BIG variacs

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.

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.

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 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.

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 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.

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

Karl

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

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.

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

Yes. That should be fine.

Good Luck, DoN.

[ ... ]

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.

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

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.

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.

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.

Mart>

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):

Jon