Stacking Cores

I intend to wind my own transformer core for a custom application. I understand that with higher power requirements and higher currents through the transformer windings means that you need a physically larger core to handle all the flux without saturation.

Assuming you had 2 or more toroidal transformer cores, could you stack them to make something that looks less like a donut and more like a sort of "tube" and wind around them all at once to make for a larger core if you can't find one the size you need to begin with?
 
The problem with stacking too many cores is that the winding resistance goes up. This is because the allowed flux goes roughly as the cross-sectional area of the core, but the winding resistance goes as the length of the periphery. You get the best ratio by having a core which is roughly round/square in cross-section.

So stack two or maybe three. After that you will get diminishing returns.
 
are you talking about stacking toroid cores?

I think the power capability of the transformer and it's core varies as the square of the core area.
If that square rule applies and you stack two identical cores, then you increase the power roughly by four times.
That would need double the thickness (4 times the area) of wire to give four times the current and four times the power with the same voltage.

But you would have to re-wind the primary for this four times capability. Are you able to wind the primary/ies safely?
 
http://www.diyaudio.com/forums/powe...formers-power-requirements-2.html#post2337162

to clarify "side note, power scales with flux squared, which is the biggest reason why cheap stamped E core transformers run 10-20% higher than they should, wasting 20-40% the cost to manufacture in electricity.. every year..." should read "10-20% higher flux density"

in the case of two toroids.. ymmv.. you're only saving up to 25% of the Cu that two separate transformers would use up, assuming the core is square... but most of the cores i've unwound are rectangular, 1:1.5 , so you'll save even less copper, somewhere between 20 and 10%.

you would be much better off unwrapping the second core and wrapping it around the first one.
 
Andrew-

It reads like you're stuck trying to apply the numbers to "size", when it seems clear that stacking two same toroids or doubling the lam stack height is "doubling the size of the transformer", which allows twice the power capacity at the same frequency.

power sine excitation:

P=.707JfWaB x 10e-8

J - current density Amperes sq. cm
f - frequency
W - winding window area sq. cm
a - core cross sectional area sq. cm
B - flux density Gauss
 
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Andrew-

It reads like you're stuck trying to apply the numbers to "size", when it seems clear that stacking two same toroids or doubling the lam stack height is "doubling the size of the transformer", which allows twice the power capacity at the same frequency.

power sine excitation:

P=.707JfWaB x 10e-8

J - current density Amperes sq. cm
f - frequency
W - winding window area sq. cm
a - core cross sectional area sq. cm
B - flux density Gauss
How do J & a interact?

core cross sectional area is doubled.
What happens to J, the current density?
Stacking two toroids leaves f (mains frequency), W (window area) and B (flux density) unchanged.

What, exactly, is "power sine excitation"?
 
I suspect that in the examples you gave the winding area (W in Eckardt's equation) also changes.
I don't think so. I have downloaded the exe after you responded and no where does it ask for window area. It does ask for core cross-sectional area or VA and any 3 out of the following 4: Mains voltage, Mains frequency, Max flux density, Primary turns.
 
Should have maybe written "Power: sine excitation".

The equation is a straight product of factors. Current density is only how much current you pass through winding cross sectional area. However, if you double core cross sectional area, B is halved, so available power throughput is doubled.

Stacking two same cores halves B. Or for fixed B, doubles power...
 
The equation is a straight product of factors. Current density is only how much current you pass through winding cross sectional area. However, if you double core cross sectional area, B is halved, so available power throughput is doubled.

..................... Or for fixed B, doubles power...
I wonder if this is where you are both going wrong?

B (core flux) stays the same.
If you retain the same number of turns then to get the same flux density you need to double the current. If you double the current you need to double the wire cross sectional area (40% increase in diameter or bi-fillar wound using same diameter).
Now the ampere turns has increased by a factor of two.

What does that do to the VA capability?
 
The window area, current density thing can make it unnecessarily complex to think about flux density. You can also use the turns per volt arrangement to think more directly just about core saturation.

N/V= 10e-8/4.4faB

N - number of turns
V - volts
F - frequency
a - core cross sectional area
B - Gauss
 
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