variac carbon brush material?

[coppercone2]

So I realized my mistake in a variac restoration of mistaking the almost-disintegrated carbon brush residue on a VERY used variac for a regular dirty contact. It works fine but I have been reading that there is likely excessive contact wear in a non brushed variac, despite lubed brass on copper contact... to my defense though its silky smooth and during assembly after my restorations I measured the resistance to be low.. so I thought I was in the clear.. (before restoration I noticed the brush was sparking during traverse, it felt like it was grinding and the resistance was often unreasonably high.)

Is it possible to get a small cylinder of this material so that it can be glued to  the contact with high conductivity silver epoxy to 'rebrush' it? Or do I need to get the new part that is sintered or whatever (meaning custom repair mod job). The remains of the old contact is basically a little dome, so it seems that if I got a cylinder of carbon brush material, carved a bowl into it with a diamond burr on a dremel, and glued it to the brass mount so it sat flush, it should work. This is for a 3A max variac.

How important is this carbon brush anyway? Like outside of literature I was 100% satisfied with my result.

[TimNJ]

Maybe something here: https://www.mcmaster.com/carbon-brushes/

[coppercone2]

I figured it should be round, but I need to attach it to brass, is silver epoxy even an option here?
The variac brush replacement with a standard new brush seems like a total pain in the ass, there is a web page some where about it. I really wanna be able to use a glue without the smithing/fitting.

[t1d]

I researched this very question. Here are my threads about it. I forget which one is most specifically on topic, but they all will likely be helpful to your restoration.
https://www.eevblog.com/forum/repair/i-need-an-education-on-electric-brushes-for-a-variac-aka-electric-motor-brush/msg3471324/#msg3471324
https://www.eevblog.com/forum/projects/do-you-know-this-ys-or-sy-taiwan-manufacturer-logo-see-picture/msg3492320/#msg3492320
https://www.eevblog.com/forum/repair/bk-precision-1655a-ac-power-supply-heres-a-replacement-brush-that-might-fit/msg3496578/#msg3496578

Bottom line:
- No, you can not use electric motor brush material to DIY a proper autotransformer brush. See links as to why.
- Proper autotransformer brushes are difficult to find. There are only a few USA after-market distributors. I think I included the names of the distributors that I found in the links. The manufacturer may, or may not, carry them.
- Proper autotransformer brushes are expensive. After-market is cheaper than OEM, if they carry what you need. Beware Ebay/Amazon/foreign suppliers/etc. as fakes seem likely.

I hope that helps. Please post your findings and results. We would be most interested.

[coppercone2]

oh boy 

I did not think about the shorted turn thing in the web page. seems like the graphite dimensions is a design choice based on the coil dimensions. probobly better to replace the variac then

wonder how long that will take to manifest itself without a brush

[t1d]

probobly better to replace the variac then

I had a little difficulty understanding your short post... But, if you are giving up on the repair, I would ask a couple of questions... 1) Please explain a bit more thoroughly as to why you think it is not worth searching for a brush. 2) I thought that I posted the after-market sources that I had found. If I did not, and that is keeping you from searching for the part, let me know and I will get you the links. I would even help you with the search, if necessary.

Warning: My opinion follows
A repaired unit from a reputable manufacturer is way better than a new Asian manufactured unit. Yes, these usually red-colored units are cheaper on Ebay/Amazon. However, they are known to have overstated ratings, undersized and incorrect wiring, no fuse, or fuse in the wrong place, improper grounding, etc. You are not going to be able to tell which units might be okay, from the sales documentation. The best that I have seen of them did not have so many internal problems, but their ratings were still overstated.

I hope that helps.

[coppercone2]

well my concern is more that given the way it was found with how much it was sparking, i thought that this was limited to heating near the top, but the short circuit turn thing means the coil can be over heated, so I wonder if the insulation is compromised. Thats somewhat of a new damage mode for a transformer.

What I probobly should do is get a megger and check the insulation resistance on it. Thats why I thought a new one might just be better.

 I am kinda wondering if there is a bunch of thermal damage on the core after reading that stuff. I was thinking like the top interface more overheat and so long I did not see a 'band' or patches of overheating insulation near the electrical contact it was alright, but now it looks like maybe you can have single overheating turns because of  bad contacts. I am kind of wondering if some of the coils turns are on the way out. I only considered "hot contact" not the shorted turn thing due to lack of carbon pad.

[David Hess]

I did not think about the shorted turn thing in the web page. seems like the graphite dimensions is a design choice based on the coil dimensions. probobly better to replace the variac then

wonder how long that will take to manifest itself without a brush

I have replaced a few variac brushes now by making a new one from a carbon rod extracted from a carbon-zinc D cell battery.  I file a section of the carbon rod to shape and use a triangular cross section with the narrow point contacting the windings.  Real variac brushes contact adjacent windings without problems.

Lionel used a circular brush, about the size and shape of a button cell, which rolls along the windings in their model train transformers seen here.

[coppercone2]

OK, A D or F cell has that. And I did experiment with those sparking them with 2000 amps for resistance brazing and they are robust.

How did you adhere it though? I only ever jammed those rods into stuff to make electrodes (jam into a hole in a conductor). I have some of those rods already. But I also need the megger test because its bugging me

also, you need to either burn or soak off the chemicals in the rod, when you get the carbon hot the first time from a battery it ofgasses quite a bit for a little while

I made two types of electrodes, one is a long carbon F cell rod jammed into a pipe fllled with silver bearing solder and the other is a copper rod. Never found a use for the carbon rod, it needs to be thin, if you use the whole thing its not very useful unless you have very poor regulation and your desperate to solder something in a inefficient way. On first heat it makes a thick grey white smoke and then it decreases the smoke output drastically.

If I were to cut a puck of this material, I would fire with a torch before installation.

[David Hess]

How did you adhere it though? I only ever jammed those rods into stuff to make electrodes (jam into a hole in a conductor). I have some of those rods already. But I also need the megger test because its bugging me

The variacs I have fixed all held the brush in the wiper assembly through spring tension, so I just popped it in.  The rod section was filed into the shape of the original graphite wiper.

also, you need to either burn or soak off the chemicals in the rod, when you get the carbon hot the first time from a battery it ofgasses quite a bit for a little while

I put the carbon rod from the battery into a small bowl of water saturated with baking soda and microwaved it to boil the water.

[coppercone2]

this is fitting for a episode of mac guyver

[rikkitikkitavi]

This is quite an old topic, but I have been trying to renovate an old Variac (of many) and found out:

Many carbon brushes, especially the ones with a wire inserted are made from high pressure compressed graphite powder. This in at large not anisotropic due to the randomness of graphite powder. I have as far as I tried not seen any difference in conductivity in axial or radial orientation. But the measurement is tricky, and prone to errors. Say a 1:2 variation in results.

Also, making the graphite with the wire inserted from resin that is graphitised will not work, as the later process requires >2000K to happend and no sane conducting wires can survive that temperature.

Pure anisotropic graphite is available, but at horrendous costs due to its manufacturing method (vacuum decomposition of hydrocarbons against hot surfaces)  Its anisotropy is however remarkable, there could be a 1:100 ratio of the thermal/electrical conductivity. A sheet 30X100mm , 6 mm costs abou 100Euros or even more.

Motor brushes is generally a no-no, due to the metal powder content.

I ended up with a ordinary moro brush which was pure graphite, with wires inserted and a connector. Worked fine, but I am not pushing the Variac to the limit. It is by now 60years old, dating back to early 60ies  but hey, they never grow old.

I have in total 2X25A, 1X20, 4X10A, 1X3A (insulated) Variacs , all with 230VAC. Some are meterology nuts. I am a variac nut.

[coppercone2]

Do you know the formulation for the 2000F method of creation of the carbon cylinder? I can do that temperature range now

[rikkitikkitavi]

2000F is not sufficient. 2000C , or 2300K is necessary, that is about 3600F.

The graphite is made by running current through a mix of coal powder, resins and thick petroleum materials in the form of a paste, that is well insulated untill all volatiles are evaporated and the carbon is graphitised. Google Acheson process.

Silica carbide is made this way aswell.

Most graphite items today is hoever made from high pressure/temperature forming of powder with some binders and subsequent heating to drive of volatiles. The more hydrocarbons the more porous.

Many million tonnes of graphite is also produced from mining natural sources.

[coppercone2]

oh that was a Kelvin


But I wonder if you do the green press method, can you induction heat them to ridiculous temperatures maybe? instead of baking? Its a fairly solid part at that point.

and you might not need super temperature control, its not like you can over heat the part and melt it. and it has low thermal mass so its not like heating up a kilo of copper

[rikkitikkitavi]

Higher temp just means the sintering goes faster, or rather the decomposition of the resin and graphitisation of formed amorphous carbon.

I dont know if induction heating works well, I doubt resistance is low enough for any reasonable power transfer.