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DCGeneratorsandsolidstatecontrollertherefore

I.OldDCdynamosandhowtheywork

MostcommonDCdynamosforuseonmotorcyclesareeitherof

a)thethreebrushtype,orofb)theelectromechanicallycontrolledtype.Acommontypeofsuchdynamos,whichfromtheoutsidealllookessentiallythesame,isshownbelowinFig1

a)thethreebrushtype

Thethreebrushtypedynamo,normallyhasanelectromechanicalcutoutswitch,butlacksanyseparateexternalvoltageregulationmeans.Voltageregulationisinherenttoitsconstructionandinfactdependsontheamountofdistortionofthemagneticfieldunderdifferentelectricloads(seeFigure2).However,thismannerofregulationislimitedinitsabilitytokeepthevoltageandcurrentinlinewiththeactualrequirements.

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ExamplesofelectricalcircuitsofdifferenttwopoleandfourpoledynamosofthistypeareshownbelowinFigure3:

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InthesecircuitsLeisthefieldwinding,SgafuseandMgthecutoutsolenoidswitch.ThebrusharrangementshowninFigure4(twopolearrangement)issuitableforeasychangeofdirectionofrotationofthedynamo.

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b)theelectromechanicallycontrolledtype

Theelectromechanicallycontrolledtype,orvibratingregulatortype,permitsthedynamotomaintainthesameorconstantvoltagethroughoutthehigherspeedsand,ifalsoequippedwithcurrentregulatingcontacts,limitsvoltageandcurrenttoamaximumallowablevalue.Sucharegulatorproducesnearlyperfectbatterychargingcharacteristics.However,becauseofthevibratingcontactstheseregulatorsarepronetowearandcanberepairedonlybyaspecialisthavingsufficientknowledgeofthespecifictypeofregulator.Furthermore,oftenthedynamofoundatanautojumbleiswithoutitsregulatororthewrongtypeismountedandreplacementprovesverydifficulttofindnowadaysevenifonehappenstoknowwhattypeofregulatorhadbeenusedoriginally.

AnexampleofoneofthegreatvarietyofsuchregulatorsanditselectricalcircuitisshowninFigure5

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III.AdditionalconsiderationswhenrestoringaDCDynamoforamotorbike

Iftheoriginallymountedgeneratorisoftheelectromechanicallycontrolledtypeandworkswelloriscompleteandstillrestorable,thereisnormallynoneedforaconversionoftheregulator,especiallythemotorcycle'soriginalitycanthusbepreserved.

However,forthosewhohavenochoicebecauseessentialpartsaremissingorarebeyondrepairor,ifitisthoughtnecessarytoimprovethereliabilityofthepoweroutput(inparticularofathreebrushtypeofgenerator),itissuggestedthatuseshouldbemadeofthesolidstateregulatorpresentedinthisarticle.

InsomeEuropeancountrieselectriclightingismandatoryevenforveryoldmotorcyclesiftheyareusedonthepublicroadsandinsuchacaseconversiontoelectriclightingandanelectricpowergeneratorbecomesunavoidable.Theoldestgenerators,soldseparatelyatthetimeasanaccessoryandthereforepreferredforsuchaconversion,arenearlyalwaysofthe6Voltsthreebrushtype.Theylooknice,aresmallandinstallationisnormallynottoodifficult,buttheelectricoutputisverylow.However,whenusingtheregulatorpresentedinthisarticlesuchageneratorcanbeadaptedtobecomea12Voltsgeneratortherebyeffectivelydoublingitspoweroutput!Itisalsoeasytochangethedirectionofrotationoftheadaptedgeneratorbyjustinterchangingtheconnectionsofthefiledwindings.

Becausethefieldwindingsarecontrolleddirectlybytheregulatorthethirdbrush,thepositionofwhichwithrespecttothepositivebrush(seefigures3and4)normallydeterminesthesenseofrotation,isnotneededanymore.

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III.The"homemade"regulator

Theregulatorcircuitshownonthelastpageofthisarticle,wasdevelopedfromastateoftheartregulatorformodernalternatorsandgivesasolutiontomostoftheabovestatedproblems.ItcanbeusedincombinationwithalmostanyD.C.dynamoandcaneasilybeadjustedtoparticularneedswithouttamperingwiththedynamoitself.Totalcostsareonlyabout15DMinGermany.TheproposedregulatorcomprisesadiodeD2whichreplacestheelectromechanicalcutoutswitch.Theregulatorcanbebuildsmallenoughtobeinsertedinplaceoftheoriginalcutoutswitchorelectromechanicalregulator.Whenusedincombinationwithathreebrushdynamothedynamocanbecontrolledinthesamemannerasanormallyregulateddynamobycontrollingthefieldvoltagedirectly.Inthatcasethethirdbrushshouldbedispensedwithaltogetherandsuchconversionalsohastheadvantagethatpowerlossesaretoagreatextentreduced.

IV.Functioningoftheregulator

Thecircuitactsasaswitch,supplyingeitherfullornovoltagetothefieldwindingofthedynamo.Whenthedynamovoltageisbelow13volts,ZenerdiodeDzdoesnotconduct,T1isoffandT2ison,thusthedynamovoltageisfullyappliedtothefieldwindingsoitsuppliesincreasingvoltagetotheregulatorcircuitand,throughresistorR5anddiodeD2,tothebattery.Whenthecircuitvoltageexceeds13volts,DzstartstoconductandthevoltageacrossresistorP1risestoapoint,dependingonthesettingofP1,whereT1startstoconductandT2stopsconducting,therebyswitchingoffthecurrenttothefieldwindingwithasubsequentdecreaseofthedynamooutputvoltage

DuringregulationthesesequencesfollowinafastmodesuchthatthedynamovoltageiskeptregulatedatthedesiredvoltagesetbyP1.

ThecomponentsR4,R5andT3formacurrentlimitingcircuit.ThevalueofR5isselectedtolimitthemaximumoutputcurrentinordertoavoidoverloadofthedynamo.ThevalueofresistorR5ischosensuchthat,atthemaximumallowablecurrentloadofthedynamo,thevoltageacrossthisresistorissothatT3startsconductingwherebythevoltageacrossP1increases,evenwithoutdiodeDzconducting,andtheoutputvoltageofthedynamoisconsequentlydecreased.

V.Howtoconverta6Voltsdynamoto12Volts

Themaximumallowablepoweroutputofad.c.dynamoismainlydependentuponthemaximumallowabletemperatureofthewindings,whichisessentiallydependentuponthecurrent(I)inaccordancewiththefunctionQ=I.R(QistheamountofheatgenerationinWattsandRistheresistanceofthefieldcoil).

Consequently,forafixedmaximumcurrentratingandwithintheratedspeedlimit,thepoweroutput(P)ofadynamoisessentiallydirectlyproportionaltotheoutputvoltage(V)inaccordancewiththefunctionP=V.I.Takingintoaccounttheseprinciplesandthefactthata6Voltsdynamoeasilygives40Voltswhenitisnotregulated,theregulatordescribedcanalsobeusedtoconverta6voltdynamotogive12volts,andthusdoubleitspoweroutput,withoutfurthermodificationstothedynamoitself!

ItishoweveradvisedtoinsertinserieswiththefieldcoilaresistorR2ofthesameresistanceasthefieldcoilresistancetolimitthefieldcurrenttothe6voltsvalueandsolimittheloadoftransistorT2andreducetheriskofoverheatingofthefieldwindingsatlowdynamospeedswhen,asfollowsfromtheaboveexplanation,fullvoltage(thusupto13Volts)issuppliedtothefieldwindings.WithsomedynamosdifficultieshavebeenencounteredwhenusedwithadiodeD2insteadofacutout.Anelectromagneticcutoutallowsthetensionofthedynamotorisebeforetheconnectiontothebatteryismadeandthereforeitiseasierforthedynamoto"startup"inparticularwhentheremainingmagnetisminthefieldpiecesissmall.Ifyouhavedifficultieswitha"blackbox"forconverting6Voltsdynamoto12Voltsorpowerisunsufficient,tryoutwhetheranelectromagneticalcutoutswitchworksbetterandalsocheckifthefieldwindingsgetenoughpower.Normallyabadadaptationoftheblackboxcontrollertothedynamoconcernedisthecauseofthetrouble.

Ifrequired,6voltsregulationmaybemaintainedbyusinga6voltsZenerdiodeinsteadofa13voltsZenerdiodeandbyadjustingthetrimpotP1togivebatteryloadingvoltageof7volts.

Itisalwayspossibletomaintaintheoriginalcutoutrelay,whichshouldthenbeinstalledinsteadofthediodeD2.

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VI.Detailsofthecircuitanditsadjustment

DiodeD2isa50volts,6amps,ordependentuponthedynamooutput(forolddynamosgenerallyintherangeof3060Watts),evenhigherratedtypewithlowinternalresistancetolimitthevoltagedropoverthediodeatthemaximumcurrentrating.

Ifmaximumcurrentshouldbelimitedto3amps(upto5amps),theresistorR5shouldbechosentobeabout0.15ohms(downto0.08ohms).Ifnotavailablesucharesistorcanbemadefromahighloadwirewoundresistor,cuttingoffawirelengthinproportiontotheknownresistanceofthewirewoundresistortoobtain0.15ohmsandwrappingthiswireonaresistorofsay,atleast100ohms,asasupportforthewireandforeasymountinginthecircuit.

ThevalueofresistorR2isselectedtobeequalorslightlylowerthanthefieldcoilresistance(tobemeasuredwithanOhmmeter)forthereasonsmentionedinpointIVabove.

TemperaturecompensationcanbeobtainedbymountingoneortwodiodesinserieswiththezenerdiodeDz.InsteadoftheTransistorT2alsoadarlingtonpaircanbeused,suchasBD645.However,inpractisethesefurtheradaptationsprovedunnecessaryandarementionedonlyasalternativesforthosewholiketoexperiment.

Adjustmentofthevoltageshouldbecarriedoutwithafullychargedbatteryofthetypetobeusedonthemotorbike.Themaximumvoltage,normallybetween14and14.5Volts,shouldbeadjustedsothatthereisstillasmallchargingcurrent,theintensityofwhichdependsonthesizeofthebattery(usuallybetween0.1Ampsforsmalland0.5Ampsforbiggerbatteries).

Useahighqualitytrimpot(cermetorequivalent)forP1and,aftersettingittotherequiredbatteryvoltage,sealthesettingscrewwithablobofpaint.

Adjustedinthismannertheriskofoverloadingthebatteryiseffectivelypreventedanditisensuredthatthemaximumcapacityofthebatteryisalwaysavailable.

T2shouldbemountedonasmallheatsink.

Agreatadvantageoftheregulatoristhat,incontrasttothethreebrushtypeofregulation,itsfunctioningisindependentofthebatteryandcanthusalsobeusedincasethebatteryisdisconnectedwithoutariskofblowingthebulbs.

12Voltsregulatorcircuit

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