Lieutenant Scott wrote:
On Tue, 11 Dec 2012 13:18:22 -0000,
wrote:

Lieutenant Scott wrote:
On Mon, 10 Dec 2012 14:18:53 -0000,
wrote:

Lieutenant Scott wrote:
On Tue, 27 Nov 2012 14:49:56 -0000,
wrote:

Lieutenant Scott wrote:

Snip


Back to the beginning, forget that you have a Variac, just think of a
transformer with four connection points. The Primary winding is
connected to the mains and the secondary winding is connected to the
load.

Current flows from the wall socket, into one terminal of the Primary
winding, flows through the Primary, and current then flows back to
the
wall socket. No if's, no maybe's.

The alternating current flowing in the Primary produces a fluctuating
magnetic field around the Primary winding. This fluctuating magnetic
field also cuts the windings of the Secondary winding. This induces a
voltage into the Secondary winding. If there is no load connected,
there
is no load current flowing, just voltage at the Secondary winding
terminals. Because the is no load, load current will be zero!

Now when you connect a load to the Secondary winding, the voltage
induced into the Secondary winding causes a current to flow from one
end
of the Secondary winding, out through the load and back in the other
end
of the secondary winding. Totally separate from the Primary.

The Primary current has not gone anywhere near the secondary winding,
but the Primary current has caused the magnetic field which then
caused
the Secondary current.

Daniel

Yes I understand a normal transformer. But... in your example
above, is
the current flowing in the same direction in the primary and
secondary?
I know it's AC, but think of the first half of the sine wave.

Now think of the Variac. The "secondary" is actually half of the
primary. One half of the coil is just a bit of primary. The other
half
of the coil is BOTH half the primary and all the secondary. So this
part of the winding carries TWO currents. It matters if the currents
are in the same direction, as they might either add or subtract from
each other.


Not possible, Scotty, you cannot have both Primary and Secondary
currents flowing in one part of the (primary & Secondary) winding and
just Primary current flowing in the remainder of the (primary) winding.

Cannot happen!!

As for the phase relationships between totally separated Primary and
Secondary windings, this can depend on the load connected to the
Secondary (i.e. is the load purely resistive or capacitively
reactive or
inductively reactive. And the phase relationship would also depend if
the "top" or the "bottom" of the Secondary is connected to the
bottom of
the Primary winding!!

I know, because one cancels the other out, it's more like the magnetic
force pushing against the existing current and reducing it. But you can
think of it as adding and subtracting currents.

Forget a transformer.....think of two series resistors with a centre
take-off point, say a nine ohm resistor and a one ohm resistor with ten
volts applied across the combination. One amp of current would be
flowing through the two resistors, with nine volts developed across the
nine ohm resister and one volt across the one ohm resistor.

Now, if you connect another resistor across the one ohm resistor, you
don't get an increase of current flowing through the one ohm
resistor to
provide the current that would flow through the additional resistor.

I find that more confusing - as a transformer is actually creating the
voltage on the output.


"a transformer is actually creating the voltage on the output" in
exactly the same way as, in your impractical transformer, the primary
current flowing through the "secondary" winding creates the secondary
current .......... *ain't going to happen!!*

Daniel

http://petersphotos.com/temp/transformer.jpg

Correct so far?

Where I've written "?A", it has to add up to 1.5A upwards, otherwise
you'd be getting current from nowhere - 3 amps has to come out of the
centre tap.

I see this as the 1.5A flowing down (round the source circuit), plus the
3A flowing up round the load circuit. 1.5 down plus 3 up = 1.5 up.

Sorry I've been away so long!!

Your diagram is not going to happen.....ever!!

Primary power = 240 Vp times 1.5 Ip equals 360 Watts
Secondary power = 120 Vs times 3.0 Is equals 360 watts

Secondary power equals Primary power, so no (i.e. zero, zilch) power can
be dissipated in the top half of the transformer, so zero voltage
developed across the top half of the transformer, so Vs must equal Vp,
i.e. 240 V not the 120 V your diagram shows.

Not going to happen....ever!! Sorry!!

Daniel