At one location, I cover lots of ABB DC motor and I check the temperature at one location on each motor. The location is marked by a circle about 1" in diameter.

It just so happens that on one of the motors, the temperature inside the circle is about 194 deg F and just outside the circle it cools rapidly to about 130 deg F.

What could be the cause of such localized heating?

Can you post a pic? Would be nice to see IR image of the whole thing. It might be where a coil is bolted to the frame and it is acting as a good heat sink there(?).

Bill,

I don't have access to an IR camera. Just spot checking with a handheld IR thermometer.

It would be interesting to see, though.

Danny

I would be inclined to agree with Bill. Got a small ball peen hammer. You may be able to peck around and notice a difference in thud sound and separate the voids from the solid contact. But, then again; much depends on that. Size and can you make it without chipping paint. Do they keep white jackets w/long sleeves with decorcative ties at the end? But point of contact is 98% likely.

I found a couple of more pieces of information in my notes that I failed to present.

The motor was operating at 952 rpm with 1000 max.

There was visible arcing from the bottom brush which is worse at higher speeds.

There appears to be uneven laminations in the commutator, but that is only a visual observation through the window. I can only see the top and bottom brushes and the top does not appear to be arcing.

Alignment is suspect also, but I don't have any phase data. There is a significant leak in the input shaft seal of the gearbox.

Thanks again,

Some follow-up information:

This motor failed and was replaced. The only information I was given was that it "burned up".

I'm not sure of the alignment procedures used, but I have a higher axial running speed amplitude than before.

The really strange thing is that the hot spot is gone, but the bottom brush in the replacement motor is also arcing.

Any ideas?

Thanks,

A great troubleshooting flowchart for dc motor brush sparking in Reliance Publication H-7000 ("Maintenance and Troubleshooting of Electric Motors") page 26-28. Also the detailed flowchart is available here (scroll down to "Problem M" = "Brushes sparking excessively":

http://www.reliance.com/prodserv/motgen/h7000ch3.htm#Problem%20M


Note the flowchart is a little complex... 28 decision blocks. One that catches my eye is: "Is motor overloaded?" That provides a possible link to your previous symptom of overheated motor and apparently can also cause brush sparking. But of course there are many other possible causes: brush condition, tension, angle, commutator condition, neutral plane and much more.

This message has been edited. Last edited by: electricpete,

Pete,

Thanks for the link. I'll check it out later today when i have time.

Given that two different motors are experiencing arcing from the same brushes, I'm leaning towards overload also.

I'll let you know if I learn anything more.

Thanks again,

Danny

Pete,

Thanks again for the link to the REliance trouble shooting chart.

If I can get them to follow it they will probably find something around M5 to M20 that might fix their problem.

Danny

Dear Danny,

I worked at ABB Canada for a great number of years in the design of DC motors, large and small.

The main body of the motor ( we called it the frame ) is made out of steel.
It's usually rolled from a thick steel plate and seam welded if it is not an overly large frame; a steel casting would be used otherwise. That would be a very rare occurence though. The preference always being a rolled steel frame. We had some large ones rolled out of a 4 inch plate needless to specify that it was intended for a very large diameter frame.

The outer shell is always very thick so as to allow for unrestricted flux path, avoiding saturation at all cost.

If it were a cast frame, an undetected flaw in the casting would create a hot spot locally.
Cast frames are always magnafluxed, etc. to detect such flaws.

But, it is now a story of the past since it burned out, so you say in your last post. However, one brush is still arcing, so you've also wrote.
It is odd to have just the one brush arcing on a motor. It is either stuck, or too tight into the brush holder. The spring over it may be weak or insufficiently sized.
The sets of brushes could be unequally spaced radially around the commutator also... possibly.

Hope to have been of some help.

Marko Leo

I am assuming most of your ABB drive motors are on a system such as a paper machine drive-line. Often these type motors are supplied with external cooling ducts. If the air flow is hampered in any way this could aid in your localized heating. I’ve even heard of cases where to much air flow caused excessive commutator wearing. I never really understood the excessive airflow theory, but that is what the consulting experts came up with. Irregular commutator wear can also cause arching. I referred to it as brush chatter, but that’s not a technical term. One place I worked would alternate brush designs in an effort to smooth out the ruff spots. Apparently there are brush designs that have abrasive properties.

**** Vendor Warning *****

Good information presented on the hot spot issue for DC Motors but the thread seemed to take off on a discussion on brush arcing, cooling air, and commutator conditions. So, I thought that I would not sit on the sideline and be a guest and attempt to contribute. Note the key word, "attempt". Sorry ahead of time on the length of my post.

An arcing brush is most likely a result of a weak spring as Marko Leo advises. Brushes are generally changed on a regular basis as they wear but springs are usually neglected and, quite often, the original springs that are supplied on the motor or generator are never changed even after years and years of service. Whether a constant force spring (not really constant force - also called a coil spring) or a clock type spring, they both need to be inspected occasionally to check their force. Clock type springs can be adjusted but there is no adjustment to coil springs. Too little force introduces electrical wear on the brush and the commutator - caused from the brushes not keeping intimate contact with the commutator and, hence, arcing. Too much force will cause mechanical wear as the spring will "grind" the brush into the commutator or slipring - usually not the reason a comm or slipring has premature wear as the brush takes all of the abuse. Differential spring force from brush holder to brush holder will also cause uneven comm wear. The brush holders should be inspected occasionally as arcing inside the brush box can cause pitting of the sliding surface and cause a brush to stick in the holder. One other common problem is different brush grades in the same motor. This, too, will cause one brush to arc if that brush is of a different material. Brush grades even from the same manufacturer need to be the same grade or selectivity (brushes not wearing evenly) will become a problem.

The term chatter is commonly used in our corner of the world and happens when a brush holder is not oriented in the proper direction or at the correct angle. This causes a phenomenon known as friction chatter and is usually on motors with a "leading" or "stubbing" type of brush holder (the holder is angled into the direction of rotation). This causes the brush to almost resonate in the holder at a high frequency and will cause premature wear. You could possibly measure this with vibration analysis but the tell tale sign is premature brush wear and excessive polish on the sides (thickness and width) of the brushes. This phenomenom will happen on radial holders if the holder is angled as little as one degree in the leading direction.

Other causes of arcing are contaminated atmospheres which would be prevalent in a paper mill application. Overloading will also cause arcing which is not so prevelant in paper mill applications. Neither of these conditions will cause one brush to arc.

The issue of too much cooling air is usually not a problem for brushes unless the force is so great that the brush leads are actually being moved around and making contact with other parts of the motor. This can happen and is seen on occasion - if a little air movement is good for the motor than a lot must be better. The real problem with cooling comes when it is coming from an air conditioned source (some elevator equipment rooms) or the air just has low humidity content. Humidity or water vapor is one of the contributing factors of laying down brush film and the lack of humidity can severely damage a commutator and decrease brush life. Think aircraft applications, or Chilean mountain mining applications, or Texas in the winter. Lack of humidity has to be compensated for in a brush grade or there will always be problems. Again, in paper mill applications there is almost always lots of humidity in the air. Go to a paper mill in Georgia or Alabama in July and August and you will swear it is actually misting rain inside the mill.

As mentioned in other posts on this forum, I, too, am in awe of the talent and experience posted here. This is a very valuable forum and my hat is off to all of the people that contribute and make this a success and a great resource. I hope that the readers suffer me taking this in a direction not intended, and also are patient with the epistle but, hey, life is about passion right!

Dear Acklin,

I've noted that this is only your second post on this site.
I, personaly and other members of this forum, I am sure, see great value in your ' to the point ' comments.

I would appreciate reading more of your comments on this forum.

Marko Leo

Marco,

You are correct that the hot spot is a thing of the past. They replaced the motor with a brand new identical unit (yes they are ABB ) and the arcing is coming from the exact same location and apparently only from that one brush. I suspect that you are correct about the spring pressure being too low and have advised my client to check that as well as the other items in the troubleshooting procedure that Pete poseted. This problem is occurring in the bottom brush which is opushed upwards by the spring. It would seem that this location would require slightly more pressure than its direct opposite.

Waylon,

There were no airflow obsturctions. It is a cast frame and I suspect that there was a casting defect like Marco describes. REplacement of the motor did fix that portion of the problem. Fortunately, this is not in a sweaty old paper mill, but in an air-conditioned plastics extrusion process. Very nice place to work by comparison.

Acklin,

Thanks for the detailed explanation of the mechanics of dc motor brushes/commutators. It is very helpful to understand exactly what is happening.

Regards and thanks to all,

Danny

cudo's to you Acklin
Some of people like myself need all the help we can get from post's just like your's.Don't be shy about piping in on any post it is the only way to become truly involved.Your post shed a lot of light on a subject I know little of but it is information I require to do my job as best I can.Anyone can sit in the wings and watch and there is nothing wrong with that.I beleive the people who post here are open minded and willing to learn new things even if it is their specialty.No one knows it all but we all know something and by participating in these forums we share a lot of knowledge.AND THE MEMBERS JUST KEEP ON A COMIN !!!

Danny,

If the hot spot was located over the winding area of the motor, the hot spot was most likely caused by a shorted field coil. Would there be any chance of getting the motor repair shop information as to what and where the problem was in relation to the hot spot?

How long did the motor run after you found the hot spot and before it "burned up"?

Jim,

Yes the hot spot was over the winding. I was not able to get any info from the motor repair shop.

It ran no more than 2 weeks after I showed both conditions to the maintenance manager.

This message has been edited. Last edited by: Danny Harvey,

**** Vendor Warning ****

Because I am not the best proofreader and posted my comments quickly, I forgot to comment on abrasive brushes. These are called Surface Rounding Brushes and can be a stop gap tool to install in a motor to help round a rough commutator. I can send a flyer to anyone on email if they would like - just post your email address and I can send.

Also, thanks for the kind comments.

Thanks Acklin,

Here is my e-mail add!

xcm811799@hadeed.com.sa

Have a nice day!!