Thanks all. I have contacted the vendor Bert listed. We'll see how it works
out.
Paul
-----Original Message-----
From: phil
Sent: Friday, February 10, 2017 1:12 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.
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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