Iron sheet laminated gapped core inductor variation

[Easy peasy]

but using a very low range of magnetic flux density also makes it vary, as you just saw from the above posts

there is no evidence to support this - I have never observed this - it also goes against the principles of physics relating to magnetic materials.

[c_mitra]

it also goes against the principles of physics relating to magnetic materials.

Please elaborate; this is not correct.

If the quantity called permeability is a constant, then the B-H curve would be a perfect straight line. That it saturates means that the permeability is NOT a constant.

The fact that the hysteresis curve encloses some finite area also says that the permeability is also a complex number.

The graph seen in post #7 is nothing but a derivative of the B-H curve. The peak corresponds to the midpoint of the rising part of the B-H graph.

Something like the Debye equation for the dipole moments (but much more complex) should apply here but I do not recall the equations.

In the B-H curve, there is a small toe (close to zero) - that is important- but that disappears in subsequent cycles.

But nowhere it goes against the principles of physics as you claim.

But it can get messy.

[Easy peasy]

there is no published paper or article that says the slope or behaviour the BH curve at low excitation is any different to the slope/ behaviour at slightly higher excitation - nor can c-mitra supply one. Also simple ( accurate ) measurement - bears this out ...

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permeability is a complex number simply to add in the effect of losses of the B-H loop - losses do not affect intrinsic inductance ... very small B-H excursions have the same net average slope ( reactive permeability, i.e. uo x ue ) as slightly larger ones for a gapped inductor - which is what we are describing here ...

[c_mitra]

the slope or behaviour the BH curve at low excitation is any different to the slope/ behaviour at slightly higher excitation - nor can c-mitra supply one....

You are correct.

For small excursions of H (i.e., the current in this case), far from the saturation limit, the B-H curve can be considered linear and hence the mu should be approximately constant.

The actual current has not been provided. If I use 50mH at 50Hz, the impedance will be around 15 Ohm and the current at 50mv will be few mA at most. That is not sufficient to calculate H but the inductor is rated at 1A.

The question is whether the current excursion at 50mv and 300mv will get the core into the non-linear region of the B-H curve? The author claims more than 10% change. I doubt but I also do not know.

Visually, the hysteresis curve, close to zero, far from saturation looks very linear. In particular with low excitation current.

What is the role of the gap in this case? Qualitatively speaking, it will try to linearize the curve but I do not have the patience to recalculate and verify. The midpoint slope will decrease and the mu graph will become flatter.

Personally I do not like measuring an inductor rated at 1A at a current level of few mA. The measurements should be performed around the rated parameters.

My comment was on the principles of physics you mentioned.

Perhaps we are seeing the infamous Barkenhausen effect.

[Easy peasy]

Barkhausen noise would not rise to the level needed to seriously interfere with measurement at 50mV, the main issue with 50mV measurement is the likely level of environmental noise, and the unknown measurement apparatus ...

[CataM]

there is no evidence to support this - I have never observed this - it also goes against the principles of physics relating to magnetic materials.

Figure of post #7 shows that relative permeability varies a lot with applied field strength (proportional to current). Lower field strenght gives lower permeability giving lower flux density.

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there is no published paper or article that says the slope or behaviour the BH curve at low excitation is any different to the slope/ behaviour at slightly higher excitation

What does figure of post #7 tell then ?

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For small excursions of H (i.e., the current in this case), far from the saturation limit, the B-H curve can be considered linear and hence the mu should be approximately constant.

mu_ungapped/mu_effective varies according to variation in relative permeability and it does vary in our specific application.

We get the theoretical value of the inductance at high currents (~0.5 A up to 2 A) and a lot of variation in inductance with low currents (few mAs).

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and the unknown measurement apparatus

Wayne kerr 3255B

[Easy peasy]

a random data sheet has no bearing on a gapped choke made using transformer steel, an old measuring device with unknown capacitor degradation - and unknown whether it properly measures phase angles to determine true inductance rather than just total impedance, and still noise issues at 50mV