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Uses for a dead variac

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LAN_403

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Fri Dec 23 2011, 03:37AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

While cleaning out my shed the other day I unfortunately dropped my 500VA variac that I was going to use to control filament voltage on my planned VTTC. About a dozen of the turns snapped at the point where it struck the floor, really shitting me off that I'd dropped something that I don't have them money to replace. So, in an attempt to cut my losses, has anyone got any suggestions for what I can use a dead variac for?

Marko

Fri Dec 23 2011, 04:59AM

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

People have repaired variacs before. A common and even nastier accident is to have the variac accidentally shorted while the brush is very near the mains tap point, causing a huge current surge to blow it into a ball of plasma and vaporizes a portion of windings underneath.

Repairing winding that is not on the brush site should be even easier; just solder it over with some wires.

Marko

Patrick

Fri Dec 23 2011, 05:25AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Disassdemble it and post a pic of the damaged portion, maybe we can suggest a repair.

Herr Zapp

Fri Dec 23 2011, 05:55AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

DL -

Are the damaged windings very close to the start of the "low voltage" end?

If so, your can usualy connect the variac "backwards" , so zero output voltage is with the wiper at the fully clockwise position, and the output voltage increases as the knob is turned counterclockwise. Your maximum output voltage will be reduced depending on how many windings are damaged, but a variac that can produce 0-75% (or 0-XX%) of its original output voltage is more useful than no variac at all.

Its essentially impossible to repair this type of damage if the wires are actually severed.

Herr Zapp

radiotech

Fri Dec 23 2011, 07:07AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Repair it by connecting the free ends of the sections that are still good. Remove or insulate
turns that may short, turn to turn. You will have a Variac that has gaps in the span.

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Fri Dec 23 2011, 09:01AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

The wires are severed right near the end of the winding, I'm thinking I could skip the damaged part and I'd only lose <3% of the winding

Pic attached

Forty

Fri Dec 23 2011, 03:29PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

Given the location, bypassing the damage would probably be fine. You could connect both sides of the damaged part together, or one side of the damaged part to one of the outputs. Any windings left that aren't being used should be removed and a stopper added so the brush doesn't go open circuit. Less windings being used would just reduce your voltage range to about 0-105v (if it was 0-115v originally)

From the picture it doesn't look like the wires are actually severed so if they are not then you might be able to clean them up and reinsulate them with epoxy or laquer.

You could also put a big gob of solder (or solder on a piece of copper) on the exposed wire and essentially short out that whole region. This would be the same as bypassing from one side of the damage to the other except you'd have a continuous brush connection through the whole region and wouldn't have to remove any windings (and wouldn't reduce your output range by as much.)

There. hopefully that summarizes what you can do with your accident. Good luck.

Myke

Fri Dec 23 2011, 04:27PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

It looks mostly fine. Some insulation would help prevent possible shorts. This isn't a blanket fix though.
You'd have to inspect the windings for any tiny bits of metal bridging the windings and such.

You should watch out for shorted turns. With a shorted turn, you'd draw lots of current with nothing
connected which may lead to melted windings and such.

Herr Zapp

Fri Dec 23 2011, 04:45PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Radiotech, Forty -

Seriously, have either of you ever successfully repaired damaged variac windings using the techniques you are suggesting??

For the "just remove the damaged windings and reconnect the ends of the last good windings" technique, how did you deal with the brush dropping off the edge of the last turn of wire at the end of good section #1, and then having to climb up ove the first turn of of wire at the beginning of good section #2? On small variacs like the one we are discussing, the carbon brush is very brittle and the contact face that sweeps the windings is quite narrow, and just can't handle "steps" in the height of the surface it is sliding over.

For the "just short all the damaged winding togerher with solder" technique, how did you deal with heat buildup and continuous current draw of the shorted windings? While this approach might work on a damaged variable resistor, a variac is a transformer, with a voltage gain across every single turn. Shorting any turns together results in (obviously) a dead short between adjacent windings.

Herr Zapp

radiotech

Fri Dec 23 2011, 05:14PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

As to the repair techniques involving restoring the continuity, I have done this as
a temporary repair on a Variac that controlled a 'shaker' motor for a screen in
a lime kiln. The repair was just until a replacement could be shipped, (it was a three
phase 208 volt Variac).

I agree about the brush meeting the steep wire edge, however, if close to the end,
the stop could be moved to avoid the bad area. I have a low voltage power supply
with a 'linear adjustable transformer, using a carbon brush, that has a bad area just
at the start.

As to damaging a Variac, most are done by poor packing or extreme carelessness
(myself included !) They are awkward devices that really cant safely be put down
without the possibility of rolling or dropping something on them.

.

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