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Technical Discussions
Paul Wray
Paul Wray
07:27 Aug-18-2002
Skin effect in MP coil shot ?

I am developing some educational materials on NDT, but NDT is not my field. Can anyone help with this question?

My question is: does the skin effect operate during an AC coil shot in magnetic particle testing ? The common folklore in MPT seems to be that skin effect operates whenever there is AC current.
The skin effect is about the confinement of current to the surface of a conductor. When passing current along a conductor, the current is confined to the surface, and so the field is strongest at the surface, with no internal field. Thus the interior of tubes cannot be inspected using current flow.

Am I correct in saying that this does NOT apply to the coil shot ? I can see no reason why an AC magnetic field would be much stronger at the outside of a tube than at the inside, unless its something to do with induced currents.
Can anyone set me straight on this, hopefully giving a rough quantitative picture of the effect of eddy currents, if they are significant to this situation?



    
 
 Reply 
 
rodney fordham
Consultant
United Kingdom, Joined Feb 2002, 12

rodney fordham

Consultant
United Kingdom,
Joined Feb 2002
12
04:57 Aug-18-2002
Re: Skin effect in MP coil shot ?
:When an AC current flows in a conducting wire there is a tendency for the current density within the wire to be non uniform, less at the centre more at the surface. This is called the skin effect and becomes more pronounced as the AC frequency increases. In radio coils this tendency can be reduced by replacing single wire conductors by a bundle of insulated fine wires with the same cross section, called litzendraht which sounds like it was invented in Germany. At frequencies at or below the kilocycle range the effect in wires is small. Where current is induced in larger volumes through a surface by inductive coupling the effect will be increased. What frequencies are used for eddy current testing? And what thicknesses or what materials are of interest? The physical state (annealed or hardened) could also be relevant.

: I am developing some educational materials on NDT, but NDT is not my field. Can anyone help with this question?
.
: My question is: does the skin effect operate during an AC coil shot in magnetic particle testing ? The common folklore in MPT seems to be that skin effect operates whenever there is AC current.
: The skin effect is about the confinement of current to the surface of a conductor. When passing current along a conductor, the current is confined to the surface, and so the field is strongest at the surface, with no internal field. Thus the interior of tubes cannot be inspected using current flow.
.
: Am I correct in saying that this does NOT apply to the coil shot ? I can see no reason why an AC magnetic field would be much stronger at the outside of a tube than at the inside, unless its something to do with induced currents.
: Can anyone set me straight on this, hopefully giving a rough quantitative picture of the effect of eddy currents, if they are significant to this situation?
.



    
 
 Reply 
 
Tom Bloodworth
Consultant, R&D
Bloodworth Consulting Limited, United Kingdom, Joined Aug 2002, 12

Tom Bloodworth

Consultant, R&D
Bloodworth Consulting Limited,
United Kingdom,
Joined Aug 2002
12
06:25 Aug-19-2002
Re: Skin effect in MP coil shot ?
I am not sure that I understand your terminology (i.e. what the geometry is of an "ac coil shot"), but the reason that there is no field on the inside of a tube along which current flows axially is nothing to do with the skin depth effect. The same is true at DC. It can be understood by considering Ampere's circuital law.

To answer your direct question simply: yes the skin effect operates whenever you apply an AC magnetic field. This is physics, not folklore!

If you are encircling (long) tubes with an AC coil - so the coil is co-axial with the tube - at the coil location you will have stronger eddy currents on the outer surface of the coil than on the inner surface. The strength of the inner eddy currents (and that of the magnetic field) depends on the frequency, material resisitivity and permeability and the tube wall thickness. If you test at mains frequency (50Hz) then you will penetrate several millimetres in steel (see the graph on the Hocking site).

Things get more complicated for short tubes, or for locations away from the coil (remote field etc.), but the basics stay the same. I can therefore think of instances where, if I have understood the geometry, testing should be possible.

Tom


: I am developing some educational materials on NDT, but NDT is not my field. Can anyone help with this question?
.
: My question is: does the skin effect operate during an AC coil shot in magnetic particle testing ? The common folklore in MPT seems to be that skin effect operates whenever there is AC current.
: The skin effect is about the confinement of current to the surface of a conductor. When passing current along a conductor, the current is confined to the surface, and so the field is strongest at the surface, with no internal field. Thus the interior of tubes cannot be inspected using current flow.
.
: Am I correct in saying that this does NOT apply to the coil shot ? I can see no reason why an AC magnetic field would be much stronger at the outside of a tube than at the inside,unless its something to do with induced currents.
: Can anyone set me straight on this, hopefully giving a rough quantitative picture of the effect of eddy currents, if they are significant to this situation?
.




    
 
 Reply 
 
Tom Bloodworth
Consultant, R&D
Bloodworth Consulting Limited, United Kingdom, Joined Aug 2002, 12

Tom Bloodworth

Consultant, R&D
Bloodworth Consulting Limited,
United Kingdom,
Joined Aug 2002
12
06:29 Aug-19-2002
Re: Skin effect in MP coil shot ?
: I am not sure that I understand your terminology (i.e. what the geometry is of an "ac coil shot"), but the reason that there is no field on the inside of a tube along which current flows axially is nothing to do with the skin depth effect. The same is true at DC. It can be understood by considering Ampere's circuital law.
.
: To answer your direct question simply: yes the skin effect operates whenever you apply an AC magnetic field. This is physics, not folklore!
.
: If you are encircling (long) tubes with an AC coil - so the coil is co-axial with the tube - at the coil location you will have stronger eddy currents on the outer surface of the TUBE than on the inner surface. The strength of the inner eddy currents (and that of the magnetic field) depends on the frequency, material resisitivity and permeability and the tube wall thickness. If you test at mains frequency (50Hz) then you will penetrate several millimetres in steel (see the graph on the Hocking site).
.
: Things get more complicated for short tubes, or for locations away from the coil (remote field etc.), but the basics stay the same. I can therefore think of instances where, if I have understood the geometry, testing should be possible.
.
: Tom (corrected!)
.
.
:
: : I am developing some educational materials on NDT, but NDT is not my field. Can anyone help with this question?
: .
: : My question is: does the skin effect operate during an AC coil shot in magnetic particle testing ? The common folklore in MPT seems to be that skin effect operates whenever there is AC current.
: : The skin effect is about the confinement of current to the surface of a conductor. When passing current along a conductor, the current is confined to the surface, and so the field is strongest at the surface, with no internal field. Thus the interior of tubes cannot be inspected using current flow.
: .
: : Am I correct in saying that this does NOT apply to the coil shot ? I can see no reason why an AC magnetic field would be much stronger at the outside of a tube than at the inside, unless its something to do with induced currents.
: : Can anyone set me straight on this, hopefully giving a rough quantitative picture of the effect of eddy currents, if they are significant to this situation?
: .
.



    
 
 Reply 
 
Paul Wray
Paul Wray
02:41 Aug-20-2002
Re: Skin effect in MP coil shot ?
Thanks Tom for your considered response.

> what the geometry is of an "ac coil shot"

As you guessed, with the tube coaxial with the coil.

> But the reason that there is no field on the inside
> of a tube along which current flows axially is
> nothing to do with the skin depth effect. The same
> is true at DC. It can be understood by considering
> Ampere's circuital law.

You are correct, but when I say "inside", I mean not the hollow portion (through which, of course, no current flows), but the inside surface of the tube. If it were not for the skin effect (eg if we used DC), current density would be the same at all points in the tube, and the inside surface would be magnetised, making it possible to test using the MPT method, yes ?.

> Yes the skin effect operates whenever you apply an
> AC magnetic field. This is physics, not folklore!

I didnt mean to suggest that the skin effect was folklore, or that it does not operate wherever there is a current flow. But this situation is a little more complicated, isnt it ? The "folklore" is that people say AC automatically means "skin effect" whether they are talking about currents (where it does operate) or fields (where it does not, except via eddy currents).

The sequence of events (if we can call it that) is:
1) AC current in coil co-axial with tube (there will no doubt be a skin effect in the coil, although that has little effect on the magnetic field produced outside it.)
2) Oscillating longitudinal field caused by current in coil. Why would this not penetrate the entire specimen? Yes, there will be circumferential eddy currents, and they will act to weaken the overall field (Lenz's law), but surely they wont eliminate it. I have no idea what quantitative effect they would be - as you say, it would depend upon many factors.

Paul Wray



    
 
 Reply 
 
Tom Bloodworth
Consultant, R&D
Bloodworth Consulting Limited, United Kingdom, Joined Aug 2002, 12

Tom Bloodworth

Consultant, R&D
Bloodworth Consulting Limited,
United Kingdom,
Joined Aug 2002
12
09:04 Aug-20-2002
Re: Skin effect in MP coil shot ?
Paul

A few clarifications on my earlier note:

: You are correct, but when I say "inside", I mean not the hollow portion (through which, of course, no current flows), but the inside surface of the tube. If it were not for the skin effect (eg if we used DC), current density would be the same at all points in the tube, and the inside surface would be magnetised, making it possible to test using the MPT method, yes ?.


Yes - more or less- for the encircling coil. My first point had been that there is zero internal magnetic field for AXIAL tube currents. I have always thought this to be an interesting result - but irrelevant to your question!


: I didnt mean to suggest that the skin effect was folklore, or that it does not operate wherever there is a current flow. But this situation is a little more complicated, isnt it ? The "folklore" is that people say AC automatically means "skin effect" whether they are talking about currents (where it does operate) or fields (where it does not, except via eddy currents).

Didn't mean to be flippant about the folklore, I just could not resist the alliteration.

Skin depth DOES affect the magnetic field within a conductor. The eddy currents produce a field that opposes the externally-applied field that produces them. At a depth of a few skin depths where the eddy currents have decayed to near zero, this means that the alternating magnetic flux that produces them is near zero also.

.
: The sequence of events (if we can call it that) is:
: 1) AC current in coil co-axial with tube (there will no doubt be a skin effect in the coil, although that has little effect on the magnetic field produced outside it.)
: 2) Oscillating longitudinal field caused by current in coil. Why would this not penetrate the entire specimen? Yes, there will be circumferential eddy currents, and they will act to weaken the overall field (Lenz's law), but surely they wont eliminate it. I have no idea what quantitative effect they would be - as you say, it would depend upon many factors.
.

As you say, the eddy currents weaken the field - just how much depends on the skin depth. If your wall thickness is only about a skin depth or less, you will still get an axial field at the inner surface. For thicker walls, the field will be smaller.

Tom
.



    
 
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