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- since 1996 -
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Technical Discussions
Jaesul
South Korea, Joined Feb 2020, 1

Jaesul

South Korea,
Joined Feb 2020
1
08:20 Feb-20-2020
ECT: Resistance in Impedance Plane

Hello.
In Impedance Plane Graph, It seems Coil Resistance changes with test object conductivity.
As far as I know, Impedance Value Z is the combined value with inductive reactance and Resistance. isn't Coil Resistance a constant value? How come it could be changing?

Thanks

Fabrice
Engineering, -
EXTENDE, France, Joined Feb 2010, 3

Fabrice

Engineering, -
EXTENDE,
France,
Joined Feb 2010
3
11:11 Feb-20-2020
Re: ECT: Resistance in Impedance Plane
In Reply to Jaesul at 08:20 Feb-20-2020 (Opening).

Hello Jaesul,

The Impedance plane graphs reflects the electromagnetic coupling between the coil and the conductive specimen because the flux passing through the coil is both due to the generated flux and the reactive magnetic flux generated by the components thanks to eddy currents.

Therefore, R on this graph is not the coil resistance itself but a function of the induced current in the component. And this is a function of the electromagnetic coupling wich depends on the object conductivity.

Best regards
Fabrice

Frank Lund
R & D,
United Kingdom, Joined Apr 2005, 222

Frank Lund

R & D,
United Kingdom,
Joined Apr 2005
222
00:46 Feb-23-2020
Re: ECT: Resistance in Impedance Plane
In Reply to Fabrice at 11:11 Feb-20-2020 .

Yes, the EC instrument will have already been nulled to balance out the resistance, inductance and capacitance in the probe circuit.

Any deviation displayed after nulling will be a change in the EM parameters of the test piece.

The current flowing in the test piece can be represented as a single turn secondary of a transformer having the excitation coil of the probe as its primary. For perfect coupling, the impedance change in the "secondary circuit" is reflected into the "primary circuit" multiplied by the square of the turns ratio (= the number of turns on the excitation coil).

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