Phil, Bert, Yurtle,
I was not aware of the anisotropic conductivity patterns of the graphite brushes in variac wipers to which Bert referred. I will say though from what I have seen from looking inside my variac units, it would appear that the actual working contact surface of the brush is only ‘wide’ enough to obtain full contact with just one bare winding contact surface at a time. Of course, that’s not to say that it could not momentarily make simultaneous contact with two adjacent windings while traversing midway from one winding to the other, due to the very tight spacing between adjacent windings (and would likely be required to get a seamless and ‘smooth’ voltage control throughout the variac’s range). Haven’t really pulled out the magnifying glass to study this mechanism that closely, though. ;>)
David Rieben
Sent from Mail for Windows 10
From: phil
Sent: Friday, February 10, 2017 1:58 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Repair parts for variable transformers?
Beautifully explained Bert, I couldn't remember the finer points so kept
my answer brief. I have used a motor brush and while it worked for 5
mins or so, it soon shorted two adjacent windings out, and wrecked a
good variac.
Lesson learnt the hard way
On 10/02/17 17:09, Bert Hickman wrote:
> Hi Paul,
>
> "Variac" brushes are made using anisotropic (grain-oriented graphite)
> material. The electrical conductivity between stacked graphite crystal
> planes is markedly higher than the resistance parallel to the crystal
> planes. Brush manufacturers typically formulate a mixture of carbon
> and oriented carbon-graphite that provides sufficient mechanical
> strength while still maintaining a relatively high ratio of
> conductivity (~10:1) between axes.
>
> A Variac brush is oriented so that the high-resistance axis is across
> the "thin" dimension as the brush makes contact with the winding. This
> reduces turn-to-turn short circuit current when the brush bridges a
> couple of turns. Also, the main body of the brush "averages" the
> output voltage between turns, smoothly and continuously varying the
> output as the brush transitions from one turn to the next. Using a
> standard carbon brush may cause overheating of the brush tips, arcing,
> and excessive wear of the brushes and winding.
>
> Following is a possible source for the proper material - you may need
> to find a similar-size brush and then trim it to fit your unit.
>
> http://www.carbonbrush.com/variacbrush.htm
>
> Good luck,
>
> Bert
>
>
> Paul B. Thompson wrote:
>> I’ve found I need to replace the carbon contact on my Chinese variac
>> (yes, I know). I made a cursory search online without success. Anyone
>> know where I can find parts for same?
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--
Regards Phil www.hvtesla.com
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