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Re: [TCML] Tesla Digest, Vol 112, Issue 3

Hi Bert,

That's a cool bit of info... Did not know that the anisotropic nature of brushes was so pronounced, and that it's exploited in variacs to reduce turn-turn shorts!


On 2/21/2017 6:38 AM, tesla-request@xxxxxxxxxx wrote:

Date: Fri, 10 Feb 2017 11:09:53 -0600
From: Bert Hickman<bert@xxxxxxxxxxxxxxxxxxxxx>
To: Tesla Coil Mailing List<tesla@xxxxxxxxxx>
Subject: Re: [TCML] Repair parts for variable transformers?
Content-Type: text/plain; charset=UTF-8; format=flowed

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.


Good luck,


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