Eddy current question
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From: Jamie on 7 Apr 2010 06:43 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 Picture a lake of water, being calm, as a steady magnetic field and you're sitting in a row boat. Take an ore and slowly move it through the water with the paddle at its flat spot against the water. There are two things you'll notice, a slight kinetic form of energy being exerted on you and the movement of water going around the paddle in a circular formation that changes direction. Moving the ore at a faster rate through the water increase your task and thus, more kinetic energy from you and the water formation increases it's size as it circulates around the ore. Now, take a copper pipe and a neodymium magnet that will fit inside the copper pipe. Stand it vertically and drop the magnet in. You'll notice the magnet will not drop to the base of the pipe like other materials would. Most would automatically say it's just a magnet attracting itself to the pipe and clinging to it how ever, you did notice you specified copper? As you know, a magnet does not attract to it how ever, doing this experiment you'll fine the magnet falling at a slower rate than one would expect.. The better the conductor the more effect it has.. super conductors work good for these types of applications.. Any ways, as the magnet drops, Eddy currents are being developed on the walls of the pipe and roll around under the magnet. Much like you see the water on the ore rolling around and changing directions behind the paddle, This effect is like filling a void and you can think of it in the same manner with eddy currents under these conditions. And if you remember your basics with magnet's, poles of the same repel each other and this makes for a braking effect as the magnet drops.. You can find a lot of papers on the net about this. One application that is neat is the use of a shock absorber in sensitive scales. Eddy currents effects are use to dampen the scale.. Normally a natural magnet is mounted close to some part of the moving scale to create the braking effects. You have motors that employ eddy current brakes and clutches.. The faster the rotation/movement the stronger the effect..
From: Robert Baer on 7 Apr 2010 04:13 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 Oh yea; that guy i met in grade school, Eddie Current; reflected all waves towards him. Change of magnetic field induces electric field in conductor which may create a current; that current creates an opposing magnetic field. Described by Lenz's Law i think. More rapid changes means higher frequency/ies refer to Fourier series. Duration of current is NOT controlled by conductivity; refer to initial duration and Lenz's Law again.
From: neddie on 7 Apr 2010 06:07 On Apr 7, 10:13 am, Robert Baer <robertb...(a)localnet.com> wrote: > 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 > > Oh yea; that guy i met in grade school, Eddie Current; reflected all > waves towards him. > Change of magnetic field induces electric field in conductor which > may create a current; that current creates an opposing magnetic field. > Described by Lenz's Law i think. > More rapid changes means higher frequency/ies refer to Fourier series. > Duration of current is NOT controlled by conductivity; refer to > initial duration and Lenz's Law again. Thanks for all the replys. The eddy currents obviously continue to circulate after the initial source is gone. The length of time is a function of the conductivity of the metal. That is why some highly conductive metals can't be easely detected by pulse induction metal detectors.The eddy currents have dissapeared before the coils can be switched into receive mode. Cheers Rob
From: Robert Baer on 7 Apr 2010 19:34 neddie wrote: > On Apr 7, 10:13 am, Robert Baer <robertb...(a)localnet.com> wrote: >> 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 >> Oh yea; that guy i met in grade school, Eddie Current; reflected all >> waves towards him. >> Change of magnetic field induces electric field in conductor which >> may create a current; that current creates an opposing magnetic field. >> Described by Lenz's Law i think. >> More rapid changes means higher frequency/ies refer to Fourier series. >> Duration of current is NOT controlled by conductivity; refer to >> initial duration and Lenz's Law again. > > Thanks for all the replys. > The eddy currents obviously continue to circulate after the initial > source is gone. > The length of time is a function of the conductivity of the metal. > That is why some highly conductive metals > can't be easely detected by pulse induction metal detectors.The eddy > currents have dissapeared before the coils > can be switched into receive mode. > Cheers > Rob Horsefeathers; for starters there are metal detectors that DO NOT SWITCH between modes. Furthermore, the more conductive a buried meta is, the easier it is to detect.
From: neddie on 8 Apr 2010 03:45
On Apr 8, 1:34 am, Robert Baer <robertb...(a)localnet.com> wrote: > neddie wrote: > > On Apr 7, 10:13 am, Robert Baer <robertb...(a)localnet.com> wrote: > >> 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 > >> Oh yea; that guy i met in grade school, Eddie Current; reflected all > >> waves towards him. > >> Change of magnetic field induces electric field in conductor which > >> may create a current; that current creates an opposing magnetic field. > >> Described by Lenz's Law i think. > >> More rapid changes means higher frequency/ies refer to Fourier series. > >> Duration of current is NOT controlled by conductivity; refer to > >> initial duration and Lenz's Law again. > > > Thanks for all the replys. > > The eddy currents obviously continue to circulate after the initial > > source is gone. > > The length of time is a function of the conductivity of the metal. > > That is why some highly conductive metals > > can't be easely detected by pulse induction metal detectors.The eddy > > currents have dissapeared before the coils > > can be switched into receive mode. > > Cheers > > Rob > > Horsefeathers; for starters there are metal detectors that DO NOT > SWITCH between modes. > Furthermore, the more conductive a buried meta is, the easier it is > to detect. There are different types of metal detectors. Pulse induction is just one such type. |