Eddy current question
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From: neddie on 6 Apr 2010 04:40 Hi to all. I was just wondering what the characteristics of eddy currents are. For instance. If you have a pulse in a coil , as in a pulse induction metal detector,that introduces eddy currents in nearby metal , what format are the currents. Are thay ac , dc(decreacing in amplitude), what frequency. I know that the duration of the current is a factor if the conductivity of the metal , but I'm not sure about the rest. Cheers Rob
From: J.A. Legris on 6 Apr 2010 10:45 On Apr 6, 4:40 am, neddie <seegoo...(a)yahoo.com> wrote: > Hi to all. > I was just wondering what the characteristics of eddy currents are. > For instance. If you have a pulse in a coil , as in a pulse induction > metal detector,that > introduces eddy currents in nearby metal , what format are the > currents. Are thay ac , dc(decreacing in amplitude), > what frequency. I know that the duration of the current is a factor if > the conductivity of the metal > , but I'm not sure about the rest. > Cheers > Rob The duration of an eddy current depends mainly on the duration of the varying magnetic field that caused it. Perhaps you meant the *magnitude" of the current, which does depend on the conductivity of the metal, among other things. See: http://en.wikipedia.org/wiki/Eddy_current -- Joe
From: Tim Wescott on 6 Apr 2010 10:57 neddie wrote: > Hi to all. > I was just wondering what the characteristics of eddy currents are. > For instance. If you have a pulse in a coil , as in a pulse induction > metal detector,that > introduces eddy currents in nearby metal , what format are the > currents. Are thay ac , dc(decreacing in amplitude), > what frequency. I know that the duration of the current is a factor if > the conductivity of the metal > , but I'm not sure about the rest. > Cheers > Rob The eddy current will be a (mostly) linearly filtered version of the current in the coil that excited it. So a pulsed current in the coil will cause a pulsed eddy current, a single-frequency AC current in the coil will cause eddy current at that frequency, etc. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Vladimir Vassilevsky on 6 Apr 2010 11:09 neddie wrote: > Hi to all. > I was just wondering what the characteristics of eddy currents are. > For instance. If you have a pulse in a coil , as in a pulse induction > metal detector,that > introduces eddy currents in nearby metal , what format are the > currents. Are thay ac , dc(decreacing in amplitude), > what frequency. I know that the duration of the current is a factor if > the conductivity of the metal > , but I'm not sure about the rest. Speaking of PI metal detector, the induced current is instantaneously up and then exponential decay. VLV
From: markp on 6 Apr 2010 14:12
"Tim Wescott" <tim(a)seemywebsite.now> wrote in message news:RJKdnX_c5un31ibWnZ2dnUVZ_s4zAAAA(a)web-ster.com... > neddie wrote: >> Hi to all. >> I was just wondering what the characteristics of eddy currents are. >> For instance. If you have a pulse in a coil , as in a pulse induction >> metal detector,that >> introduces eddy currents in nearby metal , what format are the >> currents. Are thay ac , dc(decreacing in amplitude), >> what frequency. I know that the duration of the current is a factor if >> the conductivity of the metal >> , but I'm not sure about the rest. >> Cheers >> Rob > > The eddy current will be a (mostly) linearly filtered version of the > current in the coil that excited it. So a pulsed current in the coil will > cause a pulsed eddy current, a single-frequency AC current in the coil > will cause eddy current at that frequency, etc. > > -- It will also load the coil, there will be power dissipated in the resistance and this has to be reflected as power into the coil (change of phase of current v voltage etc). Mark. |