Repairing damaged commutator in big DC motor

I came by a beautiful DC motor, 10 HP and 1750 RPM, a nice collector’s item if you like. It weighs maybe about 300 lbs and has a heavy cast iron frame and a 1 5/8 inch shaft. It probably has about 5 or 10 years service on it and is greasy and dusty but otherwise in nice shape.

The problem is that the commutator is damaged. Under the brushes, it is depressed perhaps as much as 1/16", and visibly a little rough. The brushes are at four positions around the circumference at 90 degree angles (there are four poles in the stator). At each position there are two brushes in parallel. It looks like one of the brushes somewhere might have worn down through the carbon and brought some metal into contact with the commutator. There’s also a fine sparkly coating of copper dust in the region.

Can I repair this? Can I smooth it with successively finer sandpaper? Should I pull the rotor out and turn the commutator on a lathe? How deep can I go? Can I tell how deep I can go by looking at the end face of the commutator to see how thick the little copper segments are?

What I would like to do with the motor is clean it up and maybe play with it a little, do some experiments, that sort of thing. I don’t need to recondition it for heavy service and sell it with a guarantee.

By the way, it does not look as bad as the “grooving” example at
and I’d say my commutator is about 6" in diameter and 2 1/2" wide.

Yes is can be done with a little bit of undercutting.

Check to ensure you have enough depth of metal in the com contacts. If its too shallow then it can be relaid, but you’ll need to go to a commercial rewind service.

You need to be certain that the com isn’t falling apart, it could be that the contacts are coming up away from the rotor.

You will get a better idea of what is going on by removing the rotor.

As I reread your post, it seems to me that the brush contact area is is worn below the level of the rest of the com - if this is the case, then you should be fine to take some off with a lathe in fact is it essential that you do it. You will probably need to adjust the height of the brush holder to reduce vibration and arc erosion.

It is usual practice to take out the rotor periodically and undercut the insulating segments slightly and ensure that the build up of conductive material is controlled.

Given the weight and the rated power of the motor, it seems quite a lump, with a lot of spare capacity, after all a 10hp motor would tend to weigh a lot less than that, maybe one third - you sure about that rating?

A 1/16" is a lot to take off with sand paper. Some DC motors have a tool that is made for them to repair the commutator. If you can the tool it will mount on the motor and you spin the motor and use the tool to cut the commutator. Or the is the old hand way. Remove one set of brushes and hold a cutting tool braced against the motor at one point with the cutting end against the commutator. While the motor is running slowly move the cutting too across the commutator. The trick is make a level cut. After making the 1st cut in the first section take sand paper across the end of a stick the smoothen the cut out. Then move the brushes that were in and place in the lempty holders and repeat.

When you have the grove completely out then use sandpaper to level smoothen out the commentator. Remember to clean between the sections.
Or another choice is use sand paper with the motor running and smoothen out the commutator. Clean and cut in a new set of brushes. The grove will still be there but you want to eliminate the edges of the grove. The motor will run but it will need brush maintenance often. I did this on elevator MG sets when they would not give me the money to properly repair the units.

1750 RPM is a strange speed for a DC motor.
What voltage and how many amps is the motor. And how many leads?

When a DC motor or generator is overhauled, the rotor is pulled from the stator and the commutator is machined on a lathe. Typically, the total runout spec will be 3 mil, and the pole-to-pole spec will be 0.3 mil. So the lathe operator really has to know what he/she is doing.

As a matter of course, bearings and brushes are replaced at overhaul. Sandpaper is used to seat the brushes. The spring force of each brush spring should also be measured, and replaced if it’s outside the spec window.

The name plate says, among other things, DIRECT CURRENT, Frame 284U 1040D, HP 10, Volts 240A/150F, Amps 34/1.4. Note that these numbers are reasonable for the armature / and the field. At the bottom of the name plate is MFD BY BALDOR ELECTRIC (which I like, as Baldor seems to make much prettier motors than most others).

The physical appearance is similar to the one here, the finned larger one in the background:

except that there is a further short finned section where the covers over the brushes are.

I realize the linked motor is AC, I’m just describing the appearance. It’s really stocky.

There at least used to be shops which did nothing except repair motors - including re-wiring the coils.

I’d look around for such a shop and at least get an estimate - mfg name and frame is a start - if there are any other markings on the data plate, they may specify what exactly is inside.
Voltage and HP may be enough to give them an idea, but if they ask you to bring it in for an estimate (and charge for the estimate, which would not be unreasonable) ask simply for a “ballpark” number.

I sell Baldor motors for a living and I have a couple of cents to throw in.

Frame 284U

For general industrial use, “T” frame motors replaced “U” frame motors something like 30 years ago. So there is a very good chance this a very old motor and the insulation may have broken down. It is likely to have issues you can’t see.

Volts 240A/150F, Amps 34/1.4.

This is a field wound motor which means you must supply current to the stator windings as well. You’re going to be much better off and safer using a proper DC Motor Control than trying to bench rig some sort of DC Power Supply. 34 Amps at 240 Volts is nothing to sneeze at.

If you decide to goof around with it on your own, don’t skimp on safety. Plan for and expect the worst.

I don’t need to recondition it for heavy service and sell it with a guarantee.

No, but you don’t want to watch it tear-itself-apart/start-a-fire/explode-your-wiring either. Personally, I would take to a rewind shop and at least have them check it out professionally. They’ll be able to determine if it’s just the commutator or if it needs to be rewound.

I have to say again one last time… be careful. 10 Horsepower is serious shit. Think ten Budweiser Clydesdales pulling together as hard as they can. They could do serious damage.

Wow. Nice answer, Ornery Bob.

Thirty years? Jeez, I didn’t know that. How is a rewind shop going to determine if it needs rewinding? Use a megger to look for insulation breakdown to the frame? Is there some test for windings shorting to each other? I do have a high resistance tester, teraohms I think, but it’s only at 50 volts, it’s not intended to test for breakdown.

I was planning to power the field at its rated voltage and then apply power to the armature at low voltage and ramp the voltage up, and turn the shaft to get it started, and eventually perhaps get it all the way up. I have a “Powerstat” variable autotransformer good for 9 A at 270 V, and with no shaft load I figured the motor would spin with that. I also have a couple sources that are not variable, so it’s all in or nothing, rated for 40 A each at 240 V. I’m kind of nervous about just throwing a switch on that, though.

Am I fooling myself? Is this thing just begging for trouble?

It’s such a beauty, I just kind of hate to think of hauling it off to the metal recycler. Is there anything… I dunno… more enlightened to do with such a motor? Or with the 15 HP three phase motor sitting next to it that I never got around to making a phase converter out of?

It will run fine with your variac and a full wave rectifier, as long as you have the field power applied, and no load. Just start with the variac all the way down, and ramp it up slowly. Starting with anything but 0 vdc on the armature will most likely result in a tripped breaker or blown fuse from trying to accelerate the rotor instantaneously.

If you wanted, you could get a small speed controller from somewhere like KB controls or Carotron, which would give electronic, reversible speed control, with accel/decel ramps.

Even a 1/2 or 1HP drive will run that motor, just at a light load.

More like 20 :slight_smile: