This thread makes me all the more grateful that the Chinese variac I have, came with a spare brush. :)
> -----Original Message-----
> From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of phil
> Sent: Friday, February 10, 2017 12:13 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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