Posted by Sherman in Idaho [24.32.202.166] on Sunday, March 17, 2013 at 15:09:15 :
In Reply to: Re: Maybe it depends posted by gmharris [71.105.35.169] on Sunday, March 17, 2013 at 13:13:38 :
All good points. Carborundum is actually a semi-conductor, so I could see how it could cause problems in high-voltage machinery as well. I still hope to get in to see the Caladay hoist one of these days, where a former mine electrician told me that the brushes were bigger than bricks and it took a 10' ladder to climb up and change them.
I'm not even sure that an ohmmeter jumping around while turning the shaft definitely indicates a problem. An ohmmeter applies a very low voltage, and the graphite/copper interface actually has some strange semiconductor-like behavior at low voltages. Brushes are not a simple resistance, but are more like a constant voltage drop independent of current (to some extent). It takes a volt or so (which is all that ohm-meters provide) to get any current at all to go through the brushes.
To your last line, I would add, on our old trucks, if your generator is NOT working, it is probably something other than the commutator that's bad. Brushes are stuck or worn out, windings are open or shorted.