Re: Re. Variac brush repair

To: tesla-at-pupman-dot-com
Subject: Re: Re. Variac brush repair
From: "Robert W. Stephens" <rwstephens-at-headwaters-dot-com> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Tue, 15 Dec 1998 08:08:22 -0700
Approved: twf-at-verinet-dot-com
Tesla List wrote:
> 
> Original Poster: Gary Lau  14-Dec-1998 1014 <lau-at-hdecad.ENET.dec-dot-com>
> 
> A small bit of additional information.  I called Superior Electric in
> Bristol CT at 860-585-0600.  The model that I have, #1126, is apparently
> something like 30 years old and they tell me that they no longer have
> parts for it.  I further asked whether anything similar existed that
> might be modified, but was told that every model's brushes are very
> different compositions of metal and carbon specific to that model, MOST
> unhelpful.  I can't believe that it could be THAT unique, but I wonder if
> variac brush compositions are necessarily different than motor brushes
> which I might be able to modify.  It would be one a shame to
> junk an otherwise perfectly good 2KW variac for lack of a brush.
> 
> >Original Poster: Gary Lau  13-Dec-1998 1211 <lau-at-hdecad.ENET.dec-dot-com>
> >
> >I have  a Superior Electric Powerstat 120V/15A variac whose brushes are
> >just about at the end of their life.  While my first choice would be to
> >simply buy a new set of brushes, I fear this may be hard to come by.  If
> >anyone knows if it is possible to buy replacement brushes, please do let
> >me know!
> >
> >Anyway, I was considering the possibility of repairing the old brushes.
> >They appear to be brass blocks with a carbon brush soldered into a notch.
> >While I'm sure I could find another carbon brush and machine it down to
> >size, I'm not certain about how it's secured in that brass notch, I doubt
> >that carbon is solderable unless it's copper plated.  Has anyone else
> >attemped such a repair?
> >
> >Gary Lau
> >Waltham, MA USA

Gary, All,

The wannabe rocket scientist at the factory wants to convince you to buy
original replacent parts, or a brand new variac.  I have a 23 Amp, 230
volt variac that has a heavy nickel plated copper wiper that travels
around the outside circumference of the winding which is the cleaned
winding working surface.  There are 5 carbon/graphite type brushes in
spring backed holders each with a very short copper braid leadwire tied
to this heavy moving wiper assembly with a screw down. When I acquired
this variac it was practically metal on metal. I went to a local motor
rewind shop and purchased some motor brushes from their misc. box. I
took them, using them merely as sources of graphite block, and machined
them with a sanding disc on an angle grinder (cuts fast) and then fine
tuned with a hand file, to press fit into the original holders.  No
epoxy, just friction fit.  The nature of the finished brush assembly
keeps the graphite slug trapped anyhow under the applied spring pressure
against the variac coil working surface.

With 5 brushes in electrical parallel wiping on a distributed length of
variac winding there should be a differential voltage between the first
and last brush which the variac manuals will tell you is a very bad
thing.  The entire wiper package also shorts adjacent turns as the
variac is varied. This apparently is also a bad thing.

My repaired variac has worked faultlessly with no signs of localized
winding heating at 1.5 times rated power ever since this repair.

BTW, and this may be objectionable to some, working with graphite
materials like motor brushes really gets your hands blackened.  In case
this seems like a problem, be assured that ordinary hand-soap and water
works just fine to clean up.

Simple problems don't always have to be overly complicated. Rolling up
your sleeves and getting your hands dirty can often save an otherwise
expensive repair job, and even if the attempt fails, the effort is
more-often-than-not worth the education gleaned.

Robert W. Stephens