Hi PF.. I am an industrial electrician (w/ a electrical engineering technology diploma) I work in a iron ore mine in northern canada... we operate mining shovels. they are purely electric there are two motors used for hoisting the bucket, they are DC shunt wound motors. We recently built a new shovel, and one of the hoist motors is not operating correctly. It is obvious to me the brush sets are not correctly aligned to commutate current while the armature rests in the neutral plane. My question is... would you all agree w/ me in saying that if the brushes were spaced directly over the splits of the comm rings, you could move them, as a set all around the 360 degrees of the end bell and you would achieve satisfactory commutation? AND.. that only if one of the brushes is not aligned well enough, will the current arc across the brush to the comm on the supply positive end? I'm trying to explain as best I can it is difficult without a model... let me try again... would you agree in saying... that the only reason the motor is not working correctly... and that a brush set is arcing... is because one brush set may be aligned correctly, but another may not be lining up with the splits of the comm while the other is... therefore current is arcing through the comm and back to the positive DC supply? I mean, the brushes as a set may be not far enough away from the comm at the point of commutation (where they should be conducting NO current... only preparing for the next reversal..). and therefore they are arcing? that's what I believe... and I believe I need a unit to test my armature current at the point of commutation to ensure the current in my armature is zero.... PS> any help is appreciated!... but I dont need a crash course in DC machines... I understand what's going on with the exception of what causes the arcing of the brushes, and how it relates to the neutral plane.