Bridging on the commutator DC-Motor precious metal brushes

Thread Starter

Maurizio Gambacorta

Joined Oct 20, 2019
4
Hello all.

I would like to thank everyone who can help me with a problem I'm facing at the job.
I received from a customer a small 12V DC- Motor as described in the subject that ran a couple of hours only but with a strange commutation signal and a low resistance at the contacts.
When I took off the rear cover with the brushes, I fond some metallic particles between the air gap among all the commutator's bars. This material seems came from the commutator and/or brushes.

The strange things are listed here:
1) Each brush has three fingers that touch the commutator (of course on different positions of its length)
2) The fingers have the wear dramatically different: the one on the top (the farther from the coil) is worn-out by 30%, the middle one is worn-out by 5% and the last one has 0% of worn out. Be aware that the same situation can be found on both brushes.
3) The particles accumulated, which generate shortcuts between the bars, are positioned only at the same level where the finger more worn-out rubs against the commutator.

So, I'm starting with the Root Cause Analysis, and I want definitely look for the causes and taking the corrective actions.
I though some possible areas where the investigations should be focused:
A) Commutator error (shape)
B) Different pressure of the fingers on the commutator
C) Possible mounting error
D) Layer (gold) of the commutator
E) Possible arching?

I hope someone can help me, every advice is really appreciated.

Regards
 

MaxHeadRoom

Joined Jul 18, 2013
19,958
Modern motors have a wearable substance between the comm bars, older motors had Mica, which had to be undercut occasionally when the comm would wear.
Max.
 

Thread Starter

Maurizio Gambacorta

Joined Oct 20, 2019
4
Modern motors have a wearable substance between the comm bars, older motors had Mica, which had to be undercut occasionally when the comm would wear.
Max.
Consider that the commutator is diameter around 3mm., no presence of Mica. All commutation area is covered by grease in order to keep the local temperature as low as possible (because of sparking, I guess) and lubricate the rubbing between brushes and commutator.
 

KeithWalker

Joined Jul 10, 2017
648
Consider that the commutator is diameter around 3mm., no presence of Mica. All commutation area is covered by grease in order to keep the local temperature as low as possible (because of sparking, I guess) and lubricate the rubbing between brushes and commutator.
The brushes are obviously mis-aligned.
 

Thread Starter

Maurizio Gambacorta

Joined Oct 20, 2019
4
Hello all.

Yes, the motor is 12mm diameter and 24mm long. The commutator is around 3mm.
There are no damages on the brushes as well as on the commutator, but you can have a look at the pictures attached the status.
You can clearly see the one and only finger worn-out and the bridging on the commutator.
During our Root Cause Analysis, we measured the correct alignment of the brushes, as well as the commutator cylindricity.

The bad news is that the same customer is shipping back another motor with the same symptom, my fear is that the issue can enlarge it... but this is another aspect that has to be taken into the account.

Every advice is really appreciated.

Kind regards

MAURIZIO
 

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MaxHeadRoom

Joined Jul 18, 2013
19,958
The damaged to the commutator could be the brush material or excessive brush pressure, what is the composition of the brushes, they appear to be low in carbon.
Have other versions of these motors been used extensively so far without any problem? Or a new application?
Max.
 

Alec_t

Joined Sep 17, 2013
10,893
The commutator doesn't look particularly smooth. It has what appear to be machining ridges. Any slight surface roughness will cause accelerated wear of the contacting brush.
 

KeithWalker

Joined Jul 10, 2017
648
As I stated above, It would appear that for some reason the brushes are miss-aligned, causing the brushes to bear heavily on the top end of the commutator. How are the brushes mounted? That may indicate the cause of the problem.
 
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