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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

BIG variacs



 
 
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  #1  
Old October 10th 12, 02:26 AM posted to rec.crafts.metalworking
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Posts: 3,246
Default BIG variacs

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
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  #2  
Old October 10th 12, 02:49 AM posted to rec.crafts.metalworking
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Posts: 6,757
Default BIG variacs


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.
  #3  
Old October 10th 12, 03:17 AM posted to rec.crafts.metalworking
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Posts: 493
Default BIG variacs

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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  #4  
Old October 10th 12, 03:23 AM posted to rec.crafts.metalworking
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Posts: 493
Default BIG variacs

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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Please don't feed the trolls. Killfile and ignore them so they will go away.
  #5  
Old October 10th 12, 03:26 AM posted to rec.crafts.metalworking
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Posts: 6,757
Default BIG variacs


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.
  #6  
Old October 10th 12, 04:24 AM posted to rec.crafts.metalworking
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Posts: 6,757
Default BIG variacs


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.
  #7  
Old October 10th 12, 05:28 AM posted to rec.crafts.metalworking
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Posts: 2,077
Default BIG variacs

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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  #8  
Old October 10th 12, 05:58 AM posted to rec.crafts.metalworking
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Posts: 3,246
Default BIG variacs


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
  #9  
Old October 10th 12, 01:12 PM posted to rec.crafts.metalworking
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Posts: 6,757
Default BIG variacs


"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.
  #10  
Old October 10th 12, 01:12 PM posted to rec.crafts.metalworking
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Posts: 6,757
Default BIG variacs


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.
 




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