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Relative Permeability of Transformer Iron?

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LAN_403

Backyard Skunkworks

Sun Jun 14 2009, 09:56PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

I've searched up and down our good friend google and I can't seem to find good solid data on the relative permeability of transformer iron (the type used in slotted cores.)

I know it will be somewhere from a few hundred to a few thousand, but I can't seem to nail it down past the correct order of magnitude.

Any real mat.sci whizes out there happen to know this?

klugesmith

Sun Jun 14 2009, 11:54PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Backyard Skunkworks wrote ...

...relative permeability of transformer iron (the type used in slotted cores.)

Depends ... it's worse than asking "strength of steel (the type used in bolts)".
Not only is there a range of materials and heat treatments (cost/performance tradeoffs), they are all nonlinear. Do you want initial or maximum permeability?

Here is a reference that gives values of 2000 or 8000 for non-oriented SI steel, and 30000 or 35000 for grain-oriented Si steel (presumably measured in the good direction).

Got a datasheet on your slotted core? Is it laminated? (I thought that configuration only came in ferrite or powdered iron).

Lacking a datasheet, I think your best bet is to measure the inductance (magnetizing current)
of an inductor (transformer) with a similar core material and a known number of turns. I could help with that.

Rich

Backyard Skunkworks

Mon Jun 15 2009, 03:14AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Ah I wasn't aware it varied that much!

It does indeed sound like my best option will be to measure inductance with and without the core to figure out the exact value.

Here is my proposed experiment to figure it out:
1. Make a coil that fits on the core I want to measure
2. Take DC resistance of said coil
3. Run coil in series with AC source, measure current.
4. Run coil in series with AC source, with core, measure current.
5. Use ohms law to calculate AC impedance
6. Subtract DC resistance from the calculated impedance in 3 and 4
7. Divide impedence due to inductance of the circuit with the core by the one without the core

Would this work?

tesla500

Mon Jun 15 2009, 05:47AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Is there a reason you need to know the permeability so accurately? Usually it doesn't matter for true transformers, provided it's "high enough".

For power inductors, you would add a gap, so that the core is effectively a magnetic "short circuit" compared to the gap. Then the permeability can very wildly without changing your inductance that much.

David

Steve Conner

Mon Jun 15 2009, 08:26AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, iron permeability is like transistor beta. If you design a circuit that depends on it being a certain value, you're asking for trouble.

Backyard Skunkworks

Tue Jun 16 2009, 02:20AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Ah.

Very interesting to know.

So it sounds like I am indeed better off making an inductor and then measuring its inductance and tweaking it until its just right, versus attempting to figure out my exact design parameters before making anything?

Sulaiman

Tue Jun 16 2009, 08:53AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

If your design for inductance relies on the value of the steel permeability
that means you haven't got an airgap.
Such an inductor would be VERY non-linear and temperature dependant.
Once you have a small airgap the relative permeability of the steel is almost irrelevant,
e.g. Suppose the magnetic path length in the steel is 500mm,
if the steel relative permeability is 5000 then the steel path length is equivalent to 0.1mm airgap.

Just use L = Uo x Ae x N^2 / lp

Uo = 4 x PI x 10E-7
Ae is effective cross-sectional area of the core/gap (sq.m)
N^2 is the number of turns squared
lp is the path length (air) of the gap (plus the 0.1mm or whatever) (m)

Dr. Slack

Tue Jun 16 2009, 01:33PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Steve McConner said:

Yes, iron permeability is like transistor beta. If you design a circuit that depends on it being a certain value, you're asking for trouble.

Allow me to extend that ...

Yes, iron permeability is like transistor beta. If you design a circuit that depends on it *staying* a certain value after you've tweaked it up in the circuit, you're asking for trouble.

Temperature, mechanical strain, ageing, magnetic history, and probably the alignment of the major planets as well, will all change the relative permeability of high permaebility steel, and ferrite for that matter. Only an airgap, whether an apparent gap between the faces of the core, or the distributed gap of the epoxy between the magnetic grains of a composite core, will give any long term stability to the inductor.

The permeability of magnetic steels is not even specified as accurately as the value of smoothing lyitics (-20% +80%), the value doesn't really matter as long as it is big enough.

What accuracy and stability are you looking for?

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