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Re: variac

> I'm the proud? owner of an ancient looking 0-280 volt 28amp
> Powerstat.  It seems to work OK, but I've only tested it on a 
> 120 volt line and with a light load.  On inspection, I see that
> the wiper brushes are worn and in need of replacement.  Does 
> anyone have a source for new brushes or the address of Superior
> Electric?

Pull a brush assembly out of the housing and tote it down to your
local electrical motor repair shop. Grab three larger pieces of
carbon (one extra just in case) larger than your existing brushes
and cut them down to a perfect fit with a file or high
speed cutting tool. I have used grinder wheels, files, dremmel
tools, etc., to make many different brushes. Brushes for smaller
variacs (and small DC motors) can be cut from carbon rods pulled
out of old dry cell batteries. 

A few years back I had the opportunity to take a guided tour of
the mechanical facilities of a large/old metropolitan building.
The elevators were powered by very large stationary AC->-DC motor
generator sets. The AC side was three phase 440 or some such, the
DC side who knows... What I do know is that there were cigar
boxes heaped with old brushes from the DC generators. Brushes
pulled as far back as the 1920s were laying about. With
permission from the building engineer I cleaned up some of this
debris. :-) 

Some of these brushes seem to be made of a copper/graphite
mixture which I guess would improve the wear and conductivity
characteristics. I could almost cut three Powerstat variac
brushes out of one of these massive chunks of carbon. Each
generator used several dozen of these brushes, two generators,
24x7 duty cycle... I should have grabbed more as there were
literally hundreds of them laying about.

Richard Quick


... If all else fails... Throw another megavolt across it!
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