Maintenance of good brush performance

BRUSHES used on electric equipment have three distinct functions to perform. First, they must carry the load current into and away from the rotating element of the machine. Second, they must function as bearings on the surface of the commutator or slip ring, preferably without any applied lubrication, and at surface speeds sometimes in excess of 100 miles per hour. Third, on commutating machines, the brushes must control within safe limits the short-circuit current resulting from uncompensated voltages in the armature coils undergoing commutation and must resist the destructive action of the very high local current densities which are frequently characteristic of the commutation cycle.