brushed DC servo static motor test- is this a problem?

Preparing for a retrofit that has DC brushed servo axis drives. Resistance for motor chassis ground to motor power leads for one motor is 4.0 megohms, 0.63 M for the second and infinite for 3rd axis. All axes were tested for function and working fine prior to the test, but I'm pretty sure these readings should all be infinite. Is it possible there is some carbon dust inside from work brushes causing this? is it an issue that needs to be dealt with or can I ignore it? It is a used machine with some mileage on it but in good shape...

It will cause nuisance tripping of RCDs and GFCIs if present, and a shock hazard if the machine earth is faulty.

I have seen it deteriorate to the point of blowing up speed controllers. Not sure if servo drives are any more robust...

Give the motors a good internal clean.

I've never taken apart a servo motor; one of the motors with the issue also has a brake. I'm not too keen on taking the motor apart as I've heard rumors of that being a one way ticket to failure but I don't know if that's true or based on someone who is not very careful . I do not want to screw anything up, would the best course of action be to remove the brushes and try to vacuum out any carbon dust then use a blowgun with all the brushes out, or what? These will be used with expensive 3 axis integrated servo drive/controller that I need to protect over all...

I'm not sure sorry. These were just DC gearmotors driving conveyers, driven by cheap KBIC driver boards. They equipment was on its way out anyway and is being replaced.

The motors probably have permanent magnet fields. If you remove the armature, breaking the magnetic path, the fields will lose strength. How much depends on the magnet material. With the best it will be hardly noticeable but the old hardened steel magnets would lose so much that they had to be remagnetized. Don't take the chance. You can probably take an end bell off and blow dust out. The resistances you give will not cause much leakage.

Bill

If you just have to get the rotor out, you can machine a steel "follower" or "keeper" that fits over the shaft and goes in as the rotor comes out.

The magnet strength issue is there, but if you look at the rotor, there is a lot of rotor that "is not there", to make room for wires, etc. Plus there is an air gap between the rotor and magnet.

So the loss is not nearly what might happen in other situations even if the rotor is just pulled out. Using the follower/keeper should minimize any losses. It can be made of pipe, since the magnetic flux that gets through the airgap is not nearly what would be in a solid structure.

On tachometer generators I repaired, we just took them apart and remagnetized during reassembly. I don't know for sure what the magnet material is, but going by their vintage, probably Anico, which is bad about losing strength. The loss from breaking the path was major but we didn't worry about it because we could bring it back up. We made a slug with grooves for a couple of turns of 6 ga wire and zapped it with a large pulse from capacitors, which would knock it up to saturation. As you say, we pushed the slug out with the armature.

There seems to be a minimum magnetic flux to keep the iron molecules in line and anything above that will work. I have noticed that the keeper doesn't have to be as large cross section as the magnet and doesn't need to cover the entire pole face.

Bill

Looks like it was carbon dust causing the problem, I took off the brush caps and they were pretty loaded. I vacuumed all 4 without even removing the brushes and the chassis to power lead resistance went from 630k to 3M ohms, which is fine I think. Different motor has a different issue though which I'll address in a different thread.

FWIW I vacuumed a motor the same way that had a 4M Ohm resistance from ground to power leads and the resistance went from 4M to 16M ohms, so it seems like a helpful thing to do. The brush cap o rings had degraded and cracked due to the ozone in there as well so I replaced them. This might be worth checking on occasion with motors exposed to coolant, which was not the case with my machine.I used an air powered venturi vacuum and pulsed it to get all the carbon, I didn't even remove the brushes just the caps