How to find permeability from BH curve?

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powersys

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Doubts on determining the permeability from BH curve. The formula of permeability is

µ = B/H

... which is NOT given by the SLOPE of BH curve, right?
 

if the material is linear, thus the B/H will be a constant for all H values. and it is the permeability of that material.

if the material is not linear, B/H is H-dependent and has differenet values for different H. so we have a function for permeability: µ(H) which may be approximated by a constant for small H values (H < H_saturation). so the permeability of that material could be this approximated value.
 

µ is the slope of the B/H curve
 

The units for mu will be given by B / H

A single number mu describes only a linear system or a non-linear system within its linear range

If you are using a non-linear B vs H curve, you may adopt some other interpretations depending on your purpose.

You can use the slope of the curve at a given point (H1,B1) which perhaps would be useful if H was varying in a narrow range about that point, or you can take the slope from the origin to (H1,B1) which may give you a rough idea of equivalent me for single-frequency sinusoidal analysis (although stricly speaking you have a distorted waveform which is not completely modeled in single frequency sinusoidal analysis)>
 

powersys said:
Doubts on determining the permeability from BH curve. The formula of permeability is

µ = B/H

... which is NOT given by the SLOPE of BH curve, right?
Click to expand...

Hi,

I attached your B/H picture slightly edited with showing the CHANGE in H and the corresponding CHANGE in B, so using my numbers in the picture the permeability of this type of material is: mu= (delta)B/(delta)H=2700/10=270

Of course if you increase H further on (by exciting the core with more current), the curve flattens out, showing the nonlinear relationship with B, hence mu will also change, it will reduce to a lower value, it is said: the core starts saturating.

rgds
unkarc
 

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    powersys

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For many applications, especially power applications, where the B changes a lot, the impulse permeability is calculated as shown in the paper, µ=ΔB/ΔH.
That approximates the curve with a straight line. It is useful, because it the end points are really of interest to you: the initial and final B and the corresponding H values. Exactly HOW they change inbetween those points is almost irrelevant in most applications, especially power applications.

When you refer to the actual material, you will generally find the term "initial permeability", which refers to the portion of the curve right at the origin, that is extremely low bias.
 

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