Just about every conceivable issue related to a D.C. motor has been addressed or written about through its entire history of existence.
Today, there are numerous commutator photo charts all over the Internet that provide a visual comparison of brush/commutator problems
and offer remedies in solving brush sparking etc.
My first thought upon seeing your photograph is that the commutator is not round.
It’s sensible to believe the commutator would have been re-machined after it was rewound, but in looking at the picture,
it has an appearance of … [it was merely sanded, and called good enough].
High mica between the bars can cause a brush to hop creating sparking, but from the picture, it looks like
the mica is still below the bar surface which would rule that possibility out.
“Brush Entry”, a term related to how the motor’s winding is connected to the brush holder is a common mistake
made by repair people because of not marking their respective connection locations before dismantling.
In the situation of the brush holder being incorrectly reconnected, the motor would run fine under no load,
but then exhibit sparking when placed in service. I don’t believe that’s your problem because in that instance,
the burning would show up completely around the commutator.
My guess… is the commutator is not round.
This 1960 Re-Print shown at the Helwig Carbon site can give you additional things to consider.
