Capitulo 12 .docx

8/12/2019 Capitulo 12 .docx

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418 ELECTRICALCONTROLSYSTEMSININDUSTRY9.Describehowapro ces sin glinelikethatofFig.11'9,inwhichmaterialis

unwoundfroma reelat one en dandrewoundonanotiierreelat th eotherend,canbekeptincontinuous operation,withoutshutdown,\\'hileeitherendisstoppedwhenitisnecessarytoreplaceanem|)t\'reel(a ttheentranceend)orafullreel(at the finishedend).Indicateclearlythe functionofth ein-feedan dout-feedaccumulatorsan dthe mannerinwhichmotorsar espeededup orsloweddownduringchange-overper iod s. CHAPTER

SpecialControlCircuitsandProblems Industrial controlengineersare frequentlycalledupon tosolveunusual

problems,designspecialcircuitsthatmustfulfilldefinitespecifications, modifyexistingcircuitstoimproveoperatingconditions,ordevelopcircuitarrangementsthatemploynewtypes-ofequipment.Considerableexperienceaswellasspecializedtrainingaregenerallynecessaryifthe assignmentsindicatedaretobecarriedoutsothatthecircuitsnotonlyaresafeandreliablebutdonotmalfunctionunderthemostadverseconditionsorevenwhenthe operatorfails to followinstructionsproperl y.Also,toreducecostandminimizeservicingrequirements,thedesignermustuseasfewcomponentsaspossible.Thischapterwillconcernitselfwithagreatmanyproblemsandcircuits

ofvaryingdegreesofcomplexityandwillattempt to show,by illustrativeexamples,howcircuitsandnumericalsolutionssatisfygivenspecifications.Althoughtheoperationofthevariouscircuitdiagramswillnotbeexplained,itisrecommendedthatthestudentmakeeveryefforttoanalyzethemor,perhaps,suggestalternatesolutionsofhisown.Suchexerciseswillcertainlyservetodevelopconfidenceandskillinthehandlingofindustrialcontrolsystems.

Problem1.Threeline-startmotors aretobestartedand stoppedfromasinglepush-butt onstation.Each istohaveitsindividual across-the-linestarter,andthereistobeasmalltimedelaybetweenstartsasthemachinesaccelerateinsequence.Also,thetrippingoftheoverloadrelayononestartermuststopthepart icul armotorandnotaffecttheothertwo.Designacontrolcircuittofulfillthe given specifications.

419


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420 ELECTRICALCONTROLSYSTEMSININDUSTRY SPECIALCONTROLCIRCUITSANDPROBLEMS42 1Solution(SeeFig.12-1) Problem3.It isdesiredtocontrol twomotorsinthe followingmanner:

a.Depressingthestartbut tonatthestart-stopstationwillstartmotorNo.1.Thismotorwillcontinuetorununtilitisstoppedeitherbythepressingofthes topbut ton orbythetrippingofitsoverloadrelay.

b.WhenmotorNo.1stops,motorNo.2istostartand,afterrunningfor1min,willstop.

Drawa linediagramforthecontrolcircuitonly.Solution(SeeFig.12-3)

Problem2.Itisdesiredtostarttwoline-startmotorsfromasingleSTART-STOPstation simultaneouslyunderthe followingconditions:

a.Depressingthes topbut tonorthe trippingoftheoverloadrelayofmotorNo.1shallstop thatmotoronly,and4seclater,motorNo.2istostopautomatically.

b. ThetrippingoftheoverloadrelayonstarterNo.2shallstopbothmotorsimmediately.

Designacontrolcircuittofulfillthegiven specifications.Solution(SeeFig.12-2)

Fig.12-2 SolutiontoProb.2,

Problem4.Designacontrolcircuitthatwillpermitfivemotorstooperatein the followingmanner;

a.WhenmotorNo.1isstarted,motorNo.2willalsostartafterashorttimedelayandwillrun onlywhileNo.1isinoperation.

b.It shallnotbepossibletostartmotorsNos.3,4,or5unlessmotorsNos.1and2arerunning.

c.It shallnotbepossibletostartmotorNo.3whenNo.4isrunningorNo.4whenNo.3isrunning.d.ThetrippingoftheoverloadonmotorNo.1or2shallstopallmotors.

e.ThetrippingoftheoverloadinmotorNo.5shallstopthelatterandmotorNo.3or4,whichever isrunning.


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422 ELECTRICALCONTROLSYSTEMSININDUSTRY

Solution(SeeFig.124)

Problem5.It isdesiredtostartan dstopfourmotorsfromasingleSTART-STOPstation.Therearetobefourindependentacross-the-line starters,and th ecircuitistofunctionasfollows:

a.Eachoverloadrelayistocarryth ecurrentofone contactorcoil.b.NoSTARTorSTOPbu tt on istocarrymorethanth ecurrentofone

contactorcoil.c.Th etrippingofanyoverloadrelayshallstopalloperatingmotors. d.Theauxiliarycontactofan ystartershallno tcarrymorethanth e

current ofonecontactorcoil.Designacontrolcircuitthatwillfulfillth egivenspecifications'.Solution(SeeFig.12o)

Fig.12-5 Solutionto Prob.5

SPECIALCONTROLCIRCUITSANDPROBLEMS423Problem6.Designacontrolcircuitforafive-motorconveyorsystem

thatwilloperateinaccordancewiththe followingspecifications: a.Eachmotoristohaveitsownacross-the-line starteranditsown

START-STOPstation.b.ItshallbenecessarytostartmotorsNos.1to5innumerical

sequence.Thisshallbedonebydepressingth estartbu tt ons insuccession,'and onlyonemotorat atimeshallstartwhenastartbu tt on isdepressed.

c.Ifth epre ssi ngofth estopbu tt on orth etrippingofth eoverloadrelayofmotorNo.5stopsthe latter,it shallbeth eonlyonetostop.

d.ThestoppingofmotorNo.1shallstopallmotors.e.Thestoppingofany motorbyth epre ssi ngofas topbu tt on orthe

trippingofitsoverloadrelayshallstopthe pa rti cu larmotorand,inaddition,allhighernumberedmotors.

Thiscontrolschemeisfrequentlyusedin conveyorsystemstopr even tapile -upofmaterialshouldoneofth e"take-away"sectionsstop.

t,Solution(SeeFig.12-6)

STARTSTOPI

L_ttIOO~oIo- j -o

0L3STARTSTOP tQI"-po O--if-4MSI

5MFig.12-SolutiontoProb.6.

Problem7.Thecontrolcircuitforareversingcompoundmotor(Fig.3-15)is to bemodified sothat th emachinecan bemade to slowdownwhilerunningnormallyin forwardonly.OnemethodofaccomplishingthisistoprovideanarmatureshuntasinFig.III-CofTable7-1,page207.Showschematicallyhowthiscanbedonebyincorporatingaspecialcontactandaresistorinsuchacircuit.


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424 ELECTRICALCONTROLSYSTEMSININDUSTRY SPECIALCONTROLCIRCUITSANDPROBLEMS 42 5 Solution(SeeFig,12-7)

Fig.12*7 Solutionto Prob.7.Problem8.Asmallsynchronousmotor rotatesacamwhichrepeatedly

closesan dopensacontactevery15sec.It isdesiredtomakethiscontactac ton two independent circuitsAan dBinth efollowingmanner:

a.Fo rth efirstcontactclosing,

circuitAisenergized.

b.Whenth econtactopens,circuitAisdeenergized.c.Fo rth esecondcontactclosing,circuitBisenergized. d.Whenth econtactopens,circuitBisdeenergized. "Th eabovecycleofoperationsrepeatsitself.Designacontrolcircuitforth elistedcycleofoperations.Solution(SeeFig.12-8)

Problem9.It isdesiredto operatetwo motorsfrom asinglestart-stopstationunderth efollowingconditions;a.Th epre ssi ngofth estartbu tt on shallstartmotorNo.1,and30

seclatermotorNo.2shallstart. b.AftermotorNo.2has bee nrunningfor1min,itshallstop;motorNo.1shallcontinueto run.

c.Th epres singofth estopbu tt on shallstopbo th motors. d.If,during operation, th eoverloadonmotorNo.2trips,bo th motorsmuststop.

e.Th eoperatingcycleofmotorNo.2mustnot beaffectedbyth etrippingofth eoverload onmotorNo.1.

Designacontrolcircuittofulfillth egivenspecifications.Solution(SeeFig.12-9)

Fig.12-9 Solutionto Prob.9. Problem10.Six grid resistorsare available.Fourofthemare tappedat

centerpoi nts ,an dtwo ofthemhavetapsone-third fromth eends.a.Ifth etotalresistanceofeachofth efirstfoursectionsis6ohms

(3ohms between taps)andth e last two sections has atotalof9ohms(3ohmsbe twe entaps),howshouldth eresistorsbeconnectedtoyieldatotalresistanceof2ohms?

b.12.75ohms?Solution(SeeFig.12-10)

32 323Q 32

Fig.12-8 Solutionto Prob.8. Fig.12*10Solutionto Prob,10. 1 3n 3n1

1 30 31 311 13 1


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426 ELECTRICALCONTROLSYSTEMSININDUSTRY SPECIALCONTROLCIRCUITSANDPROBLEMS 427 Problem11.Asmallmaterial-processingdriveistobeequippedwith

threeline-startmotorswhoseoperationsaretobecontrolledat apane lcontaining(1)atwo-position selectorswitchmarkedautomatic-manual,(2)aMASTERSTART-STOPStation, and (3) an individualstart-stopstation foreachofthethreemotors.Thefollowingoperatingconditionsaretoprevai l;a.Whentheselectorswitchissetonmanual,eachmotormaybe

startedandstoppedintheconventionalmannerbypress ingtheappropriatebut ton .Sincedynamicbrakingwillbeprov ided ,arunningmotorwillcometorestquicklywhenitsstopbu tt onispressed.

b.IntheMANUALposi tionoftheselector,onlyonemotormaybeoperatedat atime,andneitheroftheother twomachines may bestarteduntilan operating machine isshut down.

c.Whenthe selectorswitchisset onautomatic, the individualstaet-STOPbu tt ons are to be ineffectiveandtheentire driveistobecontrolledatthemasterstation.Underthisconditionofoperation,thepress ingofthe bu tt onwillstartthethreemotorsinsequence,i.e.,1,2and3,withasmalltimedelaybetweenstarts.Also,whilethemotorsare running,th epres sing ofthe stopbu tt on willbring themtoastop.Shouldan overloadtripinanyofthe motorcircuits,all of themmust stop.

d.It shallnotbepossibletomakeatransferfromautomatictomanualcontrol,orviceversa,unlessallmotorsarestopped.Ifatransferisattemptedwhilemotorsarerunning,nochangeshalltakeplace ;atransfermustfirstbeprec ededbythepressing oftheappropriatebut ton.

Designacontrolcircuittofulfillthe specificationslisted.

Solution(SeeFig.1211)

Fig.12-nSolutiontoProb.11Problem12.Thefollowinginformationisgiveninconnectionwitha

hoistapplicationinwhicha380-rpmd-cseriesmotordrivesadrum,two geartrains,andaropesuspension;

Totalweighttobelifted(loadplus hookandcable)=22tonsGearratios=51to90teethand17to120teethRopereduction=2to1Diameterofdrum=27in.EstimatedoveralleEciencyofhoistmechanism=85percent

It isdesiredtospecifythe horsepowerratingofasuitablemotorfortheapplicationandtosubmitasketchillustratingthearrangementofelectrical andmechanicalequipment.


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SPECIALCONTROLCIRCUITSANDPROBLEMS 429Solution(SeeFig.12-12)

511

torqueofthemotorunderthenewoperatingconditions.Assumeaplug-ging-currentpowerfactorof0.5lagging.

So lu t i on(SeeFig.1213)90T. 17T.

120 T.

Fig.12-12 Sketchfo rProb.12. T=44,000XX~X-X

90 120 2 12X2

Fig.12-13SketchesforProb.13. /piuggint:(at220volts)=1.1X90=100amp 85imotor(forapluggingcurrentof85amp)=X220=187volts

220-Biine(perphase)==127volts,

V3187

'motor(perphase)==108voltsV3

hp 2,340X3805,250 =169.5Use175-hpmotor Referringtothephasordiagram.Fig.12-13,(127)2=[(0.5X108)-HErY+(0.866X108)

Prob l em13.Inacertainshearing-machineapplication,theprocedure istomovea steelbilletintoplacegraduallyandin stepsby startingandplugginga5-hp220-voltline-start3-phasesquirrel-cageinductionmotorrepeatedlyaboutfiveorsixtimes.Duringeachpluggingoperationitisfoundthattheinrushcurrentisapproximately10percenthigherthanthenormalstartingcurrentofabout90amp.Althoughthemotorispermitted torestabout5to8minaftereachshearingoperation,itstempera tureexceedspermissiblevalues.It is,therefore,suggestedthatthepluggingcurrentbereducedto85

ampbyconnectingpermanentresistorsinthelinewires.Thiswould,ofcourse,reducethestartingtorque,but itisfeltthatthelatterwouldbemorethanample,the220-voltstartingtorquebeingl.&Tri-Determinethevalueofeachofthe three line resistorsandthestarting

Er=32voltsTherefore

R=3%50.38ohmPercentTst(at187volts)=160X(/2o)=115.5

Prob l em14. Adual-voltage220/440-volt3-phasesquirrel-cageinductionmotorisavailablewithnine terminal leadsT1toT9brought out andlabeledinthestandardmanner(seeFig.2-7).It isproposedtooperatethemachineon220voltsbu t toconnect the windingsinseriesforstartingandinparallel for running.Designanautomaticswitchingcircuitforthemotor,anddetermine

thestarting

current

and

starting

torque

interms

ofthenormalvaluesat 220volts.


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CONTACT

430 ELECTRICALCONTROLSYSTEMSININDUSTRYSolution(SeeFig.1214) SPECIALCONTROLCIRCUITSANDPROBLEMSSolution(SeeFig.1215)

PUMP-SWITCH

431

Fig.12'14 Diagrams fo rProb.14.1st(series-star)=yiJar(two-parallel-star)Tst(series-star)= (two-parallel-star)

Problem15.Acontrolcircuitistobedesignedforawater-pumpinginstallation inwhichapres sureswitchactuatesanauxiliarycontactthat

-T.C.

TR1 TR1-HI


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CR

-0-yh

-Tm)

-Ih-CR

Fig.12-15Solutionto Prob.15. Problem16.Inatwo-wirecontrolcircuitforamotorwhichisturned

ONandOF Fat thecontactsofathermostat,anintermediatestepmustbeusedbetweenstarterand thermostat;thisisbecausethecontactson

thethermostataregenerallyincapableofhandlingthecontactor-coil current.Recognizingthislimitation,designathermostaticallycontrolled circuitforasmallline-startmotorthatwillmaketh ethermostatcontactscarryanextremelylowrelay-coilcurrentand wheretheoperationoftherelaywillstartandstopthemotor.-Twosolutionsare given forthisprob lem. Solutions(SeeFig.12-16)

startsandstopsamotoratadjustablevaluesoflowandhighwaterpressures.Sincesurgesarelikelytooccurduringstartingandstoppingand"backspin"ispossiblewhenapumpisshutoff,timing relaysshould

Thermostat Thermos ta tISbeincludedinth ecircuitforpro tec tivepurposes.Itshouldbepointed outinthisconnectionthattheforceofasuddenstopofalongcolumnofwaterproducessurgeswhich,operatingthe pres sure switch,tendtosubjectthemotorstartertochattering.Asimilarsituationpreva ilsonstartingwhenthepressureswitchtendstodropoutaslowpressure.soccurduringsurges.Suchimproperoperationcanbeavoidedbyusing OFF-DELAYand ON-DELAYtimerswhosecontactsbypassthe

CRCR

--JCRi-Jf4f0L1OLZCRI

(alFig.12-16Solutionsto Prob.16.

-'CR2

OLI0L2()

pressureswitchmomentarilyduringstartsandstops.Backspintendstooccurwhenaheadofwaterrunsbackthroughacentrifugalpumptoturnthelatterinareversedirectionjus tafteraturnoff.Shouldapumpbestartedduringabackspin,thepumpormotormightbedamaged.Theuseoftherelaysindicatedaboveautomaticallyprovidesbackspinprot ecti on.

Problem17.It isdesiredtodesignacontrolcircuitforatwo-speedsingle-winding .3-phasesquirrel-cagemotortooperateinaccordancewiththe followingspecifications:a.I tshallbepossibletostartthe motorinlowspeedonly.b.Thespeedofthemotorcanbeadvancedfromlowtohighimmedi

atelythereafterbypress ingthe highbut ton .


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,

43 2 ELECTRICALCONTROLSYSTEMSININDUSTRYc.Withthemachinerunninginhigh,ashiftmaybemadetolowbypress ingthelowbut ton,bu tth elowcontactormustpickupafteradesignatedtimedelaytope rmitthemotortodeceleratesufficiently.The latterconditionmust befulfilledbyusingaplugg ingswitchwithanN.C.contactandwhichcanbeadjustedtoclose(andopen)at anydesiredspeed(notzero).

Solution(SeeFig.2-17)

0L4CRL 0L3hi 0L2

lowSTOP

-(crh)-PLUGGINGSWITCH

I I-i--h-

CRH CRL

SPECIALCONTROLCIRCUITSANDPROBLEMS43 3Solution(SeeFig.1218)

Problem19.Specifythe properhorsepowerratingofa3-phase1,140-rpmsquirrel-cagemduction motorthat perf orms amachiningopeiationonth efollowingrepetitivedutycycle;

a.Ti=33Ib-ftforminb.T2=30Ib-ftfor50sec

CRHFig.12-17 SolutiontoProb.17.

Problem18.Asequencingcontrol systemistobedesigned foramachinetoolthatisequippedwiththreeoperatingmotordrivesandacoolant pump motor.Amasters ta r t - s t op stationistobeprovided with,and thestarters ofeachoftwo operating motorsIMand 2Mare to be actuated by,three-wirepilot devices.Thestartingandstoppingof thethirdoperatingmotorSMaretobecontrolledbyatwo-wirepilo tdevice,inthiscaseapressur eswitch.Moreover,whenanyoneofthemotorsIM,2M ,or3M isstarted,itmustautomaticallystartcoolantpumpmotor.4Mandth etrippingoftheoverloadin thelattercircuitmustshutdownthesystem.The operatingmotorsshallbeprotect edby independentoverloadrelays,the

trippingof

anyoneofwhichshall

stopthe

correspondingmachine

only.

c.Ts=48Ib-ftfor3mind.Restperiodfor4min Note:The selectionofamotorforanapphcationwhosedutycycle

repeatsitselfat regularintervalsmustbemadeonthebasisofthe rmstorqueorhorsepower.Thisvalue is then ameasureofthe thermalcapacit yofthemotor,i.e.,itsabilitytomaintainareasonabletemperature rise.Sincethereisaconsiderablereduction in coolingwhilethe motorisat rest,itiscustomarytouseone-thirdofth erest-periodtimeinmaking

,calculations.jmrX270]+[(30)X50]-F[(48)X180]~(270-f50-1-180+24%)36X1,140TT-ri,

hp=-=7. 8 Use/.5-hpmotorProblem20.Designamagnetic-amplifiercontrolcircuitsimilarto

thatofFig.9-396forcontrolling the speedofashuntmotor in an adjust-


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43 4 ELECTRICALCONTROLSYSTEMSININDUSTRY SPECIALCONTROLCIRCUITSAND PROBLEMS43 5able-voltagesystem(WardLeonard)byeliminatingthetachometergenerator.Thiscanbeaccomplishedbyusingtwocontrolwindingsonthemagneticamplifier,withoneofthemenergizedbytheloopvoltageandtheotherbytheloopcurrent.Beespeciallycarefulthatthecontrolwindingsareproper lypolar izedsothatthemotorspeedwillremainconstantforallloadchangesat apar ticul arrheostatsetting.

Solution(SeeFig.12-20)(-)

Thewindingconnections,withtheproperterminalmarkings,shouldbeincludedwiththecontrol-circuitdiagram.

Solution(SeeFig.12-21)

Fig.12-20 Solutionto Prob.20 .Problem 21.Acontrolcircuit istobedesigned fora two-winding three-

speedconstant-horsepower3-phasenonreversiblesquirrel-cagemotortofulfillthe followingoperatingconditions:

a.Withthemotorat rest,itshallbepossib letostartat an yofthe threespeeds(lowL,mediumM, orhighH)by pressingtheappropri atebut ton .

b.It shallbepossibletoadvancetoahigherspeeddirectly,i.e.,fromLtoM, LtoH,orMtoH.

c.Tochangetoalowerspeed,i.e.,fromHtoM, HtoL,orMtoL,itshallbenecessaryfirsttopress the stopbut ton.Thisprocedure permits themotortoslowdownslightlybeforeanelectricalchangetolowerspeedconnectiontakesplace.

Problem22.Acompletecontrolcircuitistobedesignedforatwo-windingfour-speedconstant-torque3-phasenonreversiblesquirred-cagemotortocomplywiththe givenspecificationsaslisted:

Ja.Withthemotorat rest,itshallbepossibletostartthemotorat anyofthefourspeedsbypressingtheappropriatebut ton .However,thefollowinglimitationsareimposed;(1)onhigh,1,200rpm,immediatelyuponclosingthemaincontrol-circuitswitch;(2)on3rd,900rpm,10secaftertheswitchisclosed;(3)on2nd,600rpm,20secaftertheswitchisclosed;(4)onlow,450rpm,30secaftertheswitchisclosed.


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. u i u -c

Hfr

(10 sec)


450 RPM

SPECIALCONTROLCIRCUITSANDPROBLEMS437

STOP oIQ

WiNOiNG

WINDING2A

600RPMQIOiiIo

90 0RPM

OU0L 2if-2CRn;iBC0

2BTR0L 30U42CR

2CTRHlrsirH

2CTRIBTR

, 'T.C..'f'T.O.(10sec)(tisec)OTR1CTR

''t .c.(10sec)(11sec)

o j Q Fig.12-22faWiringconnectionsfo rProb.22 .WiNOING12-PAR-Y iin-

1CTR -Jf-ICR2CTRHb 0L5 0L6 c.Withthe motorrunninginhigh,i.e.,1,200rpm,itshallbepossible

toshift(1)to 3rdaftera10-secdelay,(2)to2ndaftera20-secdelay,or(3)tolowaftera30-secdelay.Thismay bedone ineitheroftwo ways:(1)bykeepingtheappropriatebu tt ondepressedforthe

WINDING22-PAR-Y

1200 RPMojooio

0L7 0L8 TfH

timeperiodsindicatedor(2)byfirstpress ingthestopbu tt on and then,afterasuitabletimedelay,pressingtheappropriatebut ton .

d.Withthemotorrunningin3rd,i.e.,900rpm,itshallbepossibletoshift(1)to2ndaftera10-secdelayor(2)tolowaftera20-sec

PUSH-BUTTONSTATIONCont act ors; A,B,C,CI,0.DlControlrel ays;ICR.2CRTimingrel ays;IBTR,2BTR,1CTR,

2CTR,DTR

ICR-a-[a]

delay.Thismaybedone inamannersimilartothat indicatedinc.e.Withthemotorrunning in2nd,i.e.,600rpm,itshallbepossibleto

shifttolowaftera10-secdelay ineitherof the twowaysindicatedinc.

Solution(SeeFig.12-22aandb)Fig.12-22o Circuitconnectionsfo rProb.22 .

b.Withthemotoroperatingatanyspeed(inaccordancewiththeconditionsspecifiedina)itshallbepossib letoadvancetoahigherspeed;e.g.,itshallbepossibletoadvancedirectlyfromlow.to2nd,lowto3rd,orlowtohigh,from2ndto3rdor2ndtohigh,andfromSrdtohigh.

No te :The winding-connections diagramofFig.12-226illustrateshowthetwowindingsarenormallyopen-circuitedattheT3-T7andtheT13-T17terminals.Thus,whenwinding1isenergized,inductively coupledwinding2cancarrynocurrent,andnoinducedcurrentcanpassthroughwinding1whenwinding2isenergized.Alsoobservethatforthisconstant-torquemotor:(1)winding1givesthelowspeed,i.e.,


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43 8 ELECTRICALCONTROLSYSTEMSININDUSTRY SPECIALCONTROLCIRCUITSANDPROBLEMS 439 450rpm,whenitisconnecteddeltaby jo in in gTZtoT7throughcontact A](2)winding2givesth e2ndspeed,i.e.,600rpm,whenconnecteddeltaby joi nin gT'lStoT17throughcontactB](3)winding1givesth e3?"dspeed,i.e.,900rpm,whenconnectedtwo-parallel-star(2PY)by jo ini ngTl ,T2,T2>,an dT7through contactsCI ;(4)winding2givesth ehighspeed,i.e.,1,200rpm,whenconnectedtwo-parallel-star(2PY)by jo ini ngTil,T12,TIZ,andTilthrough contactsDl .Moreover,th ewindingsareconnectedconsequent-pole whentheypr ov id etheirrespec-tivelowspeeds,i.e.,4.'i0rp mforwinding1an d600 rp mforwinding2;th ewindingsareconnectedconventionallywhentheygivetheirrespectivehighspeeds,i.e.,900rp m forwinding1an d1,200rp mforwinding2.

Problem23.Makeawiringdiagramillustratinghowareversingseriesmotoronacranebri dgema ybepr ov id edwithdynamicbr aki ng .Thispr ac ti ceisdesirabletope rm it th emachinetoshutitselfdownquicklyinth eeventofapowerfailureortodelayth esettingofth emechanicalbr ak ewhenth emasterisreturned toth eoffpos iti on.Under th elatteroperatingcondition,severebr ak in gtorquean dshockat highspeedarepr ev en te d,andsoftdynamic-brakingactionpr ecedesth eapplicationofth emechanicalbra ke .

Unliketh edynamic-brakingpr oc edur eforshuntmotors(Fig.3-21),whereth eshunt-fieldexcitationismaintainedbyth esourceandth earmatureisconnectedtoaDBresistor,aslightlydifferentmethod mustbeemployedforseriesmotors.Here,th earmatureseriesfield,an dresistorareconnectedintoaseriesloop,and,duringth edynamic- br ak in gper iod ,th eseries-fieldexcitationismaintainedinthesame direction by th e generated voltagein th e armature, i.e., th e cemf.Although th eapplicationofthismethodiscomparativelysimplefo rnonreversingseries-motordrives,itissomewhatmoreinvolvedtoobtaindynamicbr ak in gformachinestha toperateinbo thdirections.Asimplesketchillustrating howanonreversingseriesmotorisconnectedforemergencydynamicbr ak in gisgiveninFig.12-23a.No teth euseofN.C. DBcontactswhichare springclosed(seeFig.3-22).Controllerwiring connections tha t pr ov id e emergencydynamic br ak in g

forareversingseriesmotorar egivenin Fig.12-236.No te th eJourN.C. DBcontacts,springclosed,an dth eDBresistor.Tounderstandhowth esystemoperatesitwl lbenecessarytorecognizetha t :(1)th eDBcontactor coilsareenergizeda tth eins tantth emasterismovedou tofth e OFFpos it ion and,withth emotorrunning ineitherdirection,allDBcontactsareopen;(2)inth eeventofapowerfailureorifth emasterisreturnedtoth eoffpos it ion ,onlytwocontactorsar epe rm it te dtoclosean dth eothertwoare locked ou t mechanically;(3)withth emotorrunninginaforwarddirection, acamshaft mechanismlocksou t th e 3DBandiDB

contactors bu tpe rm it sth eIDBan d2DB contactorstoclose freely; thus,whenthereisapowerfailure,onlyth eIDBand2D Bcontactsclosewhileth earmatureisrotating,anddynamicbr ak in goccursascurrentpas sesthroughth eloopmadeup ofth earmatine,th eseriesfield,th eDBresistor,andth eIDBand2DBcontacts;(4)withth emotorrunninginareversedirection,th ecamshaftmechanismlocksou tth eID Band2DB

contactors

bu tpe rm it s

th eZDBan d4DB contactors toclosefreely;thus,whenthereisapowerfailure,onlyth e3DBan diDBcontactsclosewhileth earmatureisrotating,an dth edynamic-brakingloopismadeupofth earmature,th eseriesfield,th eDBresistor,an dth e3D B an diDBcontacts;(5)fordynamicbr ak in gfromforwardorrevereerotation,thedirectionofthecurrentintheseries fie ldremainsthesame.

Solution(SeeFig.12-23aandh)

[b)Emergencydynomic-braking connectionsforareversingseriesmotorFig.12-23Circuitdiagrams forProb.23.

Problem24.Makeawiringdiagramillustratinghowaduplex,series-motordrivefo racranebri dgema ybepr ov id ed withdynamicbr ak in g


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440 ELECTRICALCONTROLSYSTEMSININDUSTRY SPECIALCONTROLCIRCUITSANDPROBLEMS 441 bytheWilson-Ritchiemethod.Thescheme,namedafteritsinventors,employs fourstandard spring-closed DB contactors forthebraki ng circuit(asinFig.12-23)andtheusualcombinationofresistorsand contactors.However,unlikethe duplexcontrolsystemofFig.4-12,wherereversingthetwoseriesmotorsisaccomplishedbychangingthe currentdirections inbotharmatures,theWilson-Ritchiemethodreversesthemotorsbyinterchangingthearmatureterminalsinonemachineandtheseries-fieldterminalsintheother.Theuniquefeatureofthisarrangementisthatspecialmechanismsareunnecessary(asinFig.12-236)andconventionaldevicesareused.The fourN.C. DB contacts ofFig.12-24openat the instantthemotorsarestartedand closeduringthe braking period. Whenthebridgecraneismovingforwardorreverse,thetwomotor

circuitsarecompletelyindependent,withthearmatureandfieldofmotorAinseriesinonepa thand thearmatureandfieldofmotorBinseries in the other.However,whilethe motorsare undergoingadynamic-bra kingstopandalldirectionalcontactorsareopen,oneloopcircuitconsistsofarmatureA,series-fieldB,and aDBresistorwhileasecondloopcircuitcomprisesarmatureB,series-fieldA,andaDB resistor.Moreover,anespeciallyimportantaspectofthesystemisthatthe series-fieldcurrentdirectionsinbo thmotorsdonotreverse,although thearmaturecurrentsdo,asthe machinesbraketoastop.

So lu t io n(SeeFig.12-24)

Problem25. Asexplained inChap.6(seeFigs.6-25to6-27),thespeedofaphas e-woundrotormotorcanbeadjustedbyinsertingvaryingvaluesofresistance in the secondary.For installations requiringconsiderablespeedrangeswithaminiinuiiinumberofcontactors,itissometimes founddesirabletoemploysetsofresistorshavingasuccessionofvaluesthatvaryaccordingtoageometricratio.Themastercontrollercanthenbewiredtoinsertresistancesequaltoonebaseunitat eachstepofthesecondarycontroller,varyingthemagnitudeofthephas eresistancefromamaximumvalue(representedbythesumofallsections)tozero.Thisschemeofspeedadjustmenthasbeenappliedsatisfactorilytolargepri nti ngpresseswherethemodeofoperationistosetthemasterat thedesiredspeedpointbeforeenergizingthe pr ima ry; apush-but tonstarteristhenactuatedtostartthemotorandbri ngituptoapresetspeed.Anadded feature forsuchcontrollers istoincludeaso-calledhigh-torquecontactorinthesecondarysothatthemotorwillalwaysdevelopsufficientstartingtorqueregardlessofthecontrollersetting.Thelatteractionisimplementedbymakingthehigh-torquecontactorpickupat the sametimethelinecontactorclosesandhaveitenergizedaslongasthe operatorkeepsthestartbu tt ondepressed;thereleaseofthe startbu tt onafterthemachinehasacceleratedcausesthehigh-torquecontactortodropout andthe insertionofthe requiredsecondary resistance.Makeawiringdiagramshowingtheconnectionsforawound-rotor

motorwithfoursectionsofresistanceineach"ofthethreephasesgraduatedaccordingtoageometricratio.Showintabularformhow16differentspeedscanbeobtainedwiththisarrangement.

Solution(SeeFig.12-25)4A 3A 2A 1A

Fig.12-24 Wilson-RitchiecircuitdiagramforProb.24 . Fig.12-25 CircuitconnectionsandtableforProb.25.

CONTACTORS MASTER POSITION1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1A2A 3A4A

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