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radiotech

Sun Feb 06 2011, 03:14PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Variac brushes are a major concern for coilers who may obtain
used Variacs for experiments. New applications, never were around
when these devices were designed. An example is switch mode
power supplies.

In the carbon brush science, there was major development and
application science, because large motors and generators used
great quantities of brushes, and the competition among brush
makers to get the contracts to supply these was fierce. Often
the chemistry was adapted to suit the environmental conditions
the machines were used in.

The electrographitics of brush to copper was they key.

4Hv does have chemistry. How about shaking the bushes for some
methodology? Either adoption of existing motor brushed or some
new formulations to make-press new good brushes.

Although, brushes are being phased out as switching controlled
AC motors are replacing the very large (500-1000 HP) drives, there
and still many around in industry.
And every few months, (life time of brush) thousands go in
trash when length drops below spec. All these brushes have pigtails, some four per brush. A respectful enquiry could yield some for
regrinding to Variac size.

Also, who knows what will pop up out the electrographitic science
for new ideas?

quicksilver

Mon Feb 07 2011, 10:50PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Just for discussion, depending upon the carbon utilized you may be placing some resistance in your contact & the attachment mechanism may present another challenge.

What repair issues have you dealt with thus far? I had an opportunity to buy some variacs and of the two I finally did buy I noticed some "flashing" (minor arcing, etc) when the rotor was spun rapidly, however this didn't affect the overall performance in SGTC usage. I am concerned as to making sure I select the correct size fuse because many of these (really old ones) have little information. In so far as the contact issue the design of the variac "coil" creates another issue as the space between the conductive surface may also be less consistent than a well made motor.

Generally, it seems like a damn good idea. Let's say that you have an obviously well used variac and would want to replace the contact; how would you do so mechanically??

Frosty90

Wed Feb 09 2011, 12:24PM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

I replaced the brush in a small variac with a carbon from a small battery.....worked perfectly...

Proud Mary

Wed Feb 09 2011, 12:38PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I needed new brushes with a circular cross section for a small 1940s variable transformer. I was able to find some larger round brushes, and putting these in the chuck of a pillar drill, ground them down to size with a diamond nail file.

Bored Chemist

Wed Feb 09 2011, 08:30PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I bet that made a mess.

radiotech

Fri Feb 11 2011, 07:51AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The brushes that carry the most current I have seen are the
brushes from car starters. Also there are othe brush schemes.
I'll try and remember 'bronz gauze' brushes from large disk
(homopolar) Faraday machines that were used in very high current low voltage machines. Faraday disk machines. Elecrtroplating?

Another thing is a carbon pressure contact metal-to-carbon
cant weld closed like metal might do on AC or DC. Ancient elevator
contactors used carbon moveable contacts.

Colin 99

Mon Mar 21 2011, 06:26PM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

I replaced the variac brush in a 10 amp variac, 120 volt with a filed down motor brush.

It worked good!

Sulaiman

Mon Mar 21 2011, 11:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I assume that you've looked at the obvious

Where I work we have a motor repair department, there are still many types of brushed motors in use in British industry, with many types of brush.
I sincerely doubt that an amateur at home will add much to the knowledge base of an industry that has been around since Tesla's time.

James

Tue Mar 22 2011, 12:56AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Motor brushes are widely available in lots of sizes, and so far as I can tell, a carbon brush is a carbon brush. I filed down some brushes I got cheap on ebay to fit the motor in my treadmill and they worked just fine. I forget how much the OEM brushes were but they were obscenely expensive.

Steve Conner

Fri Apr 01 2011, 03:12PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I heard rumours that some motor brushes are made with anisotropic resistivity to reduce sparking. That's the only thing I know about brushes.

Anisotropic means the resistivity is made low in the direction of the wanted currents, and high in the direction of the unwanted "short between segments" currents. In graphite it can be different by up to three orders of magnitude.

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