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How to calculate saturation current of rod core inductor?
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Miyuki:
Hi folks,
I have a problem I need a big current choke 5-10uH at 150A
And want to check if rod core is suitable alternative

But cannot find how to calculate at least aproximate saturation current of this tipe of inductor

And yes I know there will be some other issues with high current rod, but it will sit in metal case
DannyTheGhost:
First of all, you need to get value of highest possible current through choke during normal operation. Let's say, you estimate 20% current ripple, so 120%*150Amps= 180 Amps.
Next, you need to choose core material ( we'll use only Bsat value below for comparing purposes) and rod cross-sectional area.
Flux density in the core will be defined then with next formula (more like estimating formula) : B = L*I/S
where S - cross-sectional area, I - maximum current through inductor
If that's what you want to hear - good. But I'd rather not use rod core.
Actually, at that current rates there is possibility to use air core with enough cross-sectional area. It will cut your inductor cost and will free you from bothering about saturation.
T3sl4co1l:
The above will work if the winding is confined to a small fraction of the length, so that leakage around the rod largely avoids the winding itself.

For a distributed winding, significant leakage will span the winding itself, and the effective area will be larger.  The field will still be strongest in the middle, but the number of turns acting exactly upon the middle is fewer, and the coupling from more distant turns is less than one.  It will have a higher saturation current, but it's hard to say by how much.

In the extreme, for a rod much shorter than the winding, the reduction of magnetic path length is very small indeed, so we might use the solenoid formula to find the flux density inside the winding and assume it saturates when B_air = B_sat.  But this wouldn't be very useful as the inductance gain is also tiny (equivalent to the given, that the core length is a small fraction of the total magnetic path length [through air]).

I think I would estimate by starting with the mu_eff estimate,



and assume that mu_eff gives the average reduction in magnetic path length, and so we can assume the same ratio gives the increase in flux density over the air-cored case.

Still very rough, but perhaps good enough for use.

Easy enough to wind a few and measure saturation, at least; make a scale model if necessary.

FWIW, single turns on powdered iron toroids (mu >= 50, say) are about the right ballpark there.  Might use them like ferrite beads?

Tim
Miyuki:
Powder cores are alternative, thinks like:
MS-130026-2 stack of 8 with 6 turns
or
MS-157026-2 just two with 10 turn
But they are at limits and bigger ones are too expensive

But DannyTheGhosts idea look interesting
With 10cm diameter air core coil looks as best alternative
Kleinstein:
For an inverter welder I have seen an air coil at the output.
With just a rod code the effective increase in inductance is not that high. So even of the core goes into saturation the residual inductance could be still enough. Also saturation may be relatively soft compared to a ring core. It depends on the application and also frequency (a core with driven hard and to fast may get too hot) above some 20 kHz core loss may be more limiting that saturation.
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