What are the two reasons for arcing at the commutator?
A shorted coil or shorted coils in the rotor will cause arcing
at the commutator. It should also be noted that if the brushes are
damaged badly enough where they make contact with the commutator,
this can cause arcing. Oh, and let's not forget that a stuck brush
or brush spring will prevent a brush from making proper contact
with the commutator. Arcing can result from that, too.