Seating brushes is one of the most critical procedures when correctly repairing a DC motor. If the brushes are not seated to 100%, the neutral cannot be set properly nor can the current carrying capacity of the brush be maintained.
Wrap the complete commutator with 120 grit garnet commutator paper using painter’s tape or masking tape to secure the paper. Do not let the tape come in contact with the brushes. The garnet paper should overlap the taped end far enough to cover the tape. Install the brushes in their respective boxes, and rotate the shaft slowly several times. Stop after a few rotations checking to see what the brush contact looks like. Continue with 120 grit garnet paper until the brushes are making approximately 95% contact to the commutator. This may take several tries. Change the garnet paper to a 200 or 220 grit and finish sanding the brushes to 100% contact. Vacuum and clean all carbon out of the motor. Set neutral at this time and after all static electrical checks are performed, the motor is ready for no load test.
Apply rated voltage to the fields, and up to 50 volts to the armature such that motor speed is approximately ¼ of rated. Make sure the rotation and current are correct and no visible signs of sparking are present before trying to seat the brushes. Extreme caution must be taken while using a brush seater and use all required personal protective equipment (PPE). Using the proper size and grade brush seater, carefully make contact with the commutator placing the brush seater just ahead of the leading edge of each brush. Move the brush seater slowly across the complete commutator with a couple of passes. Each brush must be seated. Make contact with the commutator just ahead of each brush in order to seat that brush. Remove power and check to see what percentage of coverage each brush has.