Lieutenant Scott wrote:
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.

No, the load current goes to the centre point, not the bottom point

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.....


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

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

Lieutenant Scott wrote:
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:




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.

No, the load current goes to the centre point, not the bottom point

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.....


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.

--
http://petersparrots.com
http://petersphotos.com

The easiest way to find something lost around the house is to buy a replacement.

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!!

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.

Daniel

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.

--
http://petersparrots.com
http://petersphotos.com

You've heard of "Virgin Wool from New Zealand?"
It's a myth.

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

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.

--
http://petersparrots.com
http://petersphotos.com

Why are they called buildings, when they're already finished? Shouldn't they be called builts?

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