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2nd July 2019, 16:29 #1
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Iron sheet laminated gapped core inductor variation
Hello everyone,
I have a gapped (few mm of air gap) inductor wound around the E+I sheets of a laminated core Power Core M40050A.
The problem that I see is that the inductance when measured with an LCR meter varies when the amplitude of the LCR varies.
 56mH @50 Hz @ 50mVRMS
 50mH @50 Hz @ 300mVRMS
The core is not saturated because it can withstand 1ARMS without saturation, so the decrease of inductance due to increase of voltage is not due to saturation.
Could this be due to the I*R drop in the series resistance ? Why could this be ?
Any comment is appreciated !

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2nd July 2019, 23:39 #2
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Re: Iron sheet laminated gapped core inductor variation
Iron core hat strong magnitude dependency of permeability. Did you calculate if the measured inductance variation can be explained by it? I*R is a linear effect and can't affect measured inductance.
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3rd July 2019, 00:01 #3
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Re: Iron sheet laminated gapped core inductor variation
My guess is that residual magnetization can take place (rather, be more perceptible) at very weak magnetic fields; did you find out if these values hold exactly as exposed above, either increasing or decreasing the voltage?

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3rd July 2019, 05:51 #4
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Re: Iron sheet laminated gapped core inductor variation
Either per Andre above  or your measurement equip is a bit suspect ... 50mV allows noise to add in error ...
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3rd July 2019, 08:34 #5
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Re: Iron sheet laminated gapped core inductor variation
LCR meter varies when the amplitude of the LCR varies...
Most common machines assumes that the device under test is a pure L /C /R. What is the approx resistance of the coil?
Just for a test: make a simple air core inductor with a thin wire (say 100 turns on a paper former with a 3240 AWG wire).
Measure the inductance at two different applied voltage and report the result. Perhaps you will see two different values.
Could this be due to the I*R drop in the series resistance ? Why could this be? ...
Resistance measurements are usually accurate but LC measurements with series resistances are usually not.
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3rd July 2019, 10:17 #6
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Re: Iron sheet laminated gapped core inductor variation
Measure the inductance at two different applied voltage and report the result. Perhaps you will see two different values.
The main problem I think it is due to the core.
Iron core hat strong magnitude dependency of permeability. Did you calculate if the measured inductance variation can be explained by it? I*R is a linear effect and can't affect measured inductance.
J=1T (H=199 A/m) => muA (amplitude permeability) = 5927
J=0.5 T (H=108 A/m) => muA = 5276
Without air gap, the variation on permeability means L1/L2 = muA1/muA2 = 1.12 i.e. 12% increase which turns out to be exactly the values I see... I DO NOT UNDERSTAND how can this happen since our core is gapped (E laminated sheets + I laminated sheets on top).
A gapped core is supposed to reduce the influence of core variations e.g. form the magnetic permeability.

3rd July 2019, 10:29 #7
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Re: Iron sheet laminated gapped core inductor variation
Without air gap, the variation on permeability means L1/L2 = muA1/muA2 = 1.12 i.e. 12% increase which turns out to be exactly the values I see... I DO NOT UNDERSTAND how can this happen since our core is gapped (E laminated sheets + I laminated sheets on top).
A gapped core is supposed to reduce the influence of core variations e.g. form the magnetic permeability.
I presume however, that the test conditions are in much lower B range than the quoted spec, you didn't tell. See an example for a different core, your test conditions are probably further to the left.
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3rd July 2019, 23:43 #8
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Re: Iron sheet laminated gapped core inductor variation
Yes. With very low current, might not get enough magnetic field strength ("H"), hence low mu_R... Next will try variations using high current.
   Updated   
You presume correctly. I am in the B_peak=0.012 T range in the central leg and half that in the outer legs...
   Updated   
See an example for a different core, your test conditions are probably further to the left.
Will check whether the inductance varies or not at higher B_peak.

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6th July 2019, 11:36 #9
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Re: Iron sheet laminated gapped core inductor variation
Inductance measurement done in the 0.5 T range and does not vary with changes in voltage (maintaining high magnetic density flux).
Thank you all for your help.

6th July 2019, 12:27 #10
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Re: Iron sheet laminated gapped core inductor variation
J=1T (H=199 A/m) => muA (amplitude permeability) = 5927
J=0.5 T (H=108 A/m) => muA = 5276
You have to calculate the magnetic reluctance for the overall path (include the gap) and then use that to get B.
Or, am I missing something?

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6th July 2019, 14:37 #11
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Re: Iron sheet laminated gapped core inductor variation
Sure. Or assume you are in the high µ_r region and neglect core recultance: R_{core}<<R_{air gap}. Under this assumption, the overall path has a reluctance: R_{overall path under above assumption & E+I core geometry} ≈ R_{air gap}
That is what I used to design "L".
I computed "B" assuming a "L" by design.
   Updated   
Puzzled; I checked some unit conversion tables and 1T=798 kA/m.

6th July 2019, 16:11 #12
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Re: Iron sheet laminated gapped core inductor variation
And what is your point ?
the overall path has a reluctance: Roverall path under above assumption & E+I core geometry ≈ Rair gap...
If I use the graph presented in #7, the H values (199/108 A/m), the mu values you use are different. In particular, the points are to be right of the peak (undesirable).
Better to use the H value, use the mu vs H graph, then recalculate. I think there should a term like (gap length)/(total magnetic path). Please check (I may be rusted).

6th July 2019, 23:28 #13
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Re: Iron sheet laminated gapped core inductor variation
If I use the graph presented in #7, the H values (199/108 A/m), the mu values you use are different.
Better to use the H value, use the mu vs H graph, then recalculate.

7th July 2019, 05:16 #14
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Re: Iron sheet laminated gapped core inductor variation
Or another way is to use "B" (because it easy to compute), go to "B vs H", then to "mu vs H". ...
In other words, when you try to calculate B directly, you are using the constant (permeability) somehow somewhere (I do not believe you can compute B without using mu of the sample). Please elaborate.
If mu is a function of H (true for all materials except vacuum), the induced magnetism (B) is also a function of H (obviously because of the BH curve).
Therefore when you apply 50mv RMS vs 300mv RMS (respectively) to a inductor and compute the B values and then compute the H and find H 108 and 199 A/m (respectively), I guess there is some mistake.
I wonder why you quote voltages instead of current values (because that is the causative agent for the magnetisation). Current values should be proportional to the H but not to B because of permeability variations.

7th July 2019, 17:12 #15
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Re: Iron sheet laminated gapped core inductor variation
Of course there is a mistake. That was the whole point of this thread. I computed theoretical inductance for high "H" and hence "mu_r", but did not tested it correctly (at few mV RMS, the current is so little that high "mu_r" is not achieved).
   Updated   
In other words, when you try to calculate B directly, you are using the constant (permeability) somehow somewhere (I do not believe you can compute B without using mu of the sample). Please elaborate.
   Updated   
I wonder why you quote voltages instead of current values (because that is the causative agent for the magnetisation). Current values should be proportional to the H but not to B because of permeability variations.

7th July 2019, 22:21 #16
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Re: Iron sheet laminated gapped core inductor variation
having read the above  and, as some one who regularly designs iron cored chokes and transformers  there is a lot of misinformation in the above  the simple facts are at 50mV there is too much room for noise to make an accurate measurement, even 300mV is marginal.
For a choke made out of transformer steel with a very even gap  the inductance does not change with excitation until you get to > 0.5T peak
the fact that you are exercising the core very little at low excitation does not mean the instrinsic inductance changes  especially as L is dominated by the gap
If you have a little bit of steel poking out into the gap  which really shouldn't be there  then you may get a higher L reading at low excitation  but this little bit of steel will saturate at higher levels of H and the L will then fall to the nominal air gap value.
Often times "butt stacked" chokes are welded on the sides to reduce noise and hold everything together  there is no air gap for low excitation and you get high L near the zero crossings of signals  but swamped very quickly as the signal rises and the weld is saturated for most of the operating cycle ( the weld does add heat though ).
I hope this clears the matter up ...

8th July 2019, 10:36 #17
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Re: Iron sheet laminated gapped core inductor variation
For a choke made out of transformer steel with a very even gap  the inductance does not change with excitation until you get to > 0.5T peak
the fact that you are exercising the core very little at low excitation does not mean the instrinsic inductance changes  especially as L is dominated by the gap
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10th July 2019, 23:14 #18
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Re: Iron sheet laminated gapped core inductor variation
L dominated by the gap would be also my expectation if we assume typical gap dimensions.
gap length = 0.23 mm
Roverall path under above assumption & E+I core geometry ≈ Rair gap
   Updated   
the inductance does not change with excitation until you get to > 0.5T peak

10th July 2019, 23:40 #19
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Re: Iron sheet laminated gapped core inductor variation
Are you saying that inductance does not vary with lower than 0.5T but it does vary with higher than 0.5 T ?

13th July 2019, 13:49 #20
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Re: Iron sheet laminated gapped core inductor variation
that is exactly what I said ...
   Updated   
You should specify the lower end of magnetic flux density the inductance does not vary... not just use "absolute" statements, as they were true always.
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