Utilization of electrical energy Unit 4 ElectricTraction.pdf

Utilization of electrical energy

1. 4. Electric Traction
Dr. K. D. Patil
EE6I –Utilization of Electrical Energy (UEE-22626)

2. MSBTE Curriculum

3. Introduction:
Thesystemthatcausesthepropulsionofavehicleinwhichthat
drivingforceortractiveforceisobtainedfromvariousdevicessuchas
electricmotors,steamenginedrives,dieselenginedives,etc.isknownas
tractionsystem.
Tractionsystemmaybebroadlyclassifiedintotwotypes.Theyare
electric-tractionsystems,whichuseelectricalenergy,andnon-electric
tractionsystem,whichdoesnotuseelectricalenergyforthepropulsionof
vehicle.
RequirementsofIdealTractionSystem:
Therequirementsofidealtractionsystemsare:
Idealtractionsystemshouldhavethecapabilityofdevelopinghigh
tractiveeffortinordertohaverapidacceleration.
Thespeedcontrolofthetractionmotorsshouldbeeasy.
Vehiclesshouldbeabletorunonanyroute,withoutinterruption.
Equipmentrequiredfortractionsystemshouldbeminimumwithhigh
efficiency.
Itmustbefreefromsmoke,ash,durt,etc.

4. TherequirementsofidealtractionsystemsContinued:
Regenerativebrakingshouldbepossibleandbrakingshouldbeinsucha
waytocauseminimumwearonthebreakshoe.
Locomotiveshouldbeself-containedanditmustbecapableof
withstandingoverloads.
Interferencetothecommunicationlinesshouldbeeliminatedwhilethe
locomotiverunningalongthetrack.
Normally,nosingletractionsystemfulfillstherequirementsofideal
tractionsystem,whybecauseeachtractionsystemhasitsmeritsandsuffers
fromitsowndemerits,inthefieldsofapplications.

5. Different Types Traction Systems Used in India:
Broadly speaking, all traction systems may be classified as :
(a) Non-Electric Traction Systems
(b) Electric Traction Systems
(a)Non-ElectricTractionSystems:
Theydonotinvolvetheuseofelectricalenergyatanystage.
Examplesare:SteamEngineTraction,I.C.Engine(DieselEngine)
Tractionetc.
(b)ElectricTractionSystems:
Theyinvolvetheuseofelectricenergyatsomestageortheother.
Theymaybefurthersubdividedintotwogroups:
1.Firstgroupconsistsofself-containedvehiclesorlocomotives.
Examplesare:Battery-ElectricDriveandDiesel-ElectricDriveetc.
2.Secondgroupconsistsofvehicleswhichreceiveelectricpowerfroma
distributionnetworkfedatsuitablepointsfromeithercentralpower
stationsorsuitably-spacedsub-stations.
Examplesare:RailwayElectricLocomotivefedfromOverheadAC
Supply(CalledStraightElectricTraction)andtramwaysandtrolley
busessuppliedwithDCsupply.

6. Diesel-ElectricTractionSystem:
Itisaself-containedmotivepowerunitwhichemploysadiesel
enginefordirectdriveofaDCgenerator.Thisgeneratorsuppliescurrentto
tractionmotorswhicharegearedtothedrivingaxles.
InIndia,diesellocomotiveswereintroducedin1945forshunting
serviceonbroad-guage(BG)sectionsandin1956forhigh-speedmain-line
operationsonmetre-guage(MG)sections.Itwasonlyin1958thatIndian
Railwayswentinforextensivemain-linedieselisation.
Diesel-electrictractionhasthefollowingadvantages:
1.Nomodificationofexistingtracksisrequiredwhileconvertingfrom
steamtodiesel-electrictraction.
2.Itprovidesgreatertractiveeffortascomparedtosteamenginewhich
resultsinhigherstartingacceleration.
3.Itisavailableforhaulingforabout90%ofitsworkingdays.
4.Diesel-electriclocomotiveismoreefficientthanasteamlocomotive
(thoughlessefficientthananelectriclocomotive).

7. Disadvantages:Diesel-electrictractionhasthefollowingdisadvantages:
1.Forsamepower,diesel-electriclocomotiveiscostlierthaneitherthe
steamorelectriclocomotive.
2.Overloadcapacityislimitedbecausedieselengineisaconstant-kW
outputprimemover.
3.Lifeofadieselengineiscomparativelyshorter.
4.Diesel-electriclocomotiveisheavierthanplainelectriclocomotive
becauseitcarriesthemainengine,generatorandtractionmotorsetc.
5.Regenerativebrakingcannotbeemployed.
ElectricTractionSystem:
Electrictractionsystemhasmanyadvantagescomparedtonon-
electrictractionsystems.
AdvantagesofElectricTraction:
1.Electrictractionsystemismorecleanandeasytohandle.
2.Noneedofstorageofcoalandwaterthatinturnreducesthe
maintenancecostaswellasthesavingofhigh-gradecoal.
3.Electricenergydrawnfromthesupplydistributionsystemissufficient
tomaintainthecommonnecessitiesoflocomotivessuchasfansand
lights;therefore,thereisnoneedofprovidingadditionalgenerators.

8. AdvantagesofElectricTractionContinued:
4.Themaintenanceandrunningcostsarecomparativelylow.
5.Thespeedcontroloftheelectricmotoriseasy.
6.Regenerativebrakingispossiblesothattheenergycanbefedbackto
thesupplysystemduringthebrakingperiod.
7.Inelectrictractionsystem,inadditiontothemechanicalbraking,
electricalbrakingcanalsobeusedthatreducesthewearonthebrake
shoes,wheels,etc.
8.Electricallyoperatedvehiclescanwithstandforoverloads,asthesystem
iscapableofdrawingmoreenergyfromthesystem.
DisadvantagesofElectricTraction:
1.Electrictractionsysteminvolveshigherectioncostofpowersystem.
2.Interferencecausestothecommunicationlinesduetotheoverhead
distributionnetworks.
3.Thefailureofpowersupplybringswholetractionsystemtostandstill.
4.Inanelectrictractionsystem,theelectricallyoperatedvehicleshaveto
moveonlyontheelectrifiedroutes.
5.Additionalequipmentshouldbeneededfortheprovisionof
regenerativebraking,itwillincreasetheoverallcostofinstallation.

9. SystemsofTrackElectrification:
Presently,followingfourtypesoftrackelectrificationsystemsare
available:
1.DirectCurrent(DC)System:600V,750V,1500V,3000V
2.SinglePhaseACsystem:15-25kV,16⅔,25and50Hz
3.ThreePhaseACsystem:3000-3500Vat16⅔Hz
4.CompositeSystem:Twosuchcompositesystemspresentlyinuseare:
(i)SinglePhasetoThreePhaseSystemalsocalledKandoSystem
(ii)SinglePhasetoDCSystem.
1.DCSystem:
Inthissystemoftraction,theelectricmotorsemployedforgetting
necessarypropellingtorqueshouldbeselectedinsuchawaythattheyshould
beabletooperateonDCsupply.Examplesforsuchvehiclesoperatingbased
onDCsystemaretramwaysandtrolleybuses.Usually,DCseriesmotorsare
preferredfortramwaysandtrolleybuseseventhoughDCcompoundmotors
areavailablewhereregenerativebrakingisdesired.Theoperatingvoltagesof
vehiclesforDCtrackelectrificationsystemare600,750,1,500,and3,000V.

10. DCSystemContinued:
Directcurrentat600–750Visuniversallyemployedfortramwaysin
theurbanareasandformanysuburbanandmainlinerailways,1,500–3,000
Visused.Insomecases,DCsupplyfortractionmotorcanbeobtainedfrom
substationsequippedwithrotaryconverterstoconvertACpowertoDC.
ThesesubstationsreceiveACpowerfrom3-φhigh-voltagelineorsingle-
phaseoverheaddistributionnetwork.Theoperatingvoltagefortraction
purposecanbejustifiedbythespacingbetweenstationsandthetypeof
tractionmotorsavailable.Thesessubstationsareusuallyautomaticand
remotecontrolledandtheyaresocostliersincetheyinvolverotary
convertingequipment.TheDCsystemispreferredforsuburbanservicesand
roadtransportwherestopsarefrequentanddistancebetweenthestopsis
small.
2.SinglePhaseACSystem:
Inthissystemoftrackelectrification,usuallyACseriesmotorsare
usedforgettingthenecessarypropellingpower.Thedistributionnetwork
employedforsuchtractionsystemsisnormally15–25kVatreduced
frequencyof16⅔Hzor25Hz.

11. SinglePhaseACSystemContinued:
ThemainreasonofoperatingatreducedfrequenciesisACseries
motorsthataremoreefficientandshowbetterperformanceatlow
frequency.Thesehighvoltagesaresteppeddowntosuitablelowvoltageof
300–400Vbymeansofstep-downtransformer.Lowfrequencycanbe
obtainedfromnormalsupplyfrequencywiththehelpoffrequency
converter.Low-frequencyoperationofoverheadtransmissionlinereduces
thelinereactanceandhencethevoltagedropsdirectlyandsingle-phaseAC
systemismainlypreferredformainlineserviceswherethecostofoverhead
structureisnotmuchimportancemoreoverrapidaccelerationand
retardationisnotrequiredforsuburbanservices.
3.ThreePhaseACSystem:
Inthissystemoftrackelectrification,3-φinductionmotorsare
employedforgettingthenecessarypropellingpower.Theoperatingvoltage
ofinductionmotorsisnormally3,000–3,600-VACateithernormalsupply
frequencyor16⅔-Hzfrequency.

12. ThreePhaseACSystemContinued:
Usually3-φinductionmotorsarepreferablebecausetheyhave
simpleandrobustconstruction,highoperatingefficiency,provisionof
regenerativebrakingwithoutplacinganyadditionalequipment,andbetter
performanceatbothnormalandseducedfrequencies.Inadditiontothe
aboveadvantages,theinductionmotorssufferfromsomedrawbacks;they
arelow-startingtorque,high-startingcurrent,andtheabsenceofspeed
control.Themaindisadvantageofsuchtrackelectrificationsystemishigh
costofoverheaddistributionstructure.Thisdistributionsystemconsistsof
twooverheadwiresandtrackrailforthethirdphaseandreceivespower
eitherdirectlyfromthegeneratingstationorthroughtransformer
substation.
Three-phaseACsystemismainlyadoptedfortheserviceswhere
theoutputpowerrequiredishighandregenerationofelectricalenergyis
possible.

13. 4.CompositeSystem:
(i)SinglePhasetoDCSystem:
Inthissystem,theadvantagesofboth1-φandDCsystemsare
combinedtogethighvoltagefordistributioninordertoreducethelossesthat
canbeachievedwith1-φdistributionnetworks,andDCseriesmotoris
employedforproducingthenecessarypropellingtorque.Finally,1-φAC
distributionnetworkresultsminimumcostwithhightransmissionefficiency
andDCseriesmotorisideallysuitedfortractionpurpose.Normaloperating
voltageemployedofdistributionis25kVatnormalfrequencyof50Hz.This
trackelectrificationisemployedinIndia.
(ii)Single-phasetoThreePhaseSystemorKandosystem:
Inthissystem,1-φACsystemispreferredfordistributionnetwork.
Sincesingle-phaseoverheaddistributionsystemischeapand3-φinduction
motorsareemployedastractionmotorbecauseoftheirsimple,robust
construction,andtheprovisionofautomaticregenerativebraking.
Thevoltageusedforthedistributionnetworkisabout15–25kVat50
Hz.This1-φsupplyisconvertedto3-φsupplythroughthehelpofthephase
convertersandhighvoltageissteppeddowntransformerstofeedthe3-φ
inductionmotors.Frequencyconvertersarealsoemployedtogethigh-starting
torqueandtoachievebetterspeedcontrolwiththevariablesupplyfrequency.

16. TractionMechanics-BlockDiagramofanAC
Locomotive:

17. TractionMechanics-BlockDiagramofanACLocomotiveContinued:
Thevariouscomponentsofanaclocomotiverunningonsingle-
phase25-kV,50-HzacsupplyarenumberedinFigure.
1.OHcontactwire2.pantograph3.circuitbreakers4.on-loadtapchangers
5.Transformer6.rectifier7.smoothingchoke8.dctractionmotors.
Asseen,powerat25kVistakenviaapantographfromthe
overheadcontactwireandfedtothestep-downtransformerinthe
locomotive.Thelowacvoltagesoobtainedisconvertedintopulsatingdc
voltagebymeansoftherectifier.Thepulsationsinthedcvoltagearethen
removedbythesmoothingchokebeforeitisfedtodcseriestraction
motorswhicharemountedbetweenthewheels.
Thefunctionofcircuitbreakersistoimmediatelydisconnectthe
locomotivefromtheoverheadsupplyincaseofanyfaultinitselectrical
system.Theon-loadtap-changerisusedtochangethevoltageacrossthe
motorsandhenceregulatetheirspeed.

18. TractionMotors:
GeneralFeaturesorRequirementsofTractionMotors:
ElectricFeatures
Highstartingtorque
SeriesSpeed-Torquecharacteristic
Simplespeedcontrol
Possibilityofdynamic/regenerativebraking
Goodcommutationunderrapidfluctuationsofsupplyvoltage.
MechanicalFeatures
Robustnessandabilitytowithstandcontinuousvibrations.
Minimumweightandoveralldimensions
Protectionagainstdirtanddust
Notypeofmotorcompletelyfulfillsalltheserequirements.
Motors,whichhavebeenfoundsatisfactoryareD.C.seriesforD.C.
systemsandA.C.seriesforA.C.systems.WhileusingA.C.threephase
motorsareused.WiththeadventofPowerElectronicsitisveryeasyto
convertsinglephaseA.C.supplydrawnfrompantographtothreephase
A.C.

19. TractionMotorsContinued:
Asstatedabove,nosinglemotorcanhavealltheelectricaloperating
featuresrequiredfortraction.
Inearlierdays,DCmotorissuitedfortractionbecauseofthehigh-starting
torqueandhavingthecapabilityofhandlingoverloads.But,thespeedcontrolof
theDCmotorisverycomplicatedthroughsemiconductorswitches.Sothat,the
motormustbedesignedforhighbasespeedinitiallybyreducingthenumberof
turnsinthefieldwinding.Butthiswilldecreasethetorquedevelopedperampere
atthetimeofstaring.AndregenerativebrakingisalsocomplicatedinDCseries
motor;sothat,theseparatelyexcitedmotorscanbepreferredovertheseriesmotor
becausetheirspeedcontrolispossiblethroughsemi-controlledconverters.And
alsodynamicandregenerativebrakinginseparatelyexcitedDCmotorissimple
andefficient.
DCcompoundmotorsarealsopreferredfortractionapplicationssinceit
ishavingadvantageousfeaturesthanseriesandseparatelyexcitedmotors.
Butnowadayssquirrelcageinductionandsynchronousmotorsarewidely
usedfortractionbecauseoftheavailabilityofreliablevariablefrequency
semiconductorinverters.Thesquirrelcageinductionmotorhasseveraladvantages
overtheDCmotors.Theyare:(i)Robustconstruction.(ii)Highlyreliable.(iii)
Lowmaintenanceandlowcost.(iv)Highefficiency.
Synchronousmotorfeatureslieinbetweenthesquirrelcageinduction
motorandtheDCmotor.

20. DCSeriesMotor:
CharacteristicsofSeriesMotors:
1.T
a/I
aCharacteristics:Weknowthat,T
a∝ΦI
a.Inthiscase,asfield
windingsalsocarrythearmaturecurrent,Φ∝I
auptothepointofmagnetic
saturation.Hence,beforesaturation,T
a∝ΦI
aand∴Ta∝I
a
2
(Figure-1)
2.N/I
aCharacteristics:Weknowthat,N∝E
b/ΦChangeinE
b,for
variousloadcurrentsissmallandhencemaybeneglectedforthetime
being.WithincreasedI
a,Φalsoincreases.Hence,speedvariesinverselyas
armaturecurrent.(Figure-2)
3.N/T
aorMechanicalCharacteristics:Itisfoundfromabovethatwhen
speedishigh,torqueislowandvice-versa.(Figure-3)

21. DCSeriesMotorContinued:
SuitabilityofDCSeriesMotorforTraction:
Followingfeaturesofseriesmotormakeitsuitablefortraction:
1.DCseriesmotorishavinghigh-startingtorqueandhavingthe
capabilityofhandlingoverloadsthatisessentialfortractiondrives.
2.Thesemotorsarehavingsimpleandrobustconstruction.
3.Thespeedcontroloftheseriesmotoriseasybyseriesparallelcontrol.
4.Sparklesscommutationispossible,becausetheincreaseinarmature
currentincreasestheloadtorqueanddecreasesthespeedsothatthe
emfinducedinthecoilsundergoingcommutation.
5.Seriesmotorfluxisproportionaltoarmaturecurrentandtorque.But
armaturecurrentisindependentofvoltagefluctuations.Hence,the
motorisunaffectedbythevariationsinsupplyvoltage.
6.Thatismotorinputdrawnfromthesourceisproportionaltothesquare
rootofthetorque.Hence,theseriesmotorishavingself-retaining
property.
7.Ifmorethanonemotoraretoberuninparallel,theirspeed–torqueand
current–torquecharacteristicsmustnothavewidevariation,whichmay
resultintheunequalwearofdrivingwheels.

22. SinglePhaseACSeriesMotor:
Characteristics of AC Series Motors:
TheoperatingcharacteristicsoftheACseriesmotoraresimilarto
theDCseriesmotor.WeightofanACseriesmotorisoneandahalftotwo
timesthatofaDCseriesmotor.Andoperatingvoltageislimitedto300V.
SuitabilityofACSeriesMotorforTraction:
Practically,ACseriesmotorisbestsuitedforthetractionpurpose
duetohigh-startingtorque.WhenDCseriesmotorisfedfromACsupply,it
worksbutnotsatisfactorilyduetosomeofthefollowingreasons:
1.IfDCseriesmotorisfedfromACsupply,boththefieldandthearmature
currentsreverseforeveryhalfcycle.Hence,unidirectionaltorqueis
developedatdoublefrequency.
2.Alternatingfluxdevelopedbythefieldwindingcausesexcessiveeddy
currentloss,whichwillcausetheheatingofthemotor.Hence,the
operatingefficiencyofthemotorwilldecrease.
3.Fieldwindinginductancewillresultabnormalvoltagedropandlow
powerfactorthatleadstothepoorperformanceofthemotor.
4.Inducedemfandcurrentsflowingthroughthearmaturecoilsundergoing
commutationwillcausesparkingatthebrushesandcommutator
segments.

23. SinglePhaseACSeriesMotorContinued:
Hence,somemodificationsarenecessaryforthesatisfactoryoperation
oftheDCseriesmotorontheACsupplyandtheyareasfollows:
1.Inordertoreducetheinductivereactanceoftheseriesfield,thefield
windingofACseriesmotormustbedesignedforfewturns.
2.Thedecreaseinthenumberofturnsofthefieldwindingreducestheload
torque,i.e.,iffieldturnsdecrease,itsmmfdecreaseandthenflux,which
willincreasethespeed,andhencethetorquewilldecrease.Butinorderto
maintainconstantloadtorque,itisnecessarytoincreasethearmatureturns
proportionately.
3.Ifthearmatureturnsincrease,theinductivereactanceofthearmature
wouldincrease,whichcanbeneutralizedbyprovidingthecompensating
winding.
4.MagneticcircuitofanACseriesmotorshouldbelaminatedtoreduceeddy
currentlosses.
5.Seriesmotorshouldbeoperatingatlowvoltagebecausehighvoltagelow
currentsupplywouldrequirelargenumberofturnstoproducegivenflux.
6.Motorshouldbeoperatingatlowfrequency,becauseinductivereactance
isproportionaltothefrequency.So,atlowfrequency,theinductive
reactanceofthefieldwindingdecreases.

24. ThreePhaseInductionMotor:
Characteristics of Three Phase Induction Motors:
Theoperatingcharacteristicsofthethreephaseinductionmotorare
suchthat,ithasdroopingspeedcharacteristicslikeDCshuntmotorandits
startingtorqueislowandstartingcurrentishigh.However,usingmodern
powerelectronicscontrollerstheoperatingcharacteristicsofthreephase
inductionmotorareadjustable.
SuitabilityofThreePhaseInductionMotorforTraction:
Thethree-phaseinductionmotorsaregenerallypreferredfor
tractionpurposeduetothefollowingadvantages.
1.Simpleandrobustconstruction. 2.Trouble-freeoperation.
3.Theabsenceofcommutator. 4.Lessmaintenance.
5.Simpleandautomaticregeneration. 6.Highefficiency.
Three-phaseinductionmotoralsosufferfromthefollowing
drawbacks:
1.Low-startingtorque.
2.High-startingcurrentandcomplicatedspeedcontrolsystem.
3.Itisdifficulttoemploythree-phaseinductionmotorforamultiple-unit
systemusedforpropellingaheavytrain.

25. ThreePhaseInductionMotorContinued:
Three-phaseinductionmotordrawslesscurrentwhenthemotoris
startedatlowfrequencies.Whenathree-phaseinductionmotorisused,the
costofoverheaddistributionsystemincreasesanditconsistsoftwo
overheadconductorsandtrackrailforthethirdphasetofeedpowerto
locomotive,whichisacomplicatedoverheadstructureandifanyperson
comesincontactwiththethirdrail,itmaycausedangertohimorher.
Thisdrawbackcanbeovercomebyemployingkandosystem.In
thissystem,1-φsupplyfromtheoverheaddistributionstructureis
convertedto3-φsupplybyusingphaseconvertersandisfedto3-φ
inductionmotor.
Thespeedcontrollerofinductionmotorbecomessmoothandeasy
withtheuseofthyristorizedinvertercircuitstogetvariablefrequency
supplythatcanbeusedtocontrolthespeedofthree-phaseinductionmotor.
Nowadays,byovercomingthedrawbacksofthree-phaseinductionmotor,it
canbeusedfortractionpurpose.

26. ElectronicsSpeedControlThreePhaseInductionMotor:
ClassesofElectronicA.C.Drives:
ACmotors,particularly,thesquirrel-cageandwound-rotor
inductionmotorslendthemselveswelltoelectroniccontroloftheirspeed
andtorque.Suchacontrolisusuallyexercisedbyvaryingvoltageand
frequency.MajorityoftheelectronicACdrivescanbegroupedunderthe
followingbroadclasses:
1.StaticFrequencyChangers:Likecyclo-converterswhichconvert
incominghighlinefrequencydirectlyintothedesiredlowload
frequency.Cyclo-convertersareusedbothforsynchronousand
squirrel-cageinductionmotors.
2.VariableVoltageControllers:Whichcontrolthespeedandtorqueby
varyingtheAcvoltagewiththehelpofSCRsandgateturn-off
thyristors(GTOs).
3.RectifierInverterSystemswithNaturalCommutation.
4.RectifierInverterSystemswithSelfCommutation.

27. CurrentCollectionSystem
Broadlyspeaking,therearetwosystemsofcurrentcollection
byatractionunit:(i)ThirdRailSystem,and(ii)OverheadWireSystem.
Ithasbeenfoundthatcurrentcollectionfromoverheadwireisfar
superiortothatfromthethirdrail.Moreover,insulationofthirdrailathigh
voltagebecomesanimpracticablepropositionandendangersthesafetyof
theworkingpersonnel.
ThesimplesttypeofOHEconsistsofasinglecontactwireofhard
drawncoppersupportedeitherbybracketoranoverheadspan.Tofacilitate
connectiontothesupports,thewireisgrooved.Becausethereisappreciable
sagofthewirebetweensupports,itlimitsthespeedofthetractionunitto
about30km/h.Hence,singlecontactwiresystemissuitablefortramways
andincomplicatedyardsandterminalstationswherespeedsarelowand
simplicityoflayoutisdesirable.Forcollectionofcurrentbyhigh-speed
trains,thecontact(ortrolley)wirehastobekeptlevelwithoutanyabrupt
changesinitsheightbetweenthesupportingstructures.
Cross-Section of
Overhead Contact Wire

28. (i)ThirdRailorConductorRailSystem:
Inthissystem,thecurrentissuppliedtotheelectricallyoperated
vehiclethroughone-railconductororthroughtwo-railconductors.Incase
ofone-railconductor,thetrackrailisemployedasthereturnconductor.
Therailsaremountedoninsulatorsparallelwiththetrackrailsatadistance
of0.3–0.4mfromtherunningrail,whoseuppermostsurfacesactingas
contactsurfaceandarefedatsuitablepointsfromthesubstations.When
currentdrawnbythemotoratstartingisverylarge,thiscausesthewearing
ofrailconductorduetothetractionofthecollectorshoes.Theserailsare
designedbasedontheelectricalpropertiesratherthanmechanical.
Thissystemissuitableforheavycurrentcollection,topcontact
systemforvoltagesupto750V,andsidecontactsystemupto1,200V.The
mainadvantageofthissystemischeapandeasytorepairandinspection

29. (ii)OverheadWireSystem:
Threetypesofgeararein
commonuse:(i)TrolleyCollector,(ii)BowCollector,and
(iii)PantographCollector.
(i)TrolleyCollector:
Thiscollectorisemployedontramwaysandtrolleybusesandis
mountedontheroofofthevehicle.ContactwiththeOHwireismadeby
meansofeitheragroovedwheeloraslidingshoecarriedattheendofa
lighttrolleypoleattachedtothetopofthevehicleandheldincontactwith
Overhead(OH)wirebymeansofaspring.Thepoleishingedtoa
swivellingbasesothatitmaybereversedforreverserunningthereby
makingitunnecessaryforthetrolleywiretobeaccuratelymaintainedabove
thecentreofthetrack.Trolleycollectorsalwaysoperateinthetrailing
position.Thetrolleycollectorissuitableforlowspeedsupto32km/h
beyondwhichthereisariskofitsjumpingofftheOHcontactwire
particularlyatpointsandcrossing.

30. (ii)BowCollector:
Bowcollectorconsistsoftwotrolleycollectorpoles.Attheendof
thesepoles,alightmetalstripof1misplacedforcurrentcollection.The
currentcollectionmetalstripismadeupofsoftmaterialssuchascopper.
Thebowcollectoralsooperatesinthetrailingposition.Hence,it
requiresprovisionofeitherduplicatebowsoranarrangementforreversing
thebowforrunninginthereversedirection.Bowcollectorisnotsuitable
forrailwayworkwherespeedsupto120km/handcurrentsupto3000Aare
encountered.Itissobecausetheinertiaofthebowcollectorissmallto
ensuresatisfactorycurrentcollection.

31. (iii)PantographCollector:
Pantographisemployedinelectrictractionsystemforthecollection
ofcurrents.Thiscollectorisemployedforthesystemwhoseoperatingspeed
is100or130kmph,andcurrenttobecollectedareaslargeas2,000or
3,000A.
Pantographcollectorsaremountedontheroofofthevehicles
similartothebowcollector.Thiscollectorcarriesaslidingshoeforcontact
withtheovertheadtrolleywire.Thecontactshoesareusuallyabout1.2-m
long.Thematerialusedforthepantographisoftensteel,wearingplatesof
copper,orbronzeinserted.Therearebasicallytwoformsofpantograph
collectornamely‘diamond’typeandsingle-ended‘faiveley’

32. TypesofTraction/RailwayServices:
Therearethreetypesofpassengerservicesofferedbytherailways:
1.CityorUrbanService.Inthiscase,therearefrequentstops,the
distancebetweenstopsbeingnearly1kmorless.Hence,high
accelerationandretardationareessentialtoachievemoderatelyhigh
schedulespeedbetweenthestations.
2.SuburbanService.Inthiscase,thedistancebetweenstopsaverages
from3to5kmoveradistanceof25to30kmfromthecityterminus.
Here,also,highratesofaccelerationandretardationarenecessary.
3.MainLineService.Itinvolvesoperationoverlongrouteswherestops
areinfrequent.Here,operatingspeedishighandacceleratingand
brakingperiodsarerelativelyunimportant.
Ongoods(freight)trafficsidealso,therearethreetypesofservices
(i)main-linefreightservice(ii)localorpick-upfreightserviceand(iii)
shuntingservice.

33. MetroRail:
Metrorailisatypeofhigh-capacitypublictransportgenerallyfound
inurbanareas.ItisusedforRapidTransitorMassRapidTransit(MRT).
Modernserviceonrapidtransitsystemsareprovidedondesignatedlines
betweenstationstypicallyusingelectricmultipleunitsonrailtracks.
Unlikebusesortrams,rapidtransitsystemsareelectricrailwaysthatoperate
onanexclusiveright-of-way,whichcannotbeaccessedbypedestriansor
othervehiclesofanysort,andwhichisoftengrade-separatedintunnelsor
onelevatedrailways.Somesystemsuseguidedrubbertires,magnetic
levitation(maglev),ormonorail.
MonoRail:
Amonorailisarailwayinwhichthetrackconsistsofasinglerailora
beam.Thetermisalsousedtodescribethebeamofthesystem,orthetrains
travelingonsuchabeamortrack.Thetermoriginatesfromjoining"mono"
(meaningone)and"rail"(meaningrail)from1897,possiblyfromGerman
engineerEugenLangen,whocalledanelevatedrailwaysystemwithwagons
suspendedtheEugenLangenOne-railedSuspensionTramway.
Generally,theterm"monorail"isoftenusedtodescribeanyform
ofelevatedrailorpeoplemover.Moreaccurately,thetermreferstothestyle
oftrack.

34. SpeedTimeCurve:
Thecurvethatshowstheinstantaneousspeedoftraininkmph
alongtheordinateandtimeinsecondsalongtheabscissaisknownasspeed
timecurve.Theareaunderthespeedtimecurvegivesthedistancetravelled
during,giventimeinternalandslopeatanypointonthecurvetoward
abscissagivestheaccelerationandretardationattheinstance,
Typicalspeed/timecurveforelectrictrainsoperatingonpassenger
servicesisshowninfollowingfigure.

35. SpeedTimeCurveContinued:
Speedtimecurvemaybedividedintothefollowingfiveparts:
1.ConstantAccelerationPeriod(0tot
1):
Itisalsocallednotching-uporstartingperiodbecauseduringthis
period,startingresistanceofthemotorsisgraduallycutoutsothatthe
motorcurrent(andhence,tractiveeffort)ismaintainednearlyconstant
whichproducesconstantaccelerationalternativelycalled‘rheostatic
acceleration’or‘accelerationwhilenotching’.(portionOAinfigure).
2. Acceleration on Speed Curve (t
1to t
2):
Thisaccelerationcommencesafterthestartingresistancehasbeen
allcutoutatpointt
1andfullsupplyvoltagehasbeenappliedtothemotors.
Duringthisperiod,themotorcurrentandtorquedecreaseastrainspeed
increases.Hence,accelerationgraduallydecreasestilltorquedevelopedby
motorsexactlybalancesthatduetoresistancetothetrainmotion.Theshape
oftheportionABofthespeed/timecurvedependsprimarilyonthe
torque/speedcharacteristicsofthetractionmotors.(portionABinfigure).
3.Free-runningPeriod(t
2tot
3)
Thetraincontinuestorunatthespeedreachedatpointt
2.Itis
representedbyportionBCinfigureandisaconstant-speedperiodwhich
occursonleveltracks.(portionBCinfigure).

36. SpeedTimeCurveContinued:
4.Coasting(t
3tot
4)Period:
Powertothemotorsiscutoffatpointt
3sothatthetrainrunsunder
itsmomentum,thespeedgraduallyfallingduetofriction,windageetc.
(portionCD).Duringthisperiod,retardationremainspracticallyconstant.
Coastingisdesirablebecauseitutilizessomeofthekineticenergyofthe
trainwhichwould,otherwise,bewastedduringbraking.Hence,ithelpsto
reducetheenergyconsumptionofthetrain.(portionCDinfigure).
5.Braking(t
4tot
5)Period:
Atpointt
4,brakesareappliedandthetrainisbroughttorestat
pointt
5.Itmaybenotedthatcoastingandbrakingaregovernedbytrain
resistanceandallowableretardationrespectively.(portionDEinfigure).
Speed/TimeCurvesforDifferentServices:
SpeedTimeCurveforMainLineService:
Typicalspeedtimecurveofatrainrunningonmainlineserviceis
showninabovefigure.Asdiscussedabove,itmainlyconsistsofthe
followingtimeperiods:
(i)Constantacceleratingperiod.(ii)Accelerationonspeedcurve.
(iii)Free-runningperiod. (iv)Coastingperiod.
(v)Brakingperiod.

37. Speed/TimeCurvesforDifferentServicesContinued:
SpeedTimeCurveforSuburbanService:
Insuburbanservice,thedistancebetweentwoadjacentstopsfor
electrictrainislyingbetween1and8km.Inthisservice,thedistance
betweenstopsismorethantheurbanserviceandsmallerthanthemainline
service.Thetypicalspeedtimecurveforsuburbanserviceisshownin
figure.Thespeedtimecurveforurbanserviceconsistsofthreedistinct
periods:(i)Acceleration,(ii)Coasting,and(iii)RetardationPeriod.
Forthisservice,thereisnofree-runningperiod.Thecoastingperiod
iscomparativelylongersincethedistancebetweentwostopsismore.
Brakingorretardationperiodiscomparativelysmall.Itrequiresrelatively
highvaluesofaccelerationandretardation.

38. Speed/TimeCurvesforDifferentServicesContinued:
SpeedTimeCurveforCityorUrbanService:
Thespeedtimecurvecityorurbanserviceisalmostsimilarto
suburbanserviceandisshowninfollowingfigure.
Inthisservicealso,thereisnofree-runningperiod.Thedistance
betweentwostopislessabout1km.Hence,relativelyshortcoastingand
longerbrakingperiodisrequired.Therelativevaluesofaccelerationand
retardationarehightoachievemoderatelyhighaveragebetweenthestops.
Here,thesmallcoastingperiodisincludedtosavetheenergyconsumption.

39. SimplifiedSpeedTimeCurve:
Forthepurposeofcomparativeperformanceforagivenservice,the
actualspeedtimecurveisreplacedbyasimplifiedspeedtimecurvewhich
doesnotinvolvetheknowledgeofmotorcharacteristics.Suchacurvehas
simplegeometricshapesothatsimplemathematicscanbeusedtofindthe
relationbetweenacceleration,retardation,averagespeedanddistanceetc.
Thesimplecurvewouldbefairlyaccurateprovidedit(i)retainsthesame
accelerationandretardationand(ii)hasthesameareaastheactual
speed/timecurve.Thesimplifiedspeedtimecurvecanhaveeitherofthe
followingtwoshapes:

40. SimplifiedSpeedTimeCurveContinued:
Asshowninabovefigure,thesimplifiedspeedtimecurvecanhave
eitherofthefollowingtwoshapes:
(i)TrapezoidalShapeOA
1B
1C(TrapezoidalSpeedTimeCurve):
Wherespeed-curverunningandcoastingperiodsoftheactualSpeed
timecurvehavebeenreplacedbyaconstantspeedperiod.
(ii)QuadrilateralshapeOA
2B
2C(QuadrilateralSpeedTimeCurve):
Wherethesametwoperiodsarereplacedbytheextensionsofinitial
constantaccelerationandcoastingperiods.
ItisfoundthattrapezoidaldiagramOA
1B
1Cgivessimpler
relationshipsbetweentheprincipalquantitiesinvolvedintrainmovement
andalsogivescloserapproximationofactualenergyconsumedduring
main-lineserviceonleveltrack.
Ontheotherhand,quadrilateraldiagramOA
2B
2Capproximates
morecloselytotheactualconditionsincityandsuburbanservices.

41. Crest,AverageandScheduleSpeed:
Duringtrainmovement,followingthreespeedsareofimportance:
1.CrestSpeed:Itisthemaximumspeed(V
m)attainedbyatrainduring
therun.
2.AverageSpeed:Itisdefinedastheratioofdistancebetweenthestops
totheactualtimeofrun.Inthiscase,onlyrunningtimeisconsidered
butnotthestoptime.
3.ScheduleSpeed:Itisdefinedastheratioofdistancebetweenthestops
andactualtimeofrun+stoptime.
Obviously,schedulespeedcanbeobtainedfromaveragespeedby
includingthedurationofstops.Foragivendistancebetweenstations,higher
valuesofaccelerationandretardationwillmeanlesserrunningtimeand,
consequently,higherschedulespeed.Similarly,foragivendistancebetween
stationsandforfixedvaluesofaccelerationandretardation,highercrestspeed
willresultinhigherschedulespeed.Forthesamevalueofaveragespeed,
increaseindurationofstopsdecreasestheschedulespeed.

42. FactorsAffectingScheduleSpeedofTrain:
Thevariousfactorsthataffecttheschedulespeedofatrainare:
1.CrestSpeed:Itisthemaximumspeedoftrain,whichaffectsthe
schedulespeedasforfixedacceleration,retardation,andconstantdistance
betweenthestops.Withcrestspeed,theactualrunningtimedecreases.For
thelowcrestspeedoftrainitrunningso,thehighcrestspeedoftrainwill
increasesitsschedulespeed.
2.DurationofStops:Ifthedurationofstopsismore,thentherunningtime
oftrainwillbeless;sothat,thisleadstothelowschedulespeed.Thus,for
highschedulespeed,itsdurationofstopsmustbelow.
3.DistancebetweentheStops:Ifthedistancebetweenthestopsismore,
thentherunningtimeofthetrainisless;hence,theschedulespeedoftrain
willbemore.
4.Acceleration:Iftheaccelerationoftrainincreases,thentherunningtime
ofthetraindecreasesprovidedthedistancebetweenstopsandcrestspeedis
maintainedasconstant.Thus,theincreaseinaccelerationwillincreasethe
schedulespeed.
5.BreakingRetardation:Highbreakingretardationleadstothereduction
ofrunningtimeoftrain.Thesewillcausehighschedulespeedprovidedthe
distancebetweenthestopsissmall.

43. Suggested Learning Resources As per MSBTE Curriculum