Some Information on Eddy Currents

[Jackpine Savage]

and hopefully you can give me an answer in laymans terms. While on vacation out west I used a Whites Goldmaster at an unsuccessful attempt to find a nugget. What surprised me is how strong of a signal little tiny slivers of rusty wire can give. I'm talking tiny slivers not the boot tacks some talk about, those are easy to find. LOL Is it because of the freq used or is it a combination of the freq and some characteristic of the iron wire (rusty and insitu) ??

TIA
Tom

 

[BarnacleBill]

the issue without further adieu. No instrumentation besides a metal detector needed. Those crazy Canadians make steel clad coins encased in Silver. If "skin effect" was a factor, then those coins should predominately read as Silver since only the outer skin(surface) of the coin would be analyzed. I have been plagued by those buggers for some time now as they can act like a flat steel washer, or good targets mixed with iron.

On some machines the coins will read high(Silver) on one pass of the coil, and iron on the next. The Minelab X70 on the other hand reads them as straddling the 0(zero) disc point. Sometimes slightly negative and other times as slightly positive(foil). However this does present an opportunity to discover how different detectors in one's stable react to such a target. You may be able to learn how quickly a machine samples and how much it averages etc. And certainly an interesting test to see how the new SE reads such a complex target as compared to the former models.

HH 'eh
BarnacleBill

 

[Jackpine Savage]

and I wonder if it would react any differently. iron and steel being what they are. I found one of those new candian dimes a couple weeks ago and it read high as found but change the angle slightly and it can read as iron. BTW this was with the X-70 also.

 

[Steve(MS)]

that is, take a piece of commom alum. foil, gosh, I don't know what size, say 4" square. Now scan with detector and make note of conductivity. Then fold it one time and continue until it becomes coin sized. At each stage scan with detector.The more it is folded, the higher the conductivity, since this is so what do you make of that? It has the some amount of material but for some reason the conductivity goes up, maybe there is a simple answer, just wondering...

 

[Captain Kirk]

Tom,

Was the nugget you were trying to find one that you planted? I am supposing so. Otherwise, how did you know there were any nuggets present?

At any rate, my answer is "I do not have an answer, only an some opinions.". The wire probably has significantly greater conductivity than the nugget. What is the frequency of operation of the Goldmaster? Perhaps some real experts can help you out.

I have only found one gold nugget (I do not hunt for gold nuggets, but I accept them when I find it.). That was with my Whites XLT at the beach in the dry sand. The nugget is about 3/8" by 1/2" and someone had mounted a small diamond on it ($600 appraisal several years ago). I do not have a scale so can not measure the weight. My Explorer II will detect it at about 4" or so in the air.

Glenn

 

[Captain Kirk]

Steve,

Conductance is proportional to the cross sectional area (inversely proportional to the length). Therefore, when you fold the foil, then the conductance increases.

Glenn

 

[Cody]

I think we are confusing eddy currents as a results of a difference of potential connected to a conductor and eddy currents due to the influence of a electromagnetic field. If the frequency is increased for an applied voltage the electrons will increasingly flow towards to surface of the conductor. When a conductor is exposed to a electromagnetic field the swirls of current is perpendicular to the flux and resembles eddy current in water and is on the surface of the conductive metal. Increasing or decreasing the frequency will cause more or less penetration of the conductor or form the surface towards the center of the conductor.

 

[Cody]

Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil and their strength decreases with distance from the coil as shown in the image. Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect.

Skin effect arises when the eddy currents flowing in the test object at any depth produce magnetic fields which oppose the primary field, thus reducing net magnetic flux and causing a decrease in current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil's magnetic field, thereby weakening the magnetic field at greater depths and reducing induced currents.

The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d). The word 'standard' denotes plane wave electromagnetic field excitation within the test sample (conditions which are rarely achieved in practice). Although eddy currents penetrate deeper than one standard depth of penetration they decrease rapidly with depth. At two standard depths of penetration (2d), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3d) the eddy current density is down to only 5% of the surface density.

 

[Captain Kirk]

Cody,

Be careful here. They are all related (Maxwell's equations).

* A changing current will cause a changing magnetic field.
* A changing magnetic field causes a E field (voltage).
* The voltage causes a current.
* The current causes a magnetic field
* And on and on.

All of these things work together to establish an equilibrium. It makes little difference which one starts the process.

HH,
Glenn

 

[BarnacleBill]

n/t

 

[Cody]

Look down the page at my post "Some information on Eddy Currents" and see what you think.

 

[Cody]

Here is what troubles me about this. Assume we are using a coil to test for subsurface cracks in the metal of an aircraft body. We may have a probe, coil, of a couple inches in diameter. A device that will indicate the crack uses eddy current to look for defects at different depths. An operator scans the surface with the probe.

Where do we assume the eddy currents exist on the surface of the aircraft?

Also, if a conductor is passed through a magnetic flux are we now talking about the penetration from the skin down or from the core outward?

Non destructive testing of metals using eddy currents assume it is from the surface downward? An electronic circuits with a difference of potential connected to the circuit assumes it is from the core outward?

Help me to understand how to reconcile the above.

 

[BarnacleBill]

and describes increased depth with lower frequencies. Depth being related to starting on the outside and working it's way in.

http://www.ameritherm.com/aboutinduction.html

HH
BarnacleBill

 

[Cody]

Thanks, give this one a shot.

http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/cc_ec_index.htm

 

[Captain Kirk]

Cody,

You stated:
Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil and their strength decreases with distance from the coil as shown in the image. Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect.

I disagree with the part shown in red.

HH,
Glenn

 

[Captain Kirk]

BarnacleBill,

That is a very good representation of what is going on with eddy currents. You can learn about everything you need to know just looking at the pictures. Note that, at higher frequencies, the current flows and heating (resulting from eddy currents) occurs on the longitudinal surface (skin effect) of the rod and not on the end surfaces of the rod. Of course, when the surface gets hot, then the heat is transferred to the other portions of the rod by conduction and not by eddy currents.

HH,
Glenn

 

[BarnacleBill]

But I'll repeat a previous thought. Canadian coins minted after 2001 with steel cores are ideal for studying the effects using metal detectors and not laboratory instruments. I kind of consider them the targets from hell because the disc circuits just do not know what to make of them. At a high enough freq only the outer silver electroplate should be detected.

Interesting machines would be a couple of multi-freqs, EXII/SE, CZ, or DFX. The X70 would be very interesting with the LF,MF, & HF coils. A higher freq gold machine thrown into the mix would also be good. Burying the target at a saltwater beach in dry, damp, and wet sand to note changes caused by a more conductive ground mix would not be a bad idea. Or down there in some of that red Georgia clay with it's high iron content could be useful. All of course to see if it causes the conductive properties of the coin to predominate or the magnetic.

Big science project, load up the RV Cody!

HH
BarnacleBill

 

[Cody]

I think you are correct with the test.

I might have misunderstood in that I thought you were saying eddy currents flow around the rim of a coin. The idea is we could drill a coin so it looks like a ring and the ring coin would represent the eddy currents. My thought were that the eddy currents would look like a thicker or thiner coin. The entire surface has eddy currents so a thicker coin would represent deeper penetration of the eddy currents.

Does this make sense?

 

[Cody]

CaptKirk, I copied that from the site I posted above. I don't know one way or the other for sure. I thought the entire surface of coin down to a depth was a swirl of eddy currents. One factor in the depth of penetration is the frequency of the flux. I thought I was reading that eddy current are forced to the rim of a coin and do not cover the surface of a coin to a depth.

I simply want to be as sure as I can how the eddy current flow, around the rim or to a depth of the entire surface of a coin.