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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Because the maximum output power of a transformer can be calculated as core cross-section squared (at least the ratio for transformers made of the same core material), I was wondering if I pack together two similar cores and wind the windings around them, will the power the cores can handle 4x that of a single core?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Sulaiman wrote ...
I think that the maximum power throughput is proportional to core cross-sectional area hence two cores = twice the power
I think you're not right. I can't find any good reference now, the best I found is this approximation: "Core cross section area (sq inch) = .16 * sqrt( VA)" (from ). As you can see, the cross sectional area is dependent on the square root of the VA rating. I know it's like this, I was just wonfering as it seemed kinda weird to me that the cores separate are good to only 1/2 of the power than when packed together. I think the only limiting factor could be that you could not have enough winding area when "packing" cores like this...
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I know that I am correct !
Do a bit more research ...
I'm not in the mood to produce a full proof, just consider this; Assuming the same frequency and windings for two transformers, one with N times the core cross-sectional area of the other. 1) Since both have the same windings, they both have the same current ratings 2) The transformer with N times cross-sectional area can have N times the volts-per-turn, hence N times the voltage, so N times the power (power = volts x amps) q.e.d.
In practice the transformer with a larger core cross-sectional area with the same winding area, wire gauge and number of turns will require a longer length of wire to go around a larger core, so the copper losses will increase and you will not even get N times the power throughput.
Maybe you were thinking of core linear dimensions ? e.g. 3x wider and 3x deeper = 9x the area.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
As Sulaiman said it is quite a complex interaction, and the power increase depends on exactly how you make the core "bigger". There are optimum ratios for the winding window dimensions compared to the core dimensions so these generally all scale by approximately the same amount. Look at a range of ETD core sizes to see what I mean.
In general if you make all dimensions of the core 2 times bigger, then the core cross sectional area goes up by a factor of 4. This means that you only need 1/4 of the number of turns of wire on the primary to run at the same flux density. But the winding window is also bigger, so not only do you require less turns but you also have more space to fit them in! Hence you can use less turns of thicker wire to fill the window. but each turn is longer... approximately twice as long, depending on how the wire wraps around the bobbin. "Power handling" rating is dependent on power dissipation in the core and copper and the ability of the core and windings to get rid of heat. By increasing the core size you now have more volume of lossy magnetic material in the field but it has a greater surface area to radiate heat. Similarly the temperature of windings due to copper losses can be difficult to predict because increasing conductor diameter has effects on AC resistance and heat dissipation. Then there's insulation thickness...
In general, stacking of transformer cores side by side is not normally done for the reasons mentioned above. It messes up the optimum ratios of the core dimensions to the winding window dimensions. In short if you want twice the cross sectional area of the core it is best to scale everything up by sqrt(2) than to put two similar cores side-by-side because the first option will result in a shorter winding path length and therefore less copper losses than the latter. It will also provide a bigger winding window to allow thicker wire to be used.
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Double the core, quadruple power ??
