Commutator - Every 4th segment blackened 1

[edison123]

What could be the causes of this problem of every 4th commutator segment getting blackened ? The motor is 1500 HP DC, 1000 RPM base speed with 63 slots and 189 segments.

* I would go green if only I were not yellow *

 

[oftenlost]

http://www.plantengineering.com/article/CA144818.html

Take a look at this site. It appears to me to be slot bar marking related to the number of coils per slot. From the picture, it appears to be a darkening of every third bar which makes sense as you have three coils in each armature slot.

 

[edison123]

Thanks oftenlost. A great tip.

* I would go green if only I were not yellow *