# Dark commutator on DC motor **URL:** \ **Category:** Factual Questions\ **Created:** [November 6, 2009, 3:28pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577 "2009-11-06T15:28:20Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [November 6, 2009, 3:28pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/1 "2009-11-06T15:28:20Z")
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. ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [November 6, 2009, 3:35pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/2 "2009-11-06T15:35:58Z")
“If it aint broke don’t fix it.” “Ours not to wonder why…” ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [November 6, 2009, 4:19pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/3 "2009-11-06T16:19:32Z")
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? ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [November 6, 2009, 4:35pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/4 "2009-11-06T16:35:38Z")
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? ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [November 6, 2009, 4:44pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/5 "2009-11-06T16:44:03Z")
Aha… It’s “commutator film” > [@http://www.reliance.com/prodserv/motgen/c7090/](#): > > When electric current is passed between the carbon and copper in the presence of water vapor, a microscopic layer of copper carbon composite or film, is formed. Good film is chocolate brown or burnished bronze to dark brown or black and uniform in color. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [November 6, 2009, 4:48pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/6 "2009-11-06T16:48:47Z")
> [@engineer\_comp\_geek](#): > > 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? 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: > [@](#): > > When electric current is passed between the carbon and copper in the presence of water vapor, a microscopic layer of copper carbon composite or film, is formed. Good film is chocolate brown or burnished bronze to dark brown or black and uniform in color. It is not bright copper or burnt black copper. Consult a commutator color and appearance picture chart to determine the condition of the commutator. There is a condition known as false filming in which brush graphite deposits become cooked on the commutator resulting in an appearance similar to dark film. Oil can also leave a coating which resembles film. If this film can be easily wiped away, it’s not the desired good commutator film! > > Commutator filming is a continuous process. That is, the film is continuously being formed and stripped away. A good film is only 200 nano inch thick (0.000,000,2 inch or 0.000,005,08 mm). Thus the conditions required to build good film must always be present. Changes in current, humidity, etc. will affect the commutator film. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [November 6, 2009, 4:49pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/7 "2009-11-06T16:49:51Z")
> [@gotpasswords](#): > > Aha… > > It’s “commutator film” Ha, you beat me to it. 😛 ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [November 6, 2009, 5:05pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/8 "2009-11-06T17:05:16Z")
For anyone else out there who cares about this, it’s also called [skin or patina: this link is to a PDF!](http://www.engineeringcarbonproducts.com/reports/Assessing%20the%20Performance%20of%20Carbon%20Brushes.pdf) That link contains pictures illustrating various conditions and how they affect the patina formation on the commutator. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [November 6, 2009, 6:31pm UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/9 "2009-11-06T18:31:03Z")
> [@HongKongFooey](#): > > For anyone else out there who cares about this, I care. Thanks for posting it. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [November 7, 2009, 2:49am UTC](https://boards.straightdope.com/t/dark-commutator-on-dc-motor/516577/10 "2009-11-07T02:49:36Z")
> [@HongKongFooey](#): > > For anyone else out there who cares about this, it’s also called [skin or patina: this link is to a PDF!](http://www.engineeringcarbonproducts.com/reports/Assessing%20the%20Performance%20of%20Carbon%20Brushes.pdf) That link contains pictures illustrating various conditions and how they affect the patina formation on the commutator. 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.