dc spindle motor control.

I am working on a fuji electric dc spindle motor. This is on a large
Makino vertical mill, that had a 3000c control.

I would like to increase the speed of this motor. Would simply
increasing the voltage do this? This spindle motor was connected to a
fanuc 3000c controler(which is all missing). The motor itself has 5
wires coming from it. 3 from the armature area, and 2 from the
fields. The motor runs the best with the fields connected in series
with the armature. Is anyone familure with this type of motor
control? What did this 3rd wire do? ty jeff