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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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