Roughness of commutator

[WTRVS]

- Is there a rule of thumb for the ideal roughness of a commutator surface for DC-machines? I've found an explanation that was really helpful and they indicate ideal values (Ra 0,8 - 1,2 and Rz 5-8µm)

My question: Isn't this value depending by the size of the commutator? Is the ideal roughness of a 50kW machine the same as for a 500kW machine? And what about the roundness of the commutator, what are the limits for this?

- In the document there are a few tools shown to grind the commutator but the use of it is not recommended (especially rubber stones) because it leaves particles on the commutator. In case a motor has stalled and there are burn marks on the commutator, what is the best way to clear the burn marks?

Document link:

http://www.schunk-group.com/sixcms/media.php/1701/surface-roughness.pdf

Thanks!
WVS

 

[WTRVS]

Found already some more info on the out-of-roundness requirements:

http://www.ingenieurparadies.de/sixcms/media.php/1466/out-of-roundness.pdf

 

[petronila]

Hello WTRVS

The attached information could help you

Regards

Carlos

 

[WTRVS]

Thank you, Petronila

So what I understand out of this document is that the out-of-roundness tolerance is depening on the peripheral speed of the commutator (wich makes sense).

What I don't understand from this document is that the commutator surface should be as smooth as possible. Because in the Schunk information is described that a commutator with a certain roughness reduces friction because it helps to form a layer on the commutator and prevents the brushes from vibrating.

Do you also have any tips on how to remove burn marks without hurting the commutator too much?


Thanks for your help,

Wouter

 

[panter]

WTNC ,
Very interesting story because, I think , Schunk has different opinion from another manufacturer of DC motor and brushes about condition and maintenance tools for commutator.
I have attached interesting idea with Surface Rounding Brushes from National but I don’t that somebody has experience with that tool.
Good hunting

 

[petronila]

Hello WTRVS,

If you want only clean the surface( in case you don´t have signals of poor commutation or some spring pressure problem) you can use special stones to clean the surface. Example : if the motor is dismantled, put the armature on lathe and turn, use a sand paper(400) in order to clean the commutator surface.

Later when the armature was mounted, the motor working at least 350 RPM use an special stone to help to set the brushes, the stone is applyed directly on the commutator and in the same rotation direction.

Regards

Carlos