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byChrisWoodford.Lastupdated:September15,2014.

4/17/2015 Howdoeddycurrentbrakeswork?ExplainthatStuff

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Oneofthedrawbacksofgoinganywherefastisthatyoualwayshavetostopsoonerorlater.Inanemergency,whenyouhavetobrakequickly,theonlythingthatcomesbetweensafestoppinganddisasteristhesimplescienceoffriction:youslowtoahaltwhentwosurfacesrubtogether.Nowfrictionbrakeshavemorethanprovedtheirworth:you'llfindthemineverycar,bicycle,airplane,andmostfactorymachines.Buttheyhaveabigdrawbacktoo:everytimeyouusethem,theywearoutalittlebit,andthatmeansthey'rerelativelyexpensive.What'sthealternative?Oneoptionistoslowthingsdownwiththeforceofelectromagnetisminsteadoffriction.ItsoundslikesomethingoutofFlashGordonorBuckRogers,butit'sthebasicideabehindeddycurrentbrakes,whichcancosthalfasmuchtorunovertheirlifetimeastraditional,frictionbrakes.Whataretheyarehowdotheywork?Let'stakeacloserlook!

Photo:Eddycurrentbrakesinaction!Relativelyfewtrainsuseelectromagneticbraking,butthisisanexception:theJapaneseShinkansen700bullettrainrunningtheNozomiservice.Youcanseeaphotoofthebrakesusedinthistrainfurtherdownthepage.PhotobyDougBowmanpublishedonFlickrin2006underaCreativeCommonsLicence.

Howordinary(friction)brakeswork

Movingthingshavekineticenergyand,ifyouwanttostopthem,youhavetogetridofthatenergysomehow.Ifyou'reonabicyclegoingfairlyslowly,youcansimplyputyourfeetdownsotheydragontheground.Thesolesofyourfeetactasbrakes.Friction(rubbing)betweentheroughgroundandthegriponyoursolesslowsyoudown,convertingyourkineticenergyintoheatenergy(doitlongenoughandyourshoeswillgethot).Brakesonvehiclesworkprettymuchthesameway,with"shoes"thatpressrubberpads(brakeblocks)againstdiscsmountedtothewheels.(Findoutmoreaboutthisinourmainarticleonbrakes.)

Evenifyoumakebrakesfromsuperstrong,hardwearingmaterialslikeKevlar,they'restillgoingtowearoutsoonerorlater.Butthereareotherproblemswithfrictionbrakes.Thefasteryougo,thehardertheyhavetoworktogetridofyourkineticenergy,andthequickerthey'llwearout.Useyourbrakestoooftenandyoumaysufferaproblemcalledbrakefade,whereheatbuildsuptoomuchinthebrakesorthehydraulicsystemthatoperatesthemandthebrakescannolongerworkaseffectively.Whatifyourbrakescan'tstopyouintime?

Photo:Motorcyclebrakes:Likemostvehicles,thisbikebrakeswithfriction.Whenyoupullonthebrakehandle,ahydrauliccableappliesthebrakepadstothebrakerotordisc,slowingthemachinedownby

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convertingyourkineticenergytoheat.Thetiredoesn'tnormallyplaymuchpartinbrakingunlessyoubrakereallyhard:thenthewheelwilllockcompletelyandfrictionbetweenthetireandtheroadwillbringyoutoasuddenhalt,leavingarubberskidmarkontheroad.That'snotagoodwaytobrake:it'llwearoutyourtiresveryquickly.

Whatareeddycurrents?

Beforewecanunderstandeddycurrentbrakes,weneedtounderstandeddycurrents!They'repartofthescienceofelectromagnetism:electricityandmagnetismaren'ttwoseparatethingsbuttwosidesofthesame"coin"twodifferentaspectsofthesameunderlyingphenomenon.

Electricityandmagnetismgohandinhand

Whereveryougetelectricity,yougetmagnetismaswell,andviceversa.Thisisthebasicideabehindelectricitygeneratorsandelectricmotors.Generatorsusesomekindofmovement(maybeawindturbinerotorspinningaround)tomakeanelectriccurrent,whilemotorsdotheopposite,convertinganelectriccurrentintomovementthatcandriveamachine(orpropelsomethinglikeanelectriccarorelectricbike).

Bothkindsofmachine(theyarevirtuallyidentical)workontheideathatyoucanuseelectricitytomakemagnetismormagnetismtomakeelectricity.Tomakeelectricity,allyouhavetodoismoveanelectricalconductor(likeacopperwire)throughamagneticfield.That'sit!It'scalledFaraday'slawofinductionafterEnglishscientistMichaelFaraday,whodiscoveredtheeffectintheearly19thcentury.Ifyouconnectthewireuptoameter,you'llseetheneedleflickeverytimeyoumovethewire(butonlywhenyoumoveit).Ifyouwereclever,youcouldfigureoutsomewayofremovingtheelectricityandstoringit:you'dhavemadeyourselfaminiatureelectricpowerplant.

Photo:Abasicelectricmotorhasanaxle(thesilverrodinthemiddle)thatrotateswhenyoufeedelectricityintothemotor'scoppercoilthroughtwowires.Ageneratorissimilar,butyouturntheaxlemanuallyandgetelectricityoutofthewiresinstead.

Howeddycurrentsaremade

Whatiftheconductoryou'removingthroughthemagneticfieldisn'tawirethatallowstheelectricitytoflowneatlyaway?Youstillgetelectriccurrents,butinsteadofflowingoffsomewhere,theyswirlaboutinsidethematerial.Thesearewhatwecalleddycurrents.They'reelectriccurrentsgeneratedinsideaconductorbyamagneticfieldthatcan'tflowawaysotheyswirlaroundinstead,dissipatingtheirenergy

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asheat.

Oneoftheinterestingthingsabouteddycurrentsisthatthey'renotcompletelyrandom:theyflowinaparticularwaytotrytostopwhateveritisthatcausesthem.ThisisanexampleofanotherbitofelectromagnetismcalledLenz'slaw(itfollowsonfromanotherlawcalledtheconservationofenergy,andit'sbuiltintothefourequationssummarizingelectromagnetismthatweresetoutbyJamesClerkMaxwell).

Here'sanexample.Supposeyoudropacoinshapedmagnetdowntheinsideofaplasticpipe.Itmighttakeahalfsecondtogettothebottom.Nowrepeatthesameexperimentwithacopperpipeandyou'llfindyourmagnettakesmuchlonger(maybethreeorfourseconds)tomakeexactlythesamejourney.Eddycurrentsarethereason.Whenthemagnetfallsthroughthepipe,youhaveamagneticfieldmovingthroughastationaryconductor(whichisexactlythesameasaconductormovingthroughastationarymagneticfield).Thatcreateselectriccurrentsintheconductoreddycurrents,infact.Nowweknowfromthelawsofelectromagnetismthatwhenacurrentflowsinaconductor,itproducesamagneticfield.Sotheeddycurrentsgeneratetheirownmagneticfield.Lenz'slawtellsusthatthismagneticfieldwilltrytoopposeitscause,whichisthefallingmagnet.Sotheeddycurrentsandthesecondmagneticfieldproduceanupwardforceonthemagnetthattriestostopitfromfalling.That'swhyitfallsmoreslowly.Inotherwords,theeddycurrentsproduceabrakingeffectonthefallingmagnet.

It'sbecauseeddycurrentsalwaysopposewhatevercausesthemthatwecanusethemasbrakesinvehicles,engines,andothermachines.

Howdoesaneddycurrentbrakestopsomethingmoving?

Supposewehavearailroadtrainthat'sactuallyahugesolidblockofcoppermountedonwheels.Let'ssayit'shurtlingalongathighspeedandwewanttostopit.Wecouldapplyfrictionbrakestothewheelsorwecouldstopitwitheddycurrents.How?Whatifweputagiantmagnetnexttothetracksothetrainhadtopassnearby.Asthecopperapproachedthemagnet,eddycurrentswouldbegenerated(or"induced")insidethecopper,whichwouldproducetheirownmagneticfield.Eddycurrentsindifferentpartsofthecopperwouldtrytoworkindifferentways.Asthefrontpartofthetrainapproachedthemagnet,eddycurrentsinthatbitofthecopperwouldtrytogeneratearepulsivemagneticfield(toslowdownthecopper'sapproachtothemagnet).Asthefrontpartpassedby,slowingdown,thecurrentstherewouldreverse,generatinganattractivemagneticfieldthattriedtopullthetrainbackagain(again,slowingitdown).Thecopperwouldheatupastheeddycurrentsswirledinsideit,gainingthekineticenergylostbythetrainasitsloweddown.Itmightsoundlikeastrangewaytostopatrain,butitreallydoeswork.You'llfindtheproofofitinmanyrollercoastercars,whichusemagneticbrakeslikethis,mountedonthesideofthetrack,toslowthemdown.

Artwork:Here'soursimplecopperblocktrainmovingfromrighttoleft,andI'veembeddedagiantbar

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magnetinthetracktostopit.Asthetrainapproaches,eddycurrentsareinducedinthefrontofitthatproducearepulsivemagneticfield,whichslowsthetraindown.Ifthetrainismovingreallyfast,thismagnetmightnotstopitcompletely,soit'llkeepmovingbeyondthemagnet.Asitmovespasttheotherendofthemagnet,theinducededdycurrentswillworktheoppositeway.Nowthey'llproduceanattractivemagneticfieldthattriestopullthetrainbackward,butstilltryingtoslowitdown.Thebasicpointissimple:theeddycurrentsarealwaystryingtoopposewhatevercausesthem.(Notethateddycurrentsareactuallyinducedthroughthewholeofthecopperblock,butI'vedrawnonlyafewofthemforclarity.)

Typesofeddycurrentbrakes

Realeddycurrentbrakesareabitmoresophisticatedthanthis,butworkinessentiallythesameway.Theywerefirstproposedinthe19thcenturybythebrilliantFrenchphysicistJeanBernardLonFoucault(alsotheinventoroftheFoucaultpendulumandoneofthefirstpeopletomeasurethespeedoflightaccuratelyonEarth).Eddycurrentbrakescomeintwobasicflavorslinearandcircular.

Linearbrakes

Linearbrakesfeatureonthingsliketraintracksandrollercoasters,wherethetrackitself(orsomethingmountedonit)worksaspartofthebrake.

Thesimplestlinear,eddycurrentbrakeshavetwocomponents,oneofwhichisstationarywhiletheothermovespastitinastraightline.Inarollercoasterride,youmighthaveaseriesofpowerful,permanentmagnetspermanentlymountedattheendofthetrack,whichproduceeddycurrentsinpiecesofmetalmountedonthesideofthecarsastheywhistlepast.Thecarsmovefreelyalongthetrackuntiltheyreachtheveryendoftheride,wherethemagnetsmeetthemetalandthebrakeskickin.

Thiskindofapproachisnouseforaconventionaltrain,becausethebrakesmightneedtobeappliedatanypointonthetrack.Thatmeansthemagnetshavetobebuiltintothestructurethatcarriesthetrain'swheels(knownasthebogies)andtheyhavetobethekindofmagnetsyoucanswitchonandoff(electromagnets,inotherwords).Typically,theelectromagnetsmovealittlelessthan1cm(lessthan0.5in)fromtherailand,whenactivated,slowthetrainbycreatingeddycurrents(andgeneratingheat)insidetherailitself.It'sabasiclawofelectromagnetismthatyoucanonlygenerateacurrentwhenyouactuallymoveaconductorthroughamagneticfield(notwhentheconductorisstationary)itfollowsthatyoucanuseaneddycurrentbraketostopatrain,butnottoholditstationaryonceit'sstopped(onsomethinglikeanincline).Forthatreason,vehicleswitheddycurrentbrakesneedconventionalbrakesaswell.

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Photo:Thelineareddycurrentbrakesfromarollercoaster.(Thebrakesaretheblackthingsmountedonthesideofthetrack.)PhotobyStefanScheercourtesyofWikimediaCommonspublishedunderaCreativeCommonsLicence.

Circularbrakes

Likelineareddycurrentbrakes,circularbrakesalsohaveonestaticpartandonemovingpart.Theycomeintwomainkinds,accordingtowhethertheelectromagnetmovesorstaysstill.Thesimplestoneslookliketraditionalbrakes,onlywithastaticelectromagnetthatappliesmagnetismandcreateseddycurrentsinarotatingmetaldisc(insteadofsimplepressureandfriction)thatmovesthroughit.(TheShinkansenbrakesworklikethis.)Intheotherdesign,theelectromagnetsmoveinstead:there'saseriesofelectromagnetcoilsmountedonanouterwheelthatspinsaround(andappliesmagnetismto)afixed,centralshaft.(Telmafrictionless"retarder"brakes,usedonmanytrucks,buses,andcoaches,workthisway.)

Howdothesethingsworkinpractice?Supposeyouhaveahighspeedfactorymachinethatyouwanttostopwithoutfriction.Youcouldmountametalwheelononeendofthedriveshaftandsititbetweensomeelectromagnets.Wheneveryouwantedtostopthemachine,you'djustswitchontheelectromagnetstocreateeddycurrentsinthemetalwheelthatbringitquicklytoahalt.Alternatively,youcouldmounttheelectromagnetcoilsontherotatingshaftandhavethemspinaroundorinsidestationarypiecesofmetal.

Withalinearbrake,theheatgeneratedbytheeddycurrentscanbedissipatedrelativelyeasily:it'seasytoseehowitwoulddisappearfairlyquicklyinabrakeoperatingoutdoorsoverarelativelylongsectionoftraintrack.Gettingridofheatismoreofanissuewithcircularbrakes,wheretheeddycurrentsareconstantlycirculatinginthesamepieceofmetal.Forthisreason,circulareddycurrentbrakesneedsomesortofcoolingsystem.Aircooledbrakeshavemetalmeshes,opentotheair,whichusefanbladestopullcoldairthroughthem.Liquidcooledbrakesusecoolingfluidstoremoveheatinstead.

Photo:CloseupofthecirculareddycurrentbrakefromtheShinkansen700traininourtopphoto.Althoughthisresemblesthemotorcyclefrictionbrakeupabove,itworksinatotallydifferentway.Youcanseethebrownbrakediscandtheelectromagnetthatsurroundsitatthetop.Whenthebrakeisapplied,theelectromagnetswitchesonandinduceseddycurrentsinthedisc,whichcreateopposing

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magneticfieldsandstopitrotating.Unlikeinthemotorcyclebrake,thereisnocontactatallbetweentheelectromagnetandthebrakedisc:there'sanairgapofafewmillimetersbetweenthem.PhotocourtesyofWikimediaCommonspublishedunderaCreativeCommonsLicence.

Whereareeddycurrentbrakesused?

Despitebeinginventedoveracenturyago,eddycurrentbrakesarestillrelativelylittleused.Apartfromrollercoasters,oneareawherethey'renowfindingapplicationsisinhighspeedelectrictrains.SomeversionsoftheGermanInterCityExpress(ICE)trainandJapaneseShinkansen("bullettrain")haveexperimentedwitheddycurrentbrakesandfutureversionsoftheFrenchTGVareexpectedtousethemaswell.You'llalsofindeddycurrentbrakesinallkindsofmachines,suchascircularsawsandotherpowerequipment.Andthey'reusedinthingslikerowingmachinesandgymmachinestoapplyextraresistancetothemovingpartssoyourmuscleshavetoworkharder.

Advantagesanddisadvantagesofeddycurrentbrakes

Ontheplusside,eddycurrentbrakesarequiet,frictionless,andwearfree,andrequirelittleornomaintenance.Theyproducenosmellorpollution(unlikefrictionbrakes,whichcanreleasetoxicchemicalsintotheenvironment).Allthismakesthemmuchmoreattractivethannoisyfrictionbrakesthatneedregularinspectionandroutinelywearout.It'sbeenestimatedthatswitchinganelectrictrainfromfrictionbrakestoeddycurrentbrakescouldhalvethecostofbrakeoperationandmaintenanceoveritslifetime.

Thedrawbacksofeddycurrentbrakesaremoretodowithhowlittleexperiencewehaveofusingtheminrealworldsettings.AsJenniferSchykowskinotedinanexcellentreviewofthetechnologyforRailwayGazettein2008,theelectromagneticpartsofeddycurrentbrakeshavesometimescausedproblemsbyinterferingwithtrainsignalingequipment.Althoughheatdissipationinrailsshouldnot,theoretically,beanissue,ifthere'sabusysectionoftrackwheremanytrainsbrakeinquicksuccession(somethingliketheapproachtoastation),theheatingandexpansionofrailscouldprovetobeanissue,eitherreducingtheeffectivenessofthebrakesorleadingtostructuralproblemsintherailsthemselves.Anotherimportantquestioniswhethereddycurrentbrakeswilleverbecomewidespread,giventhegrowinginterestinregenerativebrakesthatcaptureandstoretheenergyofmovingvehiclesforreuse(amuchmoreenergyefficientapproachthanturningenergyintouselessheatwitheddycurrents).SomeofthelatestShinkansentrains(seriesE5)useregenerativebrakeswhereearliermodelsusededdycurrenttechnology.

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EnergyMagnetismRegenerativebrakes

Books

ElectromagneticfieldsinelectricalengineeringbySlawomirWiak.IOSPress,2002.Anintroductiontothevariouspracticalapplicationsofelectromagnetism.

Articles

Eddycurrentbraking:alongroadtosuccessbyJenniferSchykowski.RailwayGazette,2June2008.Aninterestingsurveyofhowrailroadcompanies(notablyDeutscheBahn,DB)havetriedtodeployeddycurrentbrakes,withmixedresults.Mysteriousforces:EddyCurrentsbyDaveAnsell,NakedScientists.Inthisshortarticle,Davedemonstrateswhyamagnetfallsmoreslowlydownacopperpipeduetoeddycurrents,andgoesontoexplainwhateddycurrentsactuallyare.

Videos

Eddycurrentbrakeinaction:Watchhowthischildren'shorseslowsdownasitreachestheeddycurrentbrakeattheendofthetrack.

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