Motorcycle Alternator Motorcycle Alternator Rotor Keeps Getting Build-up?

Motorcycle alternator rotor keeps getting build-up? - motorcycle alternator

I have a Yamaha FZ600 1986, that the contact plate in the rotor of the generator keeps the accumulation of carbon brushes. I cleaned the plate contact with alcohol, then # 0000 steel wool, and replaced the brushes on before 1000 miles. Now it needs to be cleaned again. What causes this and how can I solve this problem?