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rebuilding a variac

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Tiberius

Fri Jun 06 2008, 12:35AM

Registered Member #1484 Joined: Wed May 14 2008, 03:24PM
Location: Cary, NC, USA
Posts: 27

I just picked up a W50H 0-280V 25A/32.5A variac for pretty cheap off eBay "untested, as-is". The internal fuse was blown, so I imagine that's why it was so cheap and "untested". I've replaced that, corrected an alignment problem, cleaned it, it tests to the spec for coil resistance on the multimeter. Now I'm wondering about a few other things.

When the brushes slide over the contact surface, there's like a plastic-y scraping sound, almost like I'd imagine the varnish sounding if it were peeling back from the wires in places. Could this be the case, and if so should I be concerned about that?

The contact surfaces all seem to be a little oxidized, both on the windings and the contact surface under the rotor. Would it be a good idea to freshen these surfaces up with a metal file or some sandpaper? I was thinking maybe 300-600 grit sandpaper, but if I can just hit it with the file that'd be nice to know.

Steve Ward

Fri Jun 06 2008, 02:00AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

First, DONT file it, its got a special coating on it which should resist oxidizing better than bare copper. Find some metal cleaner, usually in powder form, and just scrub it with a rough sponge or tooth-brush or something.

As to the sound you hear when its moving, it does sound a bit suspicious of enamel flaking off, you should be able to visually inspect this though. Ive abused some variacs to the point where the enamel starts to chip away, but it doesnt seem to cause much harm otherwise. Some super glue might be a good option to wick in-between turns if you suspect the possibility of a short circuit due to enamel missing, just be sure not to let it get near the contact area, of course.

quicksilver

Fri Jun 06 2008, 03:16PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I had the same issue with a different device of some age (a chem-lab hotplate/mixer). It sounded like a scraping noise. It was very hard to isolate as it was INSIDE a pot. It did not affect functionality and I believe it could have been corrosion, salts deposits, or even simple wear.

You may be able to actually SEE what the issue is but I could not and since it did not affect functionality I choose to leave it alone. Steve outlined the possibilities of repair and the pitfalls. I had even thought of using a conductive lubricant but there again there may be a serious problem down the line should it collect and create a short.

I've created problems in the past where only superficialities existed by attempting to make a older device "new feeling" where I need to weigh how valuable the potential gamble is vs the result.

Side NOTE: The device is still working fine despite the noise with perhaps two decades of undergrad lab use to it's credit.....It just sounds "funky".

Tiberius

Fri Jun 06 2008, 11:00PM

Registered Member #1484 Joined: Wed May 14 2008, 03:24PM
Location: Cary, NC, USA
Posts: 27

Upon further inspection, I noticed that the enamel on the outside of the first and last turn is visibly peeling back from the wire. I haven't powered it yet so I'm not sure if there are going to be issues with shorted turns. If there aren't any shorted turns I'm not going to worry about it for now -- the windings appear to have enough space between them that I shouldn't have to.

There does appear to be a coating on the contact surfaces of the windings, almost like they're tinned. Some of the metal cleaners I've found suggest they shouldn't be used to clean alloys containing lead, which makes me wonder what exactly the content of the coating is and what sort of cleaner I should be looking for?

Update: I gave it some 120V and was able to verify full range 0-280V on the output, no heating of the windings seemed to be occuring as in a shorted turn. It throws the breaker every other time I plug it in and hums something wicked, but I assume that's just a benefit of a huge open Variac. Apparently if I give it neutral/phase backwards it'll give me +96V on the case!?! Time to find one of those polarized plugs, or should I be using a three-prong plug with phase and ground across the input?

quicksilver

Sat Jun 07 2008, 04:41PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Pictures always help with these issues; I had no idea that it didn't have a 3 tine grounded plug....I would put one on ASAP. Are you sure the noise is coming from the contact of the windings? Often the origin of these things can be confusing in a small dense device.

The breaker and humming concern me. I take it that you are not talking about a 60hz sort of deep sound but something else????? Go back to the fuse and ascertain whether what you replaced the blown one with the appropriate fuse for this application!
Also look for anything that indicates that the machine was available to run off of 120/240 and that it was set for 240 (or something to that effect) and that that was not corrected for your application.

The other thing I would try is to run the thing in the dark and look for internal arcing, etc. Some how this issue sounds familiar.....A well made working Variac is a $400+ tool and worth hassling over to get it in shape.

Tiberius

Sat Jun 07 2008, 10:55PM

Registered Member #1484 Joined: Wed May 14 2008, 03:24PM
Location: Cary, NC, USA
Posts: 27

The humming is definitely 60Hz. It didn't have any kind of cable it all attached when it came to me, it just has screw terminals for taps at 0V, 40V, 120V, 160V, 240V, 280V, and the slider terminal. I used a clipped off 2-prong cable I had in the parts box to attach across 0 and 120. I'm pretty sure the breaker being thrown is inrush, as if it's going to happen it seems to happen within the first few instants after power is applied. It doesn't make any additional noise with power applied than it makes with no power applied, aside from the humming. I ordered the fuse by part number and when it arrived it was identical to the fuse it replaced, aside from not being an open circuit. I'm pretty sure it's working fine now, but I may still clean the contact surfaces if I can find the right cleaner.

Here's the picture the seller used, and it appears to be of this very unit as the clipped red ends of wires were still attached when I received it. Shipping weight was 59lbs and according to spec sheet weight is 53lbs!

quicksilver

Sun Jun 08 2008, 05:40PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Now I understand your concern. In certain instances silver was used as contacts, plating and full silver. We know that silver oxidizes a dark (generally black but copper does also). You may want to consider compressed gas to "brush out" any particulate matter. Oxidation + varnish flakes could play havoc with a creature like that.

I think that since it's a real 60hz hum it just may be something age related like the above. may I ask how much you paid? I always wanted one and that looks pretty tough. If you can fine some, I have heard of people (using extreme care) applying micron-sized aluminum powder as a conductive lubricant; but you can't get sloppy with it of course.

Kolas

Mon Jun 09 2008, 05:38PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

It doesn't sound like there is anything wrong with your variac. I have a similarly rated variac (although it's 120V, but rated 22A) and it frequently pops the breaker when it's plugged in unloaded. The humming is also reminiscent. I've found it hums for about 10 seconds when it's initially charged, and then the hum goes away as if some sort of saturation were occurring. How long does your variac hum after being plugged in? Is it a constant hum?

Marko

Mon Jun 09 2008, 05:55PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Kolas wrote ...

It doesn't sound like there is anything wrong with your variac. I have a similarly rated variac (although it's 120V, but rated 22A) and it frequently pops the breaker when it's plugged in unloaded. The humming is also reminiscent. I've found it hums for about 10 seconds when it's initially charged, and then the hum goes away as if some sort of saturation were occurring. How long does your variac hum after being plugged in? Is it a constant hum?

Yes, large transformers can draw large inrush current when plugged in because initially created disbalance in V/s saturates the core. It settles down after couple of seconds as ohmic resistance removes this energy, creating the described decaying hum.

Transformer should be started up through a resistor to provide the initial magnetizing current, which is then shorted by a relay controlled by a timer.

If load is present there will be no inrush, but the starter circuit is again useful if we have a rectifier and large filter capacitors on the output which need to be charged quickly.

Marko

Tiberius

Mon Jun 09 2008, 11:30PM

Registered Member #1484 Joined: Wed May 14 2008, 03:24PM
Location: Cary, NC, USA
Posts: 27

The humming doesn't really seem to decay at all, so I measured its idle current, .53A at 122V input or 64.6W which is not too far off the no-load loss specification of 50W. I'm pretty convinced it's in serviceable shape at this point, although there is one nagging concern I have.

For some reason, as it's currently connected there's 98V on the chassis at any slider position. Near the terminals there is the fairly cryptic text: "FOR COMMON (GND) LINE-LOAD CONNECTION USE TERMINAL 2 OR 4". What exactly does this mean? To me it sounds like I should use terminal 2 or 4 for ground, and this was the connection used when I measured significant voltage on the chassis. When I put ground on the "40+80" tap and phase on terminal 2, I get around 7V on the chassis. The only connection to ground is via the screw terminals, right now the chassis is otherwise insulated from ground. What could be causing this voltage, is it indicative of a fault somewhere, and if not a fault would it still be unsafe to ground the chassis?

Update: I read >10mohm between the chassis and each tap so I assume this is an induced voltage that would dissipate with a ground connection.

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