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!
Thanks, Greg 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? Message-ID: <a1f21e38-3489-33d8-342a-d771f2232e81@xxxxxxxxxxxxxxxxxxxxx> 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. http://www.carbonbrush.com/variacbrush.htm Good luck, Bert
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