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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.
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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