Module- II, Lecture-2
Construction of D.C.Machine
Figshows the details of a four poleD.C. machine with both shunt and series field windings.
It consists of thefollowingparts:
Yoke:
a)Functions:
i)Itservesthepurposeofoutermostcoverofthed.c.machine.Sothattheinsulatingmaterials getprotectedfromharmfulatmosphericelementslikemoisture,dustandvariousgaseslikeSO2, acidic fumesetc.
ii)It provides mechanical support to the poles.
iii)Itformsapartofthemagneticcircuit.Itprovidesapathoflowreluctanceformagneticflux. Thelowreluctancepathisimportanttoavoidwastageofpowertoprovidesameflux.Large currentandhencethepowerisnecessaryifthepathhashighreluctance,toproducethesame flux.
b)Choiceofmaterial:Toprovidelowreluctancepath,itmustbemadeupofsomemagnetic material.Itispreparedbyusingcastironbecauseitischeapest.Forlargemachinesrolledsteel, caststeel,siliconsteelisusedwhichprovideshighpermeabilityi.e.lowreluctanceandgives good mechanical strength.Asyoke does not needanymachiningorgood finishing as it rough, castingis the best method of construction ofyoke.
Poles:
Each pole is divided into two parts Namely,
a) polecore andb) poleshoeThis is shown in fig. 27.
a)Functionofpolecoreandpoleshoe:
i) pole corebasically carries a field windingwhich is necessaryto producethe flux. ii)It directs the fluxproduced throughairgapto armature core, to thenext pole.
iii) pole shoe enlarges the area of armature core to comeacross the flux, which is necessary to producelarger induced e.m.f. to achieve this, pole shoehasgiven aparticularshape.
b)Choiceofmaterial:Itismadeupofmagneticmateriallikecastironorcaststeel.asit requiresadefiniteshapeandsize,laminatedconstructionisused.Thelaminationsofrequired size and shape are stamped together toget a pole,which is then bolted to theyoke.
Fieldwinding[F1-F2]:
Thefield windingis wound on thepole corewith a definite direction.
a)Functions: i) To carrycurrent due towhich pole core on which the windingplaced behavesas an electromagnet, producingnecessaryflux.
As it helps inproducingthe magneticfield i.e.excitingthepole aselectromagnet it is called „Field winding‟ or„Excitingwinding‟.
b)Choiceofmaterial:Asithastocarry currenthence obviouslymadeupofsome conductingmaterial.Soaluminiumorcopperisthechoice.Butfieldcoilsarerequired totakeanytypeofshapeandbendaboutpolecoreandcopperhasgoodpliabilityi.e.it can bendeasily. So copper is the proper choice.
Filedwindingisdividedintovariouscoilscalledbasfieldcoils.Theseareconnectedinseries witheachotherandwoundinsuchadirectionaroundpolecores,suchthatalternate„N‟and„S‟ poles are formed.
Byusingrighthandthumbruleforcurrentcarryingcircularconductor,itcanbeeasily
determinedthathowaparticularcoreisgoingtobehaveas„N‟or„S‟foraparticularwinding direction around it.
Armature:
It is further divided into two parts namely,I) Armature core andII) Armaturewinding
I)Armaturecoreiscylindricalinshapemountedontheshaft.Itconsistsofslotsonits periphery and the air ducts to permit the air flow througharmature which serves coolingpurpose.
a)Functions:
i)Armature coreprovides house for armature windingi.e. armature conductors.
ii) To provide apath oflow reluctance to the magneticfluxproduced bythe field winding.
b)Choiceofmaterial: A it has to provide alow Reluctancepath to theflux,itis madeup of magneticmaterial like cast iron or caststeel.
ItismadeupoflaminatedConstructiontokeepeddycurrentLossaslowaspossible.Asingle circular Lamination usedfor the construction of the armature core is shownin Fig.
II)Armaturewindingisnothingbuttheinterconnectionofthearmatureconductors,placedin theslotsprovidedonthearmaturecoreperiphery.Whenthearmatureisrotated,incaseof generator, magnetic fluxgets cut byarmature conductors and e.m.f.gets induced in them.
a)Functions:
i)Generation of e.m.f.takes placein thearmaturewindingin caseofgenerators. ii)To carrythe currentsupplied in caseof d.c.motors
iii)To do the useful work in the external circuit.
b)Choiceofmaterial:Asarmaturewindingscarriesentirecurrentwhichdependsonexternal
load, it has to be madeup of conductingmaterial, which is copper.
Armaturewindingisgenerallyformerwound.Theconductorsareplacedinthearmature slots which arelined with tough insulatingmaterial.
Commutator:
Wehaveseenearlierthatthebasicnatureofe.m.finducedinthearmatureconductorsis alternating.Thisneedsrectificationsincaseofd.c.generatorwhichispossiblebydevicecalled commutator.
a)Functions:
i)To facilitate thecollection of current from the armature conductors.
ii)To convert internallydeveloped alternatinge.m.f. to unidirectional (d.c.) e.m.f. iii)To produceunidirectional torquein case of motors.
b)Choiceofmaterial:Asitcollectscurrentfromarmature,itisalsomadeupofcopper segments.
Itiscylindricalinshapeandismadeupofwedgeshapedsegmentsofharddrawn,high conductivitycopper.Thosesegmentsareinsulatedfromeachotherbythinlayerofmica.Each commutatorsegmentisconnectedtothearmatureconductorbymeansofcopperlugorstrip. This connection is shown in the fig.
Brushesandbrushgear:
Brushesand stationaryand restingonthe surfaceof the commutator. a)Functions:
i) To collect current from commutator and makeitavailable to the stationaryexternal circuit.
b)Choiceofmaterial:Brushesarenormallymadeupofsoftmateriallikecarbon.Brushesare rectangularinshape.Theyarehousedinbrushholders,whichareusuallyofboxtype.The brushersaremadetopressonthecommutatorsurfacebymeansofaspring,whosetensioncan beadjustedwiththehelpoflever.Aflexiblecopperconductorcalledpigtailisusedtoconnect the brush to the external circuit.
Bearings:
Ballbearingsareusuallyastheyaremorereliable.Forheavyduty machines,rollerbearingsare preferred.