also the armature 3 segments can have groves worn into the copper this happens if the brushes wear down to much and either by the brush cocking or the springs contact the commutator ! always change the springs and brushes when repairing the motor and polish commutator with fine polishing cloth or very fine emery paper/ copper should look like a mirror after being polished !
actually also the field winding or armature winding cook be cooked from overheating look at the actual winding for dark color the winding should be bright color not dark!
Alan
