A case of uneven brush wear
The brushes on a 4-pole, 700 hp DC motor were not wearing at the same rate. In this case, rapid brush wear occurred on two adjacent brush rows - one positive and one negative polarity. The other brushes had minimal wear. Electrical tests found no winding faults, and the air supply was clean. Most of us suspect low current-density when rapid brush wear occurs. A lightly loaded DC motor can "dust" a set of brushes in short order. Changing the brush grade (or removing some of the brushes) will usually solve the problem.