DC generator brush problem.

[d431hood]

I have a DC generator which keeps defying our maintenace team.

In short: some of the brushes keep getting stuck in the holders. They will travel towards the generator but we are not able to pull them out without removing the brush box. This makes wear checking a big exercise.

However the last time it occured I got involved and asked the maintenance team to remove the brush boxes and remove the brushes through the comm end of the box.

Once removed it was obvious why they were jammed in. The brushes appeared to have grown along the line of the commutator in the dirrection of rotation. The growth was a few mm in width, and 2-3mm high (the box-comm gap - inside recommended tollerances for the generator) and even accross the whole width of the brush.

I can't find this particular fault in any of the usual fault finding guides, so any help to find out what is wrong woudl be appreciated.

Thank you in anticipation of your assistance.

 

[dArsonval]

Brushes could be melted or deformed at one end due to other issues taking place during commutation,
“but”… that would mean other factors would be going on related to the armature, interpoles, etc.
It doesn’t read like you have an issue with generator output… yet maybe you do.

A carbon brush is not going to “grow” per se.

Some observations to consider:

Brushes can chatter-n-wear within the brush box in a way that obstructs them from being removed.

Do you know from previous brush changing, that the brushes used to replace the worn out ones were indeed the correct size and grade for the generator?

Is there any reason to suspect over-sized brushes were filed, sanded, “re-sized” during a brush change?

Are there burrs, melted portions of the brush box causing free movement of the brushes?

As the brush boxes are re-installed, are they being properly re-spaced (positioned) from the commutator?

Brush boxes spaced too far away from the commutator will contribute to chatter, chipping, breakage and accelerated wear.

It’s doubtful… yet worth asking… Is the brush rack unknowingly loose?

There… ask one question, and get five in return ; )

Staying tuned,

John

 

[Skogsgurra]

I recently had a similar case. But not on a DC machine. This was an earthing brush that was supposed to keep voltage across the DE bearing low to avoid EDM in the bearing.

A pocket had been included in the end shield moulding and the brush and holder were fitted in that pocket. Grease from the bearing all over the place and also carbon dust. Grease and dust had got into the brush holder and baked together on the brush, which grew until it got stuck in the box. When that happened, the brush didn't do its job any more and bearing EDM started so that the motor bearing was down after a little more than a year.

You didn't mention grease in this case. But if there is oil mist in the ventilation air, it can do the same thing to your brushes. Carbon dust is always present.

For this to happen in a machine that has been run sucessfully for many years (I assume that you didn't have this problem from the beginning - or did you?) there should have been a change in ventilation. It could be an air filter that hasn't been properly replaced or it may be that the ventilation system has been changed. If there is a separate ventilator, it could even be that the air direction has been inadvertently changed.

Or it could be something completely different.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[d431hood]

Gents,

Thank you.

We have recently had a few problems with this machine, but it has run without issues for years before that.

There always has been and will be oil in the atmosphere, we do have filters in the cooling air flow but they are not able to perfectly remove all the oil vapour ingress. But this hasn't changed.

No resizing of the brushes took place on fitting from new. I did the last replacement!

The grade is correct for the generator, based on the part reference stamped on them..... And they were bought from the OEM.

The brush box and mounts are not loose. There is no evidence of chatter, no chipping on these brushes (but there was on others in the machine). There was no play in the brush boxes and the replacement parts slotted in just fine, so no burrs or carbon build up.

What sort of gap between comm and brush box is acceptable? Is it possible that too big a gap might allow them to grow sideways?

It is a very even growth...... Very uniform in both colour and density. uniformly accross the width of the brush.

When these brushes were fitted there was minimal sparking (at the brush-comm interface) and no obvious control/regulation issues.

In the lead up to the inspection when these were found, some regulation issues had been reported but nothing long term, and nothing was present when the techs went to measure the output on the plant.

Anymore further guidance and suggestions would be appreciated.....

 

[dArsonval]

Hmmm? Clarity with the term “growth” will be helpful. Not sure what that implies related to a carbon brush.

As for “What sort of gap between comm and brush box is acceptable?...”

The measurement obviously varies with the size of the machine.
Dimensions of .125 inches or approx. 3 mm is considered the maximum height recommended from a commutator, or slip ring.

I don’t know if this URL has ever been posted here or not. It may be useful.

http://www.mersen.com/en/products/e...s/carbon-brushes/technical-documentation.html

Always enjoying the forum,

John

 

[waross]

Has any RTV silicon sealant been used in the vicinity of the machine lately? The fumes given off by vulcanizing RTV sealants are deadly to brush gear.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mshuaibh]

Dear John,
As per my experience in our DC exciters brushes can be deformed/chatter due to following reasons,
• If the Brush length got short
In this case the tension of spring will reduce and it will give more room to brush chatter and may move along with its it rotation direction.
So you have to shorten the regular inspection frequency to avoid Brush getting short.
• If the gap between Commutator and the Brush holder increased. It shall not more than 3mm and check the manufacture recommendation as well.
• I hope you didn’t change the Bruch Grade. If you changed sticked to the original grade.
Hope above info will be helpful.
Best Regards
Shuaib.

 

[d431hood]

Gentlemen,

Thank you for your time.

The gap between the brush box and the commutator is 2.7mm (within specification). No RTV has been used anywhere near the unit as far as I can make out (its not part of our standard repair kits or usual procedures either).

Brush Grade is as specified by the OEM for this machine (and the brushes were purchased directly from them).

Growth: The original bust was a uniform 10.7mm wide, the deformed brush is 10.7mm wide down to the end of the brush box, but then has grown to 12.8mm wide for the gap between the brush box and the comm.

I hope this helps.

Cheers

Joe

 

[waross]

Possibly the brush composition has changed. Try a different grade or brushes from another manufacturer.

Bill
--------------------
"Why not the best?"
Jimmy Carter