How Do Eddy-current Brakes Work_ - Explain That Stuff

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  • 4/17/2015 Howdoeddycurrentbrakeswork?ExplainthatStuff

    http://www.explainthatstuff.com/eddycurrentbrakes.html 1/10

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    byChrisWoodford.Lastupdated:September15,2014.

  • 4/17/2015 Howdoeddycurrentbrakeswork?ExplainthatStuff

    http://www.explainthatstuff.com/eddycurrentbrakes.html 2/10

    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

  • 4/17/2015 Howdoeddycurrentbrakeswork?ExplainthatStuff

    http://www.explainthatstuff.com/eddycurrentbrakes.html 3/10

    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

  • 4/17/2015 Howdoeddycurrentbrakeswork?ExplainthatStuff

    http://www.explainthatstuff.com/eddycurrentbrakes.html 4/10

    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

  • 4/17/2015 Howdoeddycurrentbrakeswork?ExplainthatStuff

    http://www.explainthatstuff.com/eddycurrentbrakes.html 5/10

    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'