Does anyone know why the commutator on a DC motor discolours the way it does? I was inspecting a 20 HP motor and the comm is quite dark, almost black. It’s not drawing extra current and it’s not running hot so I’m not too concerned about it right now but I’m curious about what causes it. It’s only tuned dark in the brush path so it seems like it’s caused by the brush. There are two identical motors on the same line that have comms that are still the original copper colour. I wasn’t able to check the spring pressure on the brush but the wear on all three machines is similar. It’s possible the brush manufacturer used a different composition from one brush to the others maybe, I can’t tell just by looking at them.
“If it aint broke don’t fix it.”
“Ours not to wonder why…”
Is it dust (like carbon dust buildup) or is the metal discolored? If it’s the latter, copper oxide is dark in color. You could be getting a bit of a galvanic reaction on the commutator. What type of metal is the brush made out of?
No, it is ours to wonder why, and I have wondered the same. The motor on a 60+ year old blender that I was cleaning up had a slightly darker commutator, but the motor on my 4 month old 18v drill is all but black.
My only guess is the unidirectional DC is “plating” carbon from one of the brushes onto the copper?
Aha…
It’s “commutator film”
The metal is discoloured. The box the brush comes in just lists it as a carbon brush, no more information than that. I asked Siemens motor tech and he says this discolouration is known as known as film. I found this from Reliance’s site:
Ha, you beat me to it. 
For anyone else out there who cares about this, it’s also called skin or patina: this link is to a PDF! That link contains pictures illustrating various conditions and how they affect the patina formation on the commutator.
I care. Thanks for posting it.
Wow I wish I had acess to this when I was working on elevators. And before that when I was working on a T2 tanker.