmotorsoperateatan improvedpower factor, thereby improvingoverallsystempowerfactorandeliminatingorreducingutilitypowerfactorpenalties.Animprovedpowerfactoralsoreducesthesystemvoltagedropandthevoltagedropatthemotorterminals.

    Inductionmotor(IM):mostcommonlyusedACmotors.

    The inductionmotorderives itsnamefromthefactthatACcurrentsare induced intherotorcircuitbythe rotatingmagnetic field in the stator. The stator construction of the inductionmotor and of thesynchronousmotorisalmostidentical,buttheirrotorsarecompletelydifferent.

    Twotypesofrotorwindingare1)Squirrelcageand2)Woundrotor

    Theessentialpartsofaninductionmotoraretherotorandthestator.Thestatorisintheformofashellwithlongitudinalslotsontheinnersurface.Therotorinaninductionmotorconsistsofalaminatedironcoreinwhichareplacedlongitudinalconductors.Additionalresistancecanbeaddedtowoundrotortypethroughsliprings,whilenoresistancecanbeaddedforsquirrelcagetype.

  • 8

    Stator Winding Squirrel cage rotor

    End ring

    3 supply

    Stator Winding Wound rotor winding

    Slip ring

    Brushes

    External variable resistor

    3 supply

    (Stator Winding may be Y or connection.)

    The induction motor is the most commonly used AC motor because of its simplicity, its robustconstruction and its relatively low cost. These advantages arise from the fact that the rotor of aninductionmotor isaselfcontainedcomponentthat isnotactuallyelectricallyconnectedtoanexternalsourceofvoltage.

    Theinductionmotorhasthesamestatorasthesynchronousmotor.Therotorisdifferentinthatitdoesnotrequireanexternalsourceofpower.Currentisinducedintherotorbytheactionoftherotatingfieldcuttingthroughtherotorconductors.Thisrotorcurrentgeneratesamagneticfieldwhich interactswiththestatorfield,resultinginatorquebeingexertedontherotorandcausingittorotate.

    Therotatingmagneticfieldgeneratedinthestatorinducesane.m.f.intherotor.Thecurrentintherotorcircuitcausedbythisinducede.m.f.setsupamagneticfield.Thetwofieldsinteract,andcausetherotortoturn.Notethattherotorcircuitisaclosedloopcircuit.

    Therotorofan inductionmotorrotatesat lessthansynchronousspeed, inorderthattherotatingfieldcan cut through the rotor conductors and induce a current flow in them. This percentage differencebetweenthesynchronousspeedandtherotorspeed isknownasslip.Slipvariesvery littlewithnormalload changes, and the inductionmotor is therefore considered to be a constantspeedmotor. If thespeedswereexactlythesame,norelativemotionwouldexistbetweenthetwo,andsonoe.m.f.wouldbeinducedintherotor.

  • 9

    ThreephaseIM

    Synchronousspeed(Ns)canbecalculatedfrom2 12060s

    fN fP P

    = =

    Slip(s)canbecalculatedfrom s r

    s

    N NsN

    = whereNristherotorspeed

    Developedtorque22 2

    2 22 2

    ( )

    ksE RTR sX

    =+

    Whentheloadonaninductionmotorbecomessogreatthatthetorqueoftherotorcannotcarryit,themotorwillstop.Thisiscalledthepulloutpoint.

    Inductionmotorefficiency

    3 cosL LV I

  • 10

    Fourmajorlossesare

    1.StatorCopperlosses(PSCL):lossesfromthestatorwinding

    2.Coreloss:HysteresisandEddycurrentlosses

    3.RotorCopperlosses(PRCL):lossesfromtherotorwinding

    4.Mechanicallosses:Frictionandwindagelosses

    5.Strayloadlosses:1%ofoutputpower

    TwophaseIM

    Inthecaseofatwophaseinductionmotor,twowindingsareplacedatrightanglestoeachother.Byexcitingthesewindingswithcurrentwhichis90 degreesoutofphase,arotatingmagneticfieldcanbecreated.

    SinglephaseIM

    Asinglephaseinductionmotor,ontheotherhand,hasonlyonephase.Thistypeofmotorisextensivelyusedinapplicationswhichrequiresmalllowoutputmotors.Theadvantagegainedbyusingsinglephasemotorsisthatinsmallsizestheyarelessexpensivetomanufacturethanothertypes.Alsotheyeliminatetheneedforathreephasesupply.

    Asinglephaseinductionmotorhasonlyonestatorwinding;thereforethemagneticfieldgenerateddoesnot rotate.Asinglephase inductionmotorwithonlyonewindingcannotstart rotatingby itself.Somemeansof starting is required forall singlephase inductionmotors.Once the rotor is started rotating,however,itwillcontinuetorotateandcomeuptospeed.

  • 11

    ThreetypesofcommonlyusedsinglephaseIM

    1)Splitphase IM:This IMshouldnotstartfrequently.Ratingbetween60W250W(1/12hp1/3hp).Splitstatorwindi