On Tue, 05 Jul 2005 19:35:42 GMT, "geoff smith"
wrote:

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).
Thanks in advance geoff



There are a few suppliers that can provide brushes for variable
transformers, such as
http://www.jenkins.com/jenkins/trans...able_trans.htm
http://www.ietlabs.com/Variac/Variac_Brushes.html

It appears that they are not cheap... roughly $20 USD for a 2A model.

A Google search revealed a variety of opinions on proper brush
replacement, from "any old motor brush" to "only the proper brush
specified for the particular model". Here's an excerpt from a web
site that gives a bit more info than most concerning brush
replacements:

"Easily replaceable carbon units are used, the resistance of the brush
being carefully calculated to limit the current circulating in the
bridged turns.
It is therefore most important that only the correct replacement brush
is used in any particular Brushes are mounted in robust brush carriers
insulated from the shaft, and designed so that metal parts cannot come
into contact with the winding in the event of brush fracture"

It makes sense that the composition of the brush be such that it
afford a higher resistance than normal motor brushes in order to limit
the circulating current in adjacent shorted turns.

If you can't find a suitable source for a replacement brush, your best
alternative is to buy a complete unit from ebay or find one at a swap
meet in your area. I've bought a number of variacs on Ebay for $15 -
$20 USD each, cheaper than a replacement brush.

Cheers!!!
==============

Dave M

Never take a laxative and a sleeping pill at the same time!!