Author Topic: Generator slip ring cleaning/carbon brush maintenance, a how to and why.  (Read 7750 times)

Offline machinisttx

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I'm not an expert and don't play one on TV. I do tend to like reading service manuals and learning how to not have to fix something with simple maintenance....especially if it's easy and the repair might be quite expensive. This was my project today. I've posted in a thread or two about cleaning generator slip rings, but I don't think I've ever posted any info on how, so here we go.

On some generators, voltage regulation is accomplished by an expensive electronic control board as is the case with my Onan 6.5NHE. If the slip rings are oxidized from not being used for a while, resistance tends to spike up and the magic smoke is let out of the VR board. If the oxidation is really extreme, the carbon brushes either don't make contact or make intermittent contact and the result is intermittent/no power production at all. Even if the slip rings aren't oxidized, the carbon brushes can stick in the brush holder(s), which again results in intermittent or no power production, which is equally bad for expensive electronic controls(if present). On most generators, these parts are relatively easy to access and service. If you exercise your set once a month or so, you will probably never have a need to clean the slip rings. If the engine lasts long enough(probably not going to happen with most of the cheap 3600rpm sets), you will eventually have to replace the brushes though.  :)

This generator was originally installed in a motorhome. I purchased it from a scrapyard, so I don't have any information at all regarding it's past other than what I can deduce from looking it over. Since I don't know the history, I'm checking everything over and repairing anything that needs it before trying to start it. Someone has added two 120v RV outlets, as none were originally present with the unit hardwired into the original RV. Slip ring cleaning is not as critical on this genset as on the 6.5NHE I mentioned earlier because it lacks those expensive electronic controls, but it's still good policy to clean them. The gray scotchbrite pads you see on top are what I used. Don't use sandpaper or emery cloth as both will break down and the abrasive will embed into the soft copper slip rings and/or the brushes. Red scotchbrite pads would work a little better, but this is what I had. You can also purchase and use a commutator stone.


I didn't take pictures of the disassembly to access the brushes and slip rings on this model. It's very likely that your set is different and it wouldn't be applicable anyway. Most won't be quite as easy as this one and probably won't allow as much room to work either.

Here are the slip rings, and at the far left end of the rings is the commutator, which isn't going to be present on most sets. I'll talk a bit more about it later. I've cleaned the two slip rings on the right. These were surprisingly clean to start with, so it's harder to see the difference. I've seen far worse.


And here I've cleaned the other two, and the commutator.


In the pictures above, two of the brush holders and brushes can be seen at the top and bottom of the slip rings. There is another on the opposite side, and one was removed to access the slip rings and commutator. All of the brushes need to be checked to ensure that they move freely in the brush holder.  This is done by gently pulling the braided wire attached to the back of the brush. This set has 12 brushes in all, and some of them were very lightly stuck. Only eight of those brushes are for transmitting 120/240 volts though.  ;) All of the connections to the brush holder need to be checked to ensure they aren't loose, broken, or heat damaged. Here is a picture with the braided wire circled in red and some of the connections on this brush holder.


I spent about an hour in disassembly, cleaning, and reassembly. I could have done a few simple tests to check for other problems, but since I did not have a service manual with me, and I saw nothing that led me to believe they were necessary, I did not. That hour was well spent in my opinion as I know for sure that things are clean and all the connections are good. Any future troubleshooting will be easier with this already done.

And on to the commutator I mentioned earlier. The engine on this set does not have a conventional starter, nor does it have a manual pull rope. The "starter" is actually the generator end. Power(12v) is sent through the commutator, inducing a magnetic field, which then spins the rotor, which is attached to the engine crankshaft and rotates the engine. Once the set is running, the start switch is released and power no longer flows to the commutator.

Offline keebler

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Re: Generator slip ring cleaning/carbon brush maintenance, a how to and why.
« Reply #1 on: September 14, 2015, 04:03:42 AM »
I have the same exact generator in my old Broke Down 84..Pace Arrow Motorhome--- I ran it last year & it still had out put this year. It's Locked up.
Keebler.

Offline machinisttx

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Re: Generator slip ring cleaning/carbon brush maintenance, a how to and why.
« Reply #2 on: September 14, 2015, 07:41:06 PM »
Locked up? Do you mean the engine siezed? If you need the service manuals or parts manuals, I have them. Need to know the full model number to get you the right ones though, as there were many variations. Downside to the Onan's is that parts can be really expensive....upside is that they're generally pretty reliable. My NHE set was made in '96 I think. Haven't decoded the serial numbers on the others yet.

Offline keebler

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Re: Generator slip ring cleaning/carbon brush maintenance, a how to and why.
« Reply #3 on: September 15, 2015, 01:35:18 PM »
thanks _I have the Manual. moisture has locked the engine up,  some day I  will un bolt it &  "Rack it out (remove the exhaust,) and get it to turn over. old MH 454 threw a rod.locked it up-
but I had used the genny a few times- when the power was out. just got lazy & didn't keep it free.  it was a great generator.
Keebler.

Offline machinisttx

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Re: Generator slip ring cleaning/carbon brush maintenance, a how to and why.
« Reply #4 on: September 15, 2015, 07:01:04 PM »
Pour some marvel mystery oil in the spark plug holes and leave it sitting for a while.  :)

Saw the mains of a marine 454 that a "repair" shop ran without oil in it. I have no idea how that engine was still running. The main caps were blued from heat, as were the bearing seats in the block.  :o I don't think that shop is in business anymore.

Offline keebler

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Re: Generator slip ring cleaning/carbon brush maintenance, a how to and why.
« Reply #5 on: September 16, 2015, 03:48:45 AM »
Thanks---I have some of that- will do.
keeb