Variac Brush

I'd simply file down a motor brush to fit. It's not critical. Makes a mess but less time and effort than trying to find an exact raplcement. One I did on a power supply 25 years ago is still going strong.

--- sam | Sci.Electronics.Repair FAQ Mirror:

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I find that sandpaper between 100 and 200 grit works great for sanding down brushes to fit. Wear rubber gloves if possible, and plan on throwing the sandpaper away even if it is hardly worn. The black carbon gets into EVERYTHING!.

Sometimes these were a critical design, to prevent shorting adjacent turns, but it looks like yours isn't. Just find the nearest brush you can which is bigger and plane it down. Use a coarse file, not any sort of sandpaper or emery paper.

There's no way to avoid shorting adjacent turns. Else, there would be gaps in the output voltage! However, the width could be important to avoid shorting more than one adjacent turn. :)

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ISTR some specification for resistivity for that purpose but these days you have to use what you can get.

Whether you use a file or sandpaper, do the shaping under a stream of running water to avoid a terrific mess. Wet-or-dry carborundum paper is your friend.

Isaac

Probably is and would matter for a high power Variac. I wonder what the limiting factor is on current between adjacent turns. What other factors will be important besides resistance of the winding and brush/ contact?

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Clearly these have a high volts per turn ratio. One shorted turn is limited some by the impedance of the other windings, but really the resistivity of the brush is critical.

There are a few suppliers that can provide brushes for variable transformers, such as

It appears that they are not cheap... roughly $20 USD for a 2A model.

A Google search revealed a variety of opinions on proper brush replacement, from "any old motor brush" to "only the proper brush specified for the particular model". Here's an excerpt from a web site that gives a bit more info than most concerning brush replacements:

"Easily replaceable carbon units are used, the resistance of the brush being carefully calculated to limit the current circulating in the bridged turns. It is therefore most important that only the correct replacement brush is used in any particular Brushes are mounted in robust brush carriers insulated from the shaft, and designed so that metal parts cannot come into contact with the winding in the event of brush fracture"

It makes sense that the composition of the brush be such that it afford a higher resistance than normal motor brushes in order to limit the circulating current in adjacent shorted turns.

If you can't find a suitable source for a replacement brush, your best alternative is to buy a complete unit from ebay or find one at a swap meet in your area. I've bought a number of variacs on Ebay for $15 - $20 USD each, cheaper than a replacement brush.

Cheers!!! ==============

Dave M

Never take a laxative and a sleeping pill at the same time!!

I see some brushes are VERY expensive.

VBT-6 W50 3200-5906 $149 VBT-7 W50H 3200-5907 $149

brush.As

A nubile virgin with a flexible tongue? Or a cold shower.

I don't know about his model, but many of these use a brush which consists of a carbon brush brazed to a brass holder, or a carbon brush with a pigtail embedded.

The first is impossible to duplicate, while the second requires slightly different starting materials.

Or does someone know how to braze to carbon?

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I once saw an ad for someone who was promoting the idea of a bifilar wound variable autotransformer which was meant to "cure" this problem. Personally, it didn't seem to be much of a solution, and I've never seen them on the market. It sounds like an idea which was too hastily presented, and it must have died as soon as someone thought more seriously about it.

Anyone else remember seeing this?

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Nope. Sounds insane. After all, a Variac is really an autotransformer with a LOT of taps. You don't want to short the taps together now, do you?

It takes a lot of persistence or really strong spit.

The low current ones I've seen usually just have a brush held in place by spring pressure. Don't ask me how I know. :( :)

--- sam | Sci.Electronics.Repair FAQ Mirror:

Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.