Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

Karl

Karl Townsend wrote:

Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

Karl

The neutral needs to connect to neutral I believe, otherwise you have a
variable inductor of sorts inline, rather than an autoformer. With the
pair connected and balanced it might work without an actual neutral
connection, but I think it's better to have the neutral. In their
original three phase configuration the neutrals would be connected to
the neutral of the 208/120 wye service.

From DoN. Nichols's post in the previous thread:

With a couple of caveats for US power systems.

1) Wire both Powerstats (these are that brand, not Variac, which
was General Radio's brand) with the CCW end of the winding
connected to neutral, and the CW ends of each to the two hot
lines

2) Wire the load to the two wipers.

With this wiring, your load will remain centered around ground,
and both sides will increase together.

Oh yes -- also be sure to turn both to fully CCW before you
tighten the setscrews for the common shaft to the knob.

Hmm ... another thing to watch out for. At least the General
Radio Variacs used shafts covered by a black plastic (Bakelite, perhaps,
given when they were designed) and there was no attempt to insulate the
center of the rotor plate from the shaft, so you would have a short
between the two output sides. I forget whether Superior Electric (the
maker of the Powerstat) did the same or not. Check it out for
insulation if you are going to use a bare metal shaft. Perhaps a Delrin
shaft would be a better bet.

With other wirings, you would likely burn out one of the
variable autotransformers (Powerstats, Variacs) and/or have more
hazardous voltages.

Also, fuse both hots -- ideally with a shared circuit breaker.

Good Luck,
DoN.

In article ,
Karl Townsend wrote:

Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

Karl

Sorta depends what you are trying to do. You can wire a variac (single)
straight across 220 - usually there's even a second face to the dial for
that which reads 0-280V rather than 0-140 (most of the variacs I've met
have taps so they can run 120/240 in up to 140/280 out if desired, or be
wired such that they are limited to line voltage only, depending what
you want.)

If you want to make variable US-Spec 220V power with both sides varying
the same amount, you want 120-0-120 just like the service, which means
the "centers" (0 side of the winding) is tied to neutral, which is tied
to ground at the service entrance.

If you just need variable 220V power and the item is not using neutral,
and is properly insulated to use regular 220VAC, you can just use a
single variac and "0" will be both sides 120VAC away from ground
together, no volts between - just don't confuse "0" with "off" and get
bit.

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In article ,
"Pete C." wrote:

Hmm ... another thing to watch out for. At least the General
Radio Variacs used shafts covered by a black plastic (Bakelite, perhaps,
given when they were designed) and there was no attempt to insulate the
center of the rotor plate from the shaft

The ones I've seen (Variacs) were solid phenolic (probably cotton or
linen based) - a pretty good, pretty strong insulator, with no metal in
the shaft at all. An early composite, common in electrical items.

--
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Ecnerwal wrote:

In article ,
Karl Townsend wrote:

Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

Karl

Sorta depends what you are trying to do. You can wire a variac (single)
straight across 220 - usually there's even a second face to the dial for
that which reads 0-280V rather than 0-140 (most of the variacs I've met
have taps so they can run 120/240 in up to 140/280 out if desired, or be
wired such that they are limited to line voltage only, depending what
you want.)

If you want to make variable US-Spec 220V power with both sides varying
the same amount, you want 120-0-120 just like the service, which means
the "centers" (0 side of the winding) is tied to neutral, which is tied
to ground at the service entrance.

If you just need variable 220V power and the item is not using neutral,
and is properly insulated to use regular 220VAC, you can just use a
single variac and "0" will be both sides 120VAC away from ground
together, no volts between - just don't confuse "0" with "off" and get
bit.

I don't believe you can use a 120V rated lighting Variac on 240V singly.
Remember these are sections from a three phase lighting control, they do
0-120V, no over range like some test bench variacs and they were used
with three sections, one for each 120V to neutral phase of the 208/102
Wye service.

Ecnerwal wrote:

In article ,
"Pete C." wrote:

Hmm ... another thing to watch out for. At least the General
Radio Variacs used shafts covered by a black plastic (Bakelite, perhaps,
given when they were designed) and there was no attempt to insulate the
center of the rotor plate from the shaft

The ones I've seen (Variacs) were solid phenolic (probably cotton or
linen based) - a pretty good, pretty strong insulator, with no metal in
the shaft at all. An early composite, common in electrical items.

This bank had a metal shaft connecting the sections to the motor drive.
I don't recall any particular insulation for that shaft at the drive
end.

On 2012-10-10, Karl Townsend wrote:
Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

Note that many powerstats (these are not General Radio Variacs,
based on the photos -- use "variable autotransformer" for a generic
term) as well as genuine Variacs have extra taps, so you can boost the
line to something like 140 V with a 120 V input per autotransformer.
And because these can be wired to increase CW or CCW, they often have a
lower tap as well, so they are (in series) 20V, 100V and another 20V if
you want boost. You probably don't want boost.

Beware that a big knob on these (especially the steering wheel
style which some have) tempts small hands. I remember a case when I was
in El Salvador, and encountered a setup of US audio equipment all
plugged into a 240V variable autotransformer (I think that it was
another Powerstat, FWIW) with power coming in to the center tap
(which exists on the 240 V ones, so they could adjust to make up for the
(at that time at least) rather variable voltage. Anyway -- a kid about
six years old saw it, his eyes lit up, he ran straight to it, and spun
it clockwise -- frying the selenium rectifier in a Magnecorder.

What I would suggest is to set it and then remove the knob -- or
at least loosen the setscrews so it spins freely -- unless you need to
adjust it frequently. I don't remember for sure far enough upthread,
but I think that this was just to make something designed for a
different voltage work, and not a need for frequent adjustment.

Enjoy,
DoN.

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I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

my 220 welder circuit doesn't have a neutral, so i'll tie this to
ground. Sound good?

Karl

"DoN. Nichols" wrote:

On 2012-10-10, Karl Townsend wrote:
Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

Note that many powerstats (these are not General Radio Variacs,
based on the photos -- use "variable autotransformer" for a generic
term) as well as genuine Variacs have extra taps, so you can boost the
line to something like 140 V with a 120 V input per autotransformer.
And because these can be wired to increase CW or CCW, they often have a
lower tap as well, so they are (in series) 20V, 100V and another 20V if
you want boost. You probably don't want boost.

Lighting control variacs don't have extra taps or "boost" capability,
just simple 0-120.

Karl Townsend wrote:

I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

my 220 welder circuit doesn't have a neutral, so i'll tie this to
ground. Sound good?

Karl

Supply neutral. It doesn't matter if the welder has a neutral or not,
just the supply side.

On Wed, 10 Oct 2012 06:12:47 -0500, "Pete C."
wrote:


Karl Townsend wrote:

I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

my 220 welder circuit doesn't have a neutral, so i'll tie this to
ground. Sound good?

Karl

Supply neutral. It doesn't matter if the welder has a neutral or not,
just the supply side.

Maybe my wording is incorrect. I will plug this device into the welder
outlet. It has three wires, two hots and a ground. So, I will be using
the ground as a neutral. probably not code, but should work?

karl

Karl Townsend wrote:

On Wed, 10 Oct 2012 06:12:47 -0500, "Pete C."
wrote:


Karl Townsend wrote:

I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

my 220 welder circuit doesn't have a neutral, so i'll tie this to
ground. Sound good?

Karl

Supply neutral. It doesn't matter if the welder has a neutral or not,
just the supply side.

Maybe my wording is incorrect. I will plug this device into the welder
outlet. It has three wires, two hots and a ground. So, I will be using
the ground as a neutral. probably not code, but should work?

karl

Yes.

On 10/09/2012 07:24 PM, Pete C. wrote:

Ecnerwal wrote:

In article ,
"Pete C." wrote:

Hmm ... another thing to watch out for. At least the General
Radio Variacs used shafts covered by a black plastic (Bakelite, perhaps,
given when they were designed) and there was no attempt to insulate the
center of the rotor plate from the shaft

The ones I've seen (Variacs) were solid phenolic (probably cotton or
linen based) - a pretty good, pretty strong insulator, with no metal in
the shaft at all. An early composite, common in electrical items.

This bank had a metal shaft connecting the sections to the motor drive.
I don't recall any particular insulation for that shaft at the drive
end.

I have a triple-ganged variac stack that also uses a metal shaft. The
plate on each variac that holds the wiper is phenolic, providing
isolation from the voltage on the wiper.

Jon

On 2012-10-10, Karl Townsend wrote:

I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

my 220 welder circuit doesn't have a neutral, so i'll tie this to
ground. Sound good?

Yes. That should be fine.

Good Luck,
DoN.

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On 2012-10-10, Pete C. wrote:

"DoN. Nichols" wrote:

On 2012-10-10, Karl Townsend wrote:
Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

[ ... ]

Note that many powerstats (these are not General Radio Variacs,
based on the photos -- use "variable autotransformer" for a generic
term) as well as genuine Variacs have extra taps, so you can boost the
line to something like 140 V with a 120 V input per autotransformer.
And because these can be wired to increase CW or CCW, they often have a
lower tap as well, so they are (in series) 20V, 100V and another 20V if
you want boost. You probably don't want boost.

Lighting control variacs don't have extra taps or "boost" capability,
just simple 0-120.

O.K. I wasn't sure, so I mentioned that for completeness. If
it doesn't have them, ignore them. :-)

Somebody else may wind up dealing with a variable autotransformer (under
whatever name) and find this with a search someday. :-)

Enjoy,
DoN.

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On 2012-10-10, Jon Danniken wrote:
On 10/09/2012 07:24 PM, Pete C. wrote:

Ecnerwal wrote:

In article ,
"Pete C." wrote:

Hmm ... another thing to watch out for. At least the General
Radio Variacs used shafts covered by a black plastic (Bakelite, perhaps,
given when they were designed) and there was no attempt to insulate the
center of the rotor plate from the shaft

The ones I've seen (Variacs) were solid phenolic (probably cotton or
linen based) - a pretty good, pretty strong insulator, with no metal in
the shaft at all. An early composite, common in electrical items.

I've seen some at least with the metal core.

This bank had a metal shaft connecting the sections to the motor drive.
I don't recall any particular insulation for that shaft at the drive
end.

I have a triple-ganged variac stack that also uses a metal shaft. The
plate on each variac that holds the wiper is phenolic, providing
isolation from the voltage on the wiper.

Is that genuine Variac (General Radio) or a Superior Electric
Powerstat, or some other brand? It may be that some models have the
insulated shaft and others the insulated hubs within each brand.

Enjoy,
DoN.

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"DoN. Nichols" wrote in message
...

Is that genuine Variac (General Radio) or a Superior Electric
Powerstat, or some other brand? It may be that some models have the
insulated shaft and others the insulated hubs within each brand.

Enjoy,
DoN.

My General Radio type W20 Variac, 20A @ 120/140V, has an insulated
steel-core shaft.

On 2012-10-10, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...

Is that genuine Variac (General Radio) or a Superior Electric
Powerstat, or some other brand? It may be that some models have the
insulated shaft and others the insulated hubs within each brand.

My General Radio type W20 Variac, 20A @ 120/140V, has an insulated
steel-core shaft.

Thanks for the confirmation that at least some GR ones had the
steel core on the insulated shaft.

And the bigger ones would be the most likely to have that
composite shaft, just to add strength with the big knob hung on it.

Enjoy,
DoN.

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Those for stages and such are log windings. This is to allow
most of the travel around the circle as light dimming / bright'ing range.

I found that when repairing a stage set some years ago. They used
levers on a front panel and not knobs.

I have a large one in my general AC system. It must weigh 40 pounds.

I used 150 watt versions to control my electric trains 40 years ago.

Martin

On 10/10/2012 6:12 AM, Pete C. wrote:

"DoN. Nichols" wrote:

On 2012-10-10, Karl Townsend wrote:
Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

I would tie the neutral of each together and to the supply
neutral for safety against imbalance. Otherwise, you have it right.

Note that many powerstats (these are not General Radio Variacs,
based on the photos -- use "variable autotransformer" for a generic
term) as well as genuine Variacs have extra taps, so you can boost the
line to something like 140 V with a 120 V input per autotransformer.
And because these can be wired to increase CW or CCW, they often have a
lower tap as well, so they are (in series) 20V, 100V and another 20V if
you want boost. You probably don't want boost.

Lighting control variacs don't have extra taps or "boost" capability,
just simple 0-120.

On 10/10/2012 02:16 PM, DoN. Nichols wrote:
On 2012-10-10, Jon Danniken wrote:

I have a triple-ganged variac stack that also uses a metal shaft. The
plate on each variac that holds the wiper is phenolic, providing
isolation from the voltage on the wiper.

Is that genuine Variac (General Radio) or a Superior Electric
Powerstat, or some other brand? It may be that some models have the
insulated shaft and others the insulated hubs within each brand.

Mine are GE Volt-Pac units, 240V/8.5A each. They look similar to the
ones on this page (you can see the insulated hubs):

http://www.iowa-industrial.com/Weste...ransformer.cfm

Jon

On 2012-10-10, Pete C. wrote:

Ecnerwal wrote:

In article ,
Karl Townsend wrote:

Xmas came early this year, a couple a great big variacs showed up in
the mail today. Thank you Santa Pete.

These monsters have a common shaft so two can be turned in unison,
nice feature as i need to hook these up 220. Just want to verify the
hookup: The hot lead on each variac to each leg of the 220, the wiper
for each variac to the load, tie the nuetral on each variac together
and to nothing else (not ground that is). Correct?

[ ... ]

If you want to make variable US-Spec 220V power with both sides varying
the same amount, you want 120-0-120 just like the service, which means
the "centers" (0 side of the winding) is tied to neutral, which is tied
to ground at the service entrance.

If you just need variable 220V power and the item is not using neutral,
and is properly insulated to use regular 220VAC, you can just use a
single variac and "0" will be both sides 120VAC away from ground
together, no volts between - just don't confuse "0" with "off" and get
bit.

I don't believe you can use a 120V rated lighting Variac on 240V singly.

You can't -- but there are 240 V rated ones with a center tap so
you can feed them from 120 V and still get the whole range.

But hook 240 V across a 120 V rated one (or even one of the
over-range ones which can go to 140 V) and you will probably fry the
fuse in series with the line input (if you put a proper rating on the
fuse, of course. :-)

BTW I remembered another thing to consider when wiring up these
(both single input voltage and the the double trick to get 240V
coverage from two 120 V ones).

Use *two* fuses per variable autotransformer -- one at the
input, and one at the wiper output. The first protects you from
too much current going in when you have an overrange one set to
produce more output voltage than the input, and the second
protects you when you are putting out less voltage (often much
less) than the input.

The current rating is based on what the wire in the winding will
handle, and depending on the setting, you get more current at
the input or at the output, so protect both.

Remember these are sections from a three phase lighting control, they do
0-120V, no over range like some test bench variacs and they were used
with three sections, one for each 120V to neutral phase of the 208/102
Wye service.

Indeed so. These won't have overvoltage ranges, but still need
fuses at both input and output to be truly safe.

Enjoy,
DoN.

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"DoN. Nichols" wrote in message
....

You can't -- but there are 240 V rated ones with a center tap so
you can feed them from 120 V and still get the whole range.


I've got one of those that has considerable output droop under load
above the tap. IIRC the max current falls from 10A at the tap to
about 6A at the top end. I don't have it wired up in an enclosure, and
I'm not about to power it up exposed on the bench.

On 2012-10-13, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
....

You can't -- but there are 240 V rated ones with a center tap so
you can feed them from 120 V and still get the whole range.


I've got one of those that has considerable output droop under load
above the tap. IIRC the max current falls from 10A at the tap to
about 6A at the top end. I don't have it wired up in an enclosure, and
I'm not about to power it up exposed on the bench.

If the feed is at a 120 V tap, and the maximum voltage is 140 V,
I would expect the maximum output current to drop to 8.57A, not all the
way to 6A.

If the input tap is nominally 110 V, then the 140 V output should
be 7.86 A.

If the maximum output is something other than 140 V, then it all
needs re-calculation. :-)

But essentially, you divide the nominal current by the ratio
between maximum and input tap to get the safe output current.

Essentially, make sure that neither the input current or the
output current exceeds the nominal 10A -- check *both* to be sure.
(This is why a fuse on each. :-)

Enjoy,
DoN.

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"DoN. Nichols" wrote in message
...
On 2012-10-13, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
....

You can't -- but there are 240 V rated ones with a center tap so
you can feed them from 120 V and still get the whole range.


I've got one of those that has considerable output droop under load
above the tap. IIRC the max current falls from 10A at the tap to
about 6A at the top end. I don't have it wired up in an enclosure,
and
I'm not about to power it up exposed on the bench.

If the feed is at a 120 V tap, and the maximum voltage is 140 V,
I would expect the maximum output current to drop to 8.57A, not all
the
way to 6A.

If the input tap is nominally 110 V, then the 140 V output should
be 7.86 A.

If the maximum output is something other than 140 V, then it all
needs re-calculation. :-)

But essentially, you divide the nominal current by the ratio
between maximum and input tap to get the safe output current.

Essentially, make sure that neither the input current or the
output current exceeds the nominal 10A -- check *both* to be sure.
(This is why a fuse on each. :-)

Enjoy,
DoN.

It's a 240/280V 10A Variac. I used it with 120V on the center tap to
check a motor I wanted to turn into a three-phase converter, because I
don't have 240V on the bench. .

The 240V output wouldn't keep the 240V motor spinning. I checked
voltage vs current with resistive loads and found that the voltage
drooped badly above 6A.

jsw

On 10/13/2012 04:29 AM, Jim Wilkins wrote:

It's a 240/280V 10A Variac. I used it with 120V on the center tap to
check a motor I wanted to turn into a three-phase converter, because I
don't have 240V on the bench. .

The 240V output wouldn't keep the 240V motor spinning. I checked
voltage vs current with resistive loads and found that the voltage
drooped badly above 6A.

Hmmm, maybe some type of inductive feedback is causing that? I have
never had any trouble running my variacs at a high current, with no
significant voltage droop. This is with running a resistive or an
inductive (transformer) load, but not with a motor.

You probably already know this, but it should be mentioned that a variac
can be, and is actually designed to, run at a higher current than is
specified by the nameplate, when you run it on a duty cycle (like a
welder). The limit here is the heat generated by the windings because
of the overcurrent, not any saturation in the core.

This duty cycle is normally provided on a chart, but is generally
similar among different models. I'm too lazy to pull the picture from
the .pdf file, but if you look at "Curve 3" under "Overload Capacity" on
page three, you will find it:

http://www.weschler.com/_upload/site...ac_catalog.pdf

It should also be mentioned that the weak link in any variac is the
wiper brush, and any overcurrent capability assumes that the wiper brush
is in satisfactory condition.

Jon

On 2012-10-13, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...

[ ... ]

But essentially, you divide the nominal current by the ratio
between maximum and input tap to get the safe output current.

Essentially, make sure that neither the input current or the
output current exceeds the nominal 10A -- check *both* to be sure.
(This is why a fuse on each. :-)

[ ... ]

It's a 240/280V 10A Variac. I used it with 120V on the center tap to
check a motor I wanted to turn into a three-phase converter, because I
don't have 240V on the bench. .

O.K. So you were using it as a voltage doubler. This means
that the input current would also be doubled in the other direction
relative to the load current.

The 240V output wouldn't keep the 240V motor spinning. I checked
voltage vs current with resistive loads and found that the voltage
drooped badly above 6A.

And at 6 A out of the 240 V tap, you would be pulling 12 A into to
the 120 V input tap tap -- well above the nominal current, and probably
saturating the transformer core.

If you had fused both input and output at the nominal 10 A, you
would have blown the input fuse, which would have given you a clue what
your problem was. (Or an ammeter on both input and output as well as a
voltmeter on the output.

Enjoy,
DoN.

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"DoN. Nichols" wrote in message
...
On 2012-10-13, Jim Wilkins wrote:
...
It's a 240/280V 10A Variac. I used it with 120V on the center tap
to
check a motor I wanted to turn into a three-phase converter,
because I
don't have 240V on the bench........I checked
voltage vs current with resistive loads and found that the voltage
drooped badly above 6A.

And at 6 A out of the 240 V tap, you would be pulling 12 A into to
the 120 V input tap tap -- well above the nominal current, and
probably
saturating the transformer core.

If you had fused both input and output at the nominal 10 A, you
would have blown the input fuse, which would have given you a clue
what
your problem was. (Or an ammeter on both input and output as well
as a
voltmeter on the output.

Enjoy,
DoN.

I'm fairly sure of what happened but didn't scope the waveform for
clipping to prove it, so I just reported the observation that a
center-tapped 240V Variac won't deliver the nameplate current as a
voltage doubler.
jsw

That one would be for a constant 120V line (lights) (motor...)
and the variable zero to 240 is the output for a 240 load.

On this one but mostly others one end is common and the 240 is max.
zero to max. if you hook the supply to the tap - then the
transformer works in 'auto' mode and generates more. Normally
it is 110 to 125 or the like and is used for that voltage only.

Martin

On 10/14/2012 7:03 AM, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
On 2012-10-13, Jim Wilkins wrote:
...
It's a 240/280V 10A Variac. I used it with 120V on the center tap
to
check a motor I wanted to turn into a three-phase converter,
because I
don't have 240V on the bench........I checked
voltage vs current with resistive loads and found that the voltage
drooped badly above 6A.

And at 6 A out of the 240 V tap, you would be pulling 12 A into to
the 120 V input tap tap -- well above the nominal current, and
probably
saturating the transformer core.

If you had fused both input and output at the nominal 10 A, you
would have blown the input fuse, which would have given you a clue
what
your problem was. (Or an ammeter on both input and output as well
as a
voltmeter on the output.

Enjoy,
DoN.

I'm fairly sure of what happened but didn't scope the waveform for
clipping to prove it, so I just reported the observation that a
center-tapped 240V Variac won't deliver the nameplate current as a
voltage doubler.
jsw

Just make sure you get some XC40s to keep it fast. Check at EJOT's website.