Re: [CAD_CAM_EDM_DRO] estimating current of a per supply

carlcnc wrote:

> Hello
> I have a couple of large transformers voltage is known, how do I
> gaet an accurate "guess" of their current capability?
>
>
>

There are a couple of methods. One is the weight of the transformer
iron. I'm not too clear on the method for this, as the cross sectional
area of the core is how I learned how to do it. but, apparently everything
drops out in the equations, so that laminated transformers of conventional
design all come in at nearly the same exact core weight.

If you can see the secondary winding enough to gauge the wire size,
you can do pretty well. Due to the enclosed nature, heat is trapped,
so a wire will only be able to carry about 50% of the open-air
current capacity. Watch out for aluminum-wound transformers,
as the wire gauge will be very large compared to the current, compared
to copper.

The most accurate method is the "smoke test". Put the transformer
in the setup, with rectifier and capacitors, and apply a dummy load,
made of whatever you can come up with, such as long extension
cords, rolls of wire, etc. Load the power supply to the expected
current, and check the transformer frequently for heat. If the
winding gets hot in 15 minutes or less, reduce load by half. If the
Iron heats up faster than the winding, increase load a bit. When
the transformer heats up to roughly 50 C in one hour (we are
talking about pretty good sized units, here, 1 KW or so, and 30+
Lbs.) that is a good load rating for it. Some are designed to run
incredibly hot (Sola regulating transformers, because they are
run into the saturation region, run WAY hotter than this.) and
others run nice and cool. that is part of the design strategy of
the manufacturer, whether they designed for cost alone, or with
operating life and guarantee concerns, too.

Jon