ii design of dc machines

8/18/2019 II Design of DC Machines

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Unit IIDesign of DC Machines

ByDr. R.KarthigaivelAssociate ProfessorDepartment of Electrical and ElectronicsEngineeringPSNA College of Engineering and Technology, DindigulTamil Nadu

6040_b


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul

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C:\Users\god\Desktop\DEM\Karthi11\6.jpgParts of DC machine


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Output Equation of DC Machines

OutputEquationgivesarelationbetweenthemachineratinganditsarmaturedimensionsintermsofarmaturediameter,corelength,speed,specificelectricandspecificmagneticloadings.

Let

Pa=powerdevelopedbyarmatureinkW

Pa=generatedemf×armaturecurrent×10-3

Pa=E×Ia×10-3(1)

Generatedemfinadcmachine,

.EaZnpE.

.(2)


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.flux per pole in Wb

Z total number of armature conductors

n motor speed in rps(n=N/60)

N speed in rpm

a number of parallel paths

Number of parallel path, a = 2(for wave winding)

a = no. of poles=p(for lap winding)

Substitute (2) in (1)

310....aaIaZnpP.Let current in each conductor

aIIaZ.(3)

310)()(.....nZIpPZa.(4)


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DLpBav...Specific magnetic loading,

DLBpav...(5)

Specific electric loading,

DZIacZ..DacZIZ..(6)

WhereD armature diameter

L armature core length


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Substitute (6), (5) in (4)310.....nDacDLBPava..32210....nLDacBav.kWinnLDCPoa..2Where

Output co-efficient,

3210...acBCavo.(7)

(8)

for motor

.PPa.PPa.for generator

Power developed by the armature{Pa= i/p -losses}

{Pa= o/p + losses}

{P = o/p}


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Let

Kfform factor

Bavaverage gap density

Bg(or)Bmmaximum gap density

Form factor ,

.

.

...bBBKgavfWhere

bpole arc

pD.

..Pole pitch

.Pole pitch


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Main Dimensions

a.Armature diameter (D)b.Armature core length (L)(i)Specific magnetic loading is related with D and L(ii)Specific electric loading is related with D(iii)The output coefficient depends on the value Bavand ac

The choice of specific magnetic loading depends on :

a. Teeth flux density

b. Frequency of flux reversal

c. size of the machine


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul9

Teethfluxdensity

Fluxdensityintheairgapishigh,itmayincreaseahighfluxdensityinthearmatureteethbeyondthelimit.Thevalueoffluxdensityintheteethataminimumsectionshouldnotexceed2.2Wb/m2.

Thehighvalueoffluxdensityintheteeth

(i)Causesmoreironlosses

(ii)Requireshighermmfforpassingthefluxthroughtheteeth.Itcanleadtoincreasedfieldcopperlossesandhighercostofcopper.

Frequencyoffluxreversal

f=(np/2)

Higherfrequencyoffluxreversalresultsinincreasedironlossesinarmaturecoreandteeth.Thereforeweshoulduseasmallvalueoffluxdensityintheairgapfoeamachinehavinghigherfrequency.

Sizeofmachine

Ifsizeofthemachineincreases,thewidthofthetoothalsoincreasesduetoanincreaseindiameterofthearmature.Butthiswillnotcausethesaturationofmachinewhenthereisincreasedvalueofgapfluxdensity.


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Choice of specific electric loading (or)

Ampere conductors per metre (ac)

TemperatureriseSpeedVoltageSizeofmachineArmaturereactioncommutation


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11Theventilationofthemachineisbetterwhenamachinerotatesathighspeedandgraterlossescanbedissipated.Forahighspeedmachine,itispossibletouseahighvalueofac.

.Heatdissipatedinthemachineincreaseswhenchoosingahighvalueofacduetouseofmorearmatureconductors..Ahighvalueofaccanbeusedformachinesusinginsulatingmaterialswhichcanwithstandatemperaturerise..Thechoiceofvalueforacdependsupontypeofenclosureandcoolingtechniquesemployedinthemachine.

Temperature rise

SpeedVoltage

.Thechoiceofacvaluedependsonnumberofarmatureconductorswhichinturnsdependsonspaceavailabilityfortheprovisionofconductors..Inhighvoltagemachines,thethicknessofinsulationshouldbelarge,thereforethereislessspaceforconductorsresultsinlowvalueofac


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Size of machine

.Inlargesizemachinethereismorespaceforaccommodatingtheconductors.Soitcanbepossibletousehigherac.

Armature Reaction

.Forhigheracarmaturereactionwillbesevere.

.Inordertopreventthis,thefieldshouldbemadestronger.Themmfrequiredforthefieldhavetobeincreasedtopreventdecreaseinflux..Thisincreasethecostofconductorsused

Commutation

aca(Z/D).Forhighvalueofac,eitherincreasetheconductorsordecreasethearmaturediameter..acareactancevoltageincoilainductanceinthecoil

.Highvalueofacresultinincreasedreactancevoltagewhichdelaythecommutationcondition.(acliesbetween15000to50000)


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13SeparationofDandLinDCmachines

ThefactorswhichinfluencetherelativevalueofDandLindcmachinesarei.Machineproportionsii.Peripheralspeediii.Momentofinertiaiv.Voltagebetweenadjacentcommutatorsegments


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14Machineproportions

.Thedimensionsofthemachinearedecidedbythepolecriterion

.Thecrosssectionofpolemayberectangularofsquare

..

.Inasquaresection,widthofthepolebodyisequaltolengthofthemachine.But,thewidthofthepolebody(b)isabout0.45to0.55timesofpolepitch.L=b=(0.45to0.55).Inrectangularpoleconstruction,thelengthofthemachineisequaltotwicethewidthofpolebodyL=bto2b.Forsquarepolefaceconstruction,L=b(or)L/b=1

.

.)1.145.0(to.C:\Users\god\Desktop\DEM\Karthi11\7.jpg


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Peripheralspeed

Peripheralspeedofarmature,smnDVa/..Peripheralspeedofarmatureislimitingfactortothevalueofdiameter.Therefore,thePeripheralspeedofarmatureshouldnotexceedabout30m/sec.

Moment of inertia.Machinesusedforcontrolsystems,asmallmomentofinertiaisdesirableanditcanbeobtainedbyselectinglowvalueofdiameterforarmatureconstruction..Ifmachinesaredesignedforimpactloadapplications,thenselectionofarmaturediametershouldbelarge.


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Voltage between adjacent commutator segmentsThemaximumcorelengthofmachineisdecidedbymaximumallowableVoltagebetweenadjacentcommutatorsegments.

Maximum Voltage between adjacent commutator segments,cagmcmTLVBE2.Where

Bgm-maximum gap density under load condition

L -Core length of machine

Va-peripheral speed in m/sec

Tc-number of turns/coil


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A5kW,4pole,250V,1500rpmshuntgeneratorisdesignedtohaveasquarepoleface.Theloadingsare:averagefluxdensityinthegap=0.42Wb/m2andampereconductorspermetre=15000.findthemaindimensionsofthemachine.Assumefullloadefficiency=0.87andratioofpolearctopolepitch=0.66.


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C:\Users\god\Desktop\DEM\Karthi11\8.jpgC:\Users\god\Desktop\DEM\Karthi11\8.jpg


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19C:\Users\god\Desktop\DEM\Karthi11\8.jpgC:\Users\god\Desktop\DEM\Karthi11\8.jpgC:\Users\god\Desktop\DEM\Karthi11\9.jpgC:\Users\god\Desktop\DEM\Karthi11\8.jpg


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20C:\Users\god\Desktop\DEM\Karthi11\9.jpgL = 0.1 mAns.D=0.193mL=0.1m


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21Adesignisrequiredfora50kW,4pole,600rpm,dcshuntgenerator,thefullloadterminalvoltagebeing220V.Ifthemaximumgapdensityis0.83Wb/m2andthearmatureconductorspermetreare30000.calculatesuitabledimensionsofarmaturecoretogiveasquarepoleface.

Assumethatthefullloadarmaturevoltagedropis3%oftheratedterminalvoltage,andthatthefieldcurrentis1%ofratedfullloadcurrent.Ratioofpolearctopolepitchis0.67.


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Selection of Number of PolesFactors which affect the Selection of suitable Number of Poles are(i)Frequency(ii)Weightofironpart(iii)Weightofcopper(iv)Lengthofcommutator(v)Labourcharges(vi)Flashoverbetweenbrushes(vii)Distortionoffieldform

Frequency of flux reversal apoles airon losses


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27C:\Users\god\Desktop\DEM\Karthi11\13.jpgWeightofironpart

.Ifthenumberofpolesisdoubled,thenthefluxintheyokeishalved.

.Tokeepyokefluxdensitysame,theweightofironinyokecanreducedinproportiontoincreaseinnumberofpoles..Increasingtheno.ofpolesincreasesthefrequencyoffluxreversalinthearmaturecorewhichresultsincreasecorelosses.


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28Weight of copper

The weight of copper can be divided intoi.Armature copperii.Field copperDue to increase in no. of poles decreases armature and field copper.

Length of commutator

Whenno.ofpolesincreasesitresultsinreductioninthelengthofcommutator.

2 pole m/c4 pole m/c

Current per parallel path

Ia/2

Ia/4

Current per brush arm

Ia

2Ia/4=(Ia/2)


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29Labour Charges

Thelabourchargeswillincreaseastheno.ofpolesincreasesduetomorearmaturecoilstowind,insulateandsoldertocommutatoranddesignoffieldwindingsFlash over between brushesDistortion of field formpZIATza2..Forasmallerno.poles,ATaincreasesresultsindistortionoffieldformandreductioninfluxunderloadconditions..Toavoidthedistortionoffieldform,itisnecessarytoprovidecompensatingwindingforamachineshavingsmallno.ofpoles..Itcomplicatestheconstructionandincreasesthecostofmachine.


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Advantages of having more number of polesIncreasing the number of poles, will reduces the following.Weight of iron parts.Weight of copper.Length of commutator.Distortion of field form

Disadvantages of increasing number of poles.Frequency of flux reversal is more.High labour charge.Increase the flash over between brush arms


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Guideline for selecting number of poles

i.Frequency of flux reversalFrequency lies between 25 to 50 HzLower frequency is preferred for large machinesii.Thevalueofcurrentperparallelpathshouldbelimitedto200Aandcurrentperbrusharmshouldbelimited400A.

2npf.Wave windingLap winding

Current per parallel path

Ia/2

Ia/p

Current per brush arm

Ia

2Ia/p


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iii. The value of armature mmf per pole should not be excessivelylarge.Armature mmf/pole2.acATa.pDacATa2..Output (kW)

ATa(amp turns/m)

Upto100

5000 or less

100-500

5000 to 7500

500 -1500

7500 to 10000

Over 1500

upto12500

iv.Ifmorethanonechoiceofpolessatisfiestheabovethreeconditions,thenselectlargeno.ofpoles.Thisresultsinreductioninironandcopper.


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul33Findthesuitablediameter,corelengthandnumberofpolesofa400kW,550V,900rpmdcgeneratorhaving92%efficiency.Assumeanaveragefluxdensityintheairgapofabout0.6Wb/m2andampereconductorspermetretobe35000.C:\Users\god\Desktop\DEM\Karthi11\14.jpgC:\Users\god\Desktop\DEM\Karthi11\14.jpg


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34C:\Users\god\Desktop\DEM\Karthi11\14.jpgC:\Users\god\Desktop\DEM\Karthi11\14.jpgC:\Users\god\Desktop\DEM\Karthi11\14.jpg


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul35C:\Users\god\Desktop\DEM\Karthi11\14.jpgC:\Users\god\Desktop\DEM\Karthi11\15.jpgC:\Users\god\Desktop\DEM\Karthi11\15.jpg


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul38Findthemaindimensions,numberofpolesandlengthofairgapofa600kW,500V,900rpmgenerator.Assumeaveragegapdensityis0.6Wb/m2andampereconductorspermetre35000.Theratioofpolearctopolepitchis0.75andtheefficiencyis85%.

The following are the design constraints : peripheral speed 40 m/s,frequency of flux reversal 50 Hz, current per brush arm400A, armature mmfper pole 7800 AT. The mmfrequired for airgap is 60 percent of armature mmfand gap contraction factor 1.15.


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul39C:\Users\god\Desktop\DEM\Karthi11\17.jpgC:\Users\god\Desktop\DEM\Karthi11\17.jpg


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C:\Users\god\Desktop\DEM\Karthi11\19.jpgC:\Users\god\Desktop\DEM\Karthi11\20.jpgC:\Users\god\Desktop\DEM\Karthi11\20.jpg


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45C:\Users\god\Desktop\DEM\Karthi11\20.jpgC:\Users\god\Desktop\DEM\Karthi11\20.jpg


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Magnetic circuit calculationsMagnetic circuit calculation involves two types of problemsi.Itisrequiredtodeterminethemmfneededtoestablishadesiredfluxatagivenpointinamagneticcircuit.ii.Thefluxorfluxdensityisunknownandisrequiredtobedeterminedforagivengeometryofthemagneticcircuitandspecifiedmmf.Themagneticcircuitissplitupintoconvenientpartswhichmaybeconnectedinseriesparallel.Thecalculationofmmfinthemagneticpartsaresimplewhereasincaseofairgapandtaperedteeth,thecalculationsarecomplex.


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul47MMF calculation of air gapLet

lglength of air gap

ysslot pitch

Wsslot width

Wttooth width

L Length of core

ndnumber of radial ventilating ducts

Wdwidth of each duct

H = atg= mmf per unit length

Atg = atg×lgtotal mmf required in the air gap

Bgor Bavaverage gap flux density or specific magnetic loading

Wb/m2


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul48Reluctance of air gap with smooth armature surfaceC:\Users\god\Desktop\DEM\Karthi11\1.jpgThe reluctance of air gap for one slot pitchsgggLylAlS00....70104.....Permeability of free spaceA = Lys= area of cross section of air gap over one slot pitch

AlAlSr...0..


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49Reluctance of air gap for slotted armatureThe area of cross section of the air gap through which the flux passes isLys (or) LWt.

Effective or contracted slot pitch

tsssWWyy....)(000ssgsgggWyLlyLlAlS........C:\Users\god\Desktop\DEM\Karthi11\2.jpg


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul50ReluctanceofairgapwitheffectsofbothslotsandventilatingductsC:\Users\god\Desktop\DEM\Karthi11\5.jpgEffective or concentrated axial lengthddcdwnKLL...KcdCarter co-efficient for ductswhich depends on the ratio ofgdlwlengthgapwidthduct.sddcdgsgggywnKLlyLlAlS)(000........


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51gap contraction factor for ducts is defined as)(tanRetanRecasesmoothductswithoutgapairofcelucductswithgapairofcelucKgd.LLwnKLLKLylwnKLlddcdgdsgddcdg......00/)(/..Nowconsidertheeffectofbothslotandventilatingducts,reluctanceofthiscasegivenbysgggyLlAlS.....00..


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52Picture1Clarifications ?


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Picture1Thank you

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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul54Design of Armature (Armature winding)

i.Choice of armature windingii.Number of armature conductorsiii.Number of armature coilsiv.Number of armature slots

Choice of armature winding(a)Labwinding.No.ofparallelpathisequaltoNo.ofpoles(a=p).Thecurrentineachparallelpathsis(1/p)timesofratedarmaturecurrent(b)Wavewinding.No.ofparallelpathsisequalto2..Theconductorshavetocarryhalfoftheratedarmaturecurrent


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Dr. R.Karthigaivel, Associate Professor/EEE, PSNA CET, Dindigul55Number of armature conductors

Generated emf (or) back emf,Total no.of armature conductors,aZnpE..npEaZ..E= V+Iarm(for generator)E= V-Iarm(for motor)rm-internal resistanceIarm-internal voltage drop


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Number of armature coilsThemaximumvalueofvoltagebetweenadjacentsegmentsatloadshouldnotexceed30V.Singleturnhastwoconductors,sinceNo.ofcoilsforsingleturnwinding=(Z/2)


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ii design of dc machines

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