On Tue, 27 Nov 2012 09:55:33 -0000, wrote:

Lieutenant Scott wrote:
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:


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).


No, think of a see-saw.....The current flows in at the pivot point and
each flows towards one of the ends...the 3 amps (being heavier) goes to
the bottom end, and flows out to load and back to the pivot point (the
"centre tap").

The 1.5'ish amps flows out the top, off to the mains supply and back to
the pivot point (the "centre tap").

The only place that both currents are flowing together is at the centre
tap, and then they go their own ways!

And, at the centre tap (and only at the tap), you may have both the 1.5
amps (main winding current) and the 3 amps (secondary current) flowing,
and, depending on the phase angle between them, the amount of current
flowing at any particular time, at the centre tap could be anywhere
between 1.5 amps and 4.5 amps! But, in the windings the most current
would be 3 Amps.

Daniel

I think that's the same as what I'm saying.

So 1.5 amps comes from the source to the bottom common point (common connection to source and load), and goes through the load to the centre tap.

The other 1.5 amps the load is getting is circulating through the bottom half of the coil: centre tap to bottom, then through the load, back to the centre tap.

I make that 1.5 amps in all the coil though.

Another clue might be that the Variac states that 3A LOAD is the maximum, no matter what voltage you've selected by moving the centre tap.

There must be a physics book on this somewhere online.....

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Why isn;t the apostrophe next to the L? Who ever uses the semicolon???