On Mon, 26 Nov 2012 12:40:18 -0000, wrote:

Lieutenant Scott wrote:
On Sun, 25 Nov 2012 12:57:49 -0000,
wrote:

Lieutenant Scott wrote:
I have a variable transformer,
http://www.faqs.org/docs/electric/AC/02149.png

If the current in the load (set in the middle at say 120 volts) is 3
amps, I assume the current from the source (240 volts) will be 1.5
amps. Does this mean that the current in half the windings is 4.5 amps
(3 plus 1.5), or 1.5 amps (3 minus 1.5), or will the load current be out
of phase with the source?


Looong time since I used a variable transformer (and then it was a
stepped transformer), but you can look at it better by considering
Power, i.e. if your load is using 360 Watts (120v x 3amps), so your
primary winding must be delivering at least that much power, so
360W/240V gives 1.5Amps (In reality it will be a bit more (1.6A maybe),
due to losses with-in the transformer).

So the transformer would have two totally separate windings, one (the
Primary) creating a magnetic field which induces the voltage into the
secondary winding to supply the loads current.

How the phases relate to each other ....... I think they end up in
phase, depending on the actual load!

What confused me about the Variac was it only has one winding.

If it was a normal transformer stepping from 240 to 120 volts, the
currents are easy to work out.

But with the Variac, the current going through the load is in the same
coil as the primary current from the supply. I would think it goes the
*opposite* way if they're in phase, as source and load share a common
neutral at the bottom, so this would mean LESS current in total flows in
the bottom half of the coil? (Subtracting one current from the other).


Gee, didn't even notice you were talking about a Variac!!

O.K., what I explained above was for a normal Transformer which would
have two connections for the primary winding and two for the secondary
winding.

Now if you connect the two "Earth" or "Neutral" connections together,
you do end up with an (effective) three connection device, with both the
primary winding and the secondary sharing the third connection.

I think my mathematics still applies!!

Daniel

The thing is my 3 amps and my 1.5 amps are in the same wire. If they're opposing that's fine, if they're not it adds to 4.5 amps and gets hotter. My thinking is when the current is flowing from top to bottom from the source, that it will flow bottom to top in your secondary (or in mine back through the same wire, subtracting from the current. (The source getting the "negative" on the "earth" side at the same time as the source does the same).

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