DC motor brush failure

[Mercdog]

Hello,
I have a small DC motor that is burning up the Negative brush. See photo.
Anyone seen this before?
Any ideas what could cause this?
The motor is used by me in many linear stages, but only fails in one. The one that fails is unique only in that is has a faster lead screw. It operates at about 1/10 of it's rated peak current.

Thanks.

 

[waross]

From the looks of the other brush it may be time to change armatures. When you do that, check the connections to be sure that the wires and hardware are clean.
What is the supply? Battery, Rectified, well filtered AC. Rectified poorly filtered AC, PWM drive?

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

 

[desertfox]

Hi

What is the lead screw actually doing? is it lifting something?

 

[Mercdog]

They are PWM driven.
Yes, in this case the lead screw is lifting a mass.

 

[desertfox]

Hi
Well if you provide details of what its lifting and rotation speed + pitch of thread it might provide some clues to the failure, also if you provide duty cycles of the other loading conditions which don't cause it to fail that also might help, finally what is PWM driven.

 

[waross]

How are the bearings?
Does the lead screw present unrestrained end thrust?
How does the commutator look? The supposedly good brush doesn't look very good.
Are you able to put an oscilloscope on the output of the drive?

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

 

[BrianE22]

Could the axial position of the armature be changing? If you're running with a constant axial load then you might wear a path in the commutator. Then, under a different axial load, the armature (and commutator) moves and then the brushes ride up on the shoulder of the worn path (you'd see quite a bit of sparking when running).

 

[Clyde38]

You may be driving this motor with too light of a load. If the current density in the brush is not where it is designed to be then the proper film cannot be formed. In this case friction is higher. It is normal for the brush with a negative polarity to wear less than one with a positive polarity (flow of electrons). See attached papers.

 

[Mercdog]

Hey Clyde38,
Thanks for this paper. I believe it a brush type problem, where the film is too thin. This paper explains the phenomena very well, and gives me some things to try.
Thanks again!