Concerning permeability and relative permeability

[neferkamichael]

N5211
BHmax = 52 MGOe
Br = 14.5kG
Hci = 11.0 kOe
Hc = 10.6 kOe
U₀ = .000001256636

I am working on a project with coils and electromagnets and the information above is from a B/H chart and the values for magnetic flux density and coercive force are for neodymium N5211 metal. I know that B_r is the remaneant flux density of the metal and I'm not sure which if is that the number I should use to compute the permeability(mu)and also not sure which number for coerce force I need to use to get the correct magnitude, so here goes.

u=B/H=14.5/11.0(Hci)=1.318181818181818
u=B/H=14.5/10.6(Hc) =1.367924528301887
Relative Permeability(u_r)=u/u₀=1.31818/.000001256636=1048975.20045582 for Hci and
1088560.251337698 for Hc.

First is the numbers I used to compute the permeability the correct numbers to use to start with to calculate permeability of N5211?
Which numbers would be the correct numbers to plug into an equation to get the correct magnitutde, Hci or Hc? Any help from anyone would be greatly appreciated.
Thanks
neferkamichael

 

[hacksaw]

permanent magnets have low permeability are are generally not used in electromagnets

your estimates of 1.31 to 1.36 are good and, by the way, are the relative permeability
for the units you have

you can use the average or either one, it won't make much difference

 

[MagMike]

N5211 is not linear, there is a significant knee in the curve. The relative permeability for the material is 1.02 to 1.03 as long as the magnet is operating above the knee of the curve

Your modeling software will hopefully have the option of importing the 2nd quadrant curve, point by point. That is the only way to truly model its performance. If the software doesn't have this option, you'll have to ensure the magnet always operates above the knee of the curve.

 

[neferkamichael]

Hacksaw, MagMike, thank you for responding. Now, both of you have said that the relative permeability is 1.31 and/or 1.02 respectively. That is correct for the CGS systems of units where the permeability of a vacuum is defined as 1. In the SI system of units the relative permeability would be 1048975.20045582, the big number, is that correct?

MagMike, how did ya come up with 1.02 if ya don't mind.

Appreciated the help, thanks.
neferkamichael

 

[MagMike]

The term is 'relative' permeability, so if you want to use SI units, you factor in mu0. No magnet supplier bothers reporting it any other way.

I came up with 1.02/1.03 because magnetic modeling is my day job and I use and have catalogs from all the major NdFeB magnet producers. All of them list it as 1.02 to 1.03.

Please tell us you are not using NdFeB as the core of your electromagnet.

 

[neferkamichael]

In my initial I listed the the remanent Flux Density and Coercive Force magnitudes from Dexter Magnet Technologies. u=B/H gives you the permeabilty 1.318181818181818. Ur= u/Uo=1048975.20045582. I haven't said anything about electromagnets, but why wouldn't N5211 make a powerful electromagnet.

So MagMike, for equations that call for relative permeability and I'm using SI unit is 1048975.20045582 the relative permeability?
Thanks for responding.

 

[davidbeach]

The relative permeability of a vacuum is 1 regardless of the unit system selected. The absolute permeability, on the other hand, can vary rather significantly depending on the unit system selected.

 

[neferkamichael]

The physical constant μ0, commonly called the vacuum permeability, permeability of free space, or magnetic constant is an ideal, (baseline) physical constant, which is the value of magnetic permeability in a classical vacuum. Vacuum permeability is derived from production of a magnetic field by an electric current or by a moving electric charge and in all other formulas for magnetic-field production in a vacuum. In the reference medium of classical vacuum, µ0 has an exact defined value:[1][2]

µ0 = 4π×10−7 V·s/(A·m) ≈ 1.2566370614...×10−6 H⋅m−1 or N·A−2 or T·m/A or Wb/(A·m)
in the SI system of units.

 

[MagMike]

I believe the parameter you are looking for is recoil permeability, not relative permeability. I strongly recommend using 1.02 or 1.03 in your modeling software. Most packages use the cgs value for recoil permeability and factor in µ0 if you use SI units.

That said, please realize the limitation inherent to your original question. N5211 is non-linear in the 2nd BH quadrant so its magnetic behavior can't be completely characterized by just Br, Hc and/or µ0. The slope of the BH curve is 1.02/1.03 (cgs units) until you reach the knee of the curve. To properly model N5211 you need to manually enter a number of B,H coordinates to account for the knee of the curve.

Finally an apology: I assumed the NdFeB was the core of an electromagnet based on Hacksaw's posting. His comment didn't help the conversation much.

 

[electricpete]

I am working on a project with coils and electromagnet

I haven't said anything about electromagnets, but why wouldn't N5211 make a powerful electromagnet.

Because the relative permeability is on the order of 1.02-1.03.
Compare to iron - may be on the order of 1000 at 1T


=====================================
(2B)+(2B)' ?

 

[neferkamichael]

Electricpete, if you read any of the post to my thread I clearly wanted to know the difference between magnetic permeability and relative permeability. Thanks to all the post I've gotten a better understanding of it all. Now this is an equation between the force between and electromagnet and a ferromagnetic material, I have a about it.

Force=(NxI)²xkxa/(2g²)
SI units
N=Number of turns=100
I=current in Amperes=1
Force=Newtons
k=permeability of a vacuum=.000001256636
a=area in meters=.004559991meters²(7.068²")
g=distance between electromagnet and ferromagnetic material in meters.=.0000254 meters(001")

If you do the math the answer is 44409.5147 Newtons = 9983.657lbsf.
Now my question is, is there really almost 10000 pounds of force between and air coil core with a radius of 1.5" and a metal plate at a distance of .001" between them? I've done the math over and over.

 

[MacGyverS2000]

What happens as g approaches zero? Your force approaches infinity.

Think on that one a bit...

Dan - Owner

http://www.Hi-TecDesigns.com