If the motor is only 250 Hp then It may not have a brush ring that will allow easy turning of the brushes.
You should check the following things ( most already mentioned ) if there is no adjustment possible.
1 Are the brush boxes all uniform height above the commutator? measurement is 1.8 to 2 mm ( 75 to 85 mill )
2 Are the brush boxes uniformly spaced around the commutator? Measure around the comm surface.
3 Are all the interpols electrically ok?. Measure with a megger to ground and with a "ductoR"
4 Are all the Series elements connected correctly? Uaually a connection diagram is included in the machine documentation if it still exists. Otherwise call GE motor product service with the model number +18148753469
5 Is the motor overloaded ?
6 are all the brushes the same grade ? (do not laugh)
7 Are all the Shunt fields Ok ?. Easiest way is to do an "AC Drop Test" disconnect the field supply and connect 110 V Ac (or 240) direct from an AC outlet. The current will only be low because of the high level of inductance on the fields. Measure the voltage across each of the fields. Usually they are connected , 2 in series, (in parallel with two more) so you would expect half the supply voltage across each field. Shorted turns will change the inductance significantly and upset the voltage balance. This will not work if all fields are in parallel.
8 check the neutral while you have the AC on the fields. Measure the voltage ( milli Volts) between two consequetive brush arms, it should be somewhere between 0 and about 60 or 70 milli Volts. The voltage should be the same between any consequetive two arms. If not then your brush spacing may be out. the position of the commutator under the brushes can affect this. If it is possible to turn all the brushes together, on a ring, then do so until the voltge is at it's minimum. remember, it will change when you tighten the bolts, so it is best to turn a little, tighten, and then measure. (time consuming but still the fastest way)
with regards to stoning, Mostly hand stoning will only work if you wish to remove a film form the comm. If the com is even slightly out of round, the insecure anchoring of the stone will actually make the out or roundness worse. (many years of experience behind this one ) if it is badly pitted or burned, proper machining is the only answer
A question for you. Is there burning, and if yes, then is it uniform or is it in definite bars or patterns around the comm? If a pattern exists then It could be a pointer to your problem.
happy hunting
Tom
