So I got everything to work! it seemed that the big hang-up was the Field Failure relay not closing. With the main contactor. I nudged the contacts closed with a piece of wood while depressing the Start button and everything lit up like Rockafeller Plaza at Christmas! Now the field failure relay closes the way it's supposed to. It's worth noting there's a bit in the 10ee manual for the modular drive that mentions adjusting the sping tension on the field failure relay if it's not closing. I imagine I just overcame some mechanical stickiness that was keeping it from closing properly.
I've been trying to understand the pricipal of operation for the speed control. Prior to acquiring this machine, my understanding of DC motor speed control went no further than "volts = speed".
Here we go. Upon applying power to the main disconnect all transformers are energized at their primaries and filament current is supplied to the various tubes and the blower motor at the end of the tailstock. The 6NO60 tube (a time delay relay in glass) begins it's countdown and after 60 seconds closes a portion of the circuit that allows the "Start" button to function and close the main contactor behind the headstock. Upon pressing the Start button, the field winding is supplied current and causes the Field Failure Relay to close completing the circuit that latches the main contactor (as long as the overload coils are not tripped and the drum switch is set to neutral). If the Field Failure relay fails to close, it is likely due to the 3C23 thyratrons not providing rectified current to the shunt wired field winding, or the 6SF5 triode tube which acts as a linear amplifier for the field control circuit since the 3c23 thyratrons cannot do any amplifying on their own. This all assumes the field failure relay itself isn't broken. Current is now following through the field winding and the components in the field control circuit (the 3C23 thyratrons and the 6SF5 triode).
Now when the drum switch is kicked in either direction from neutral(with the main rheostat set at 1150 RPM), fully rectified current flows from the big C16J tubes into the armature (rotor). The current rises in the armature past 25A and causes the Field Accelerating Relay coil to trigger and short out the max speed and field control pots. Maximum current flows through the armature and the series winding of the stator, accelerating rotor from rest. When the motor reaches 1150 RPM, the current drops, the field accelerating relay opens and the field control pot regains control of the field current while the 'max speed' pot regain control of the armature voltage. As load is applied to the spindle, the current load goes up and the voltage in the secondaries of the current transformers goes up proportionally. This voltage is rectified by the 6X5 tube and used to raise the voltage across the armature to make up for the speed drop in the loaded spindle. I'm not sure if that raised voltage is directly supplied to the armature, or if it controls another circuit that that does this.
If the potentiometer is turned up from 1150 RPMs, the max voltage that can be applied to the armature(rotor) of the motor has been reached by the armature control pot and the other half of the main rheostat begins to lower the current supplied to the field (stator). Weakening the magnetic field of the stator begins to shrink the torque envelope of the motor for any given RPM above 1150. If the field collapses somehow, or the field circuit opens, the motor becomes nearly torqueless and if there's little-to-no load on the spindle/motor, it will runaway. The Field Failure relay steps in and unlatches the main contactor. If the field isn't present and the motor tries to start, there won't be a magnetic field for the current to push against and you're basically heating up a filament in atmosphere (the motor burns up). The Field Failure relay prevents all of this.
I'm sure I've made some assumptions that are wrong about how the WIAD works or even fundamentals about electricity, but if someone came in and corrected those assumptions, I'd be super grateful.
Some other things I'm curious about: The schematic for the modular drive identifies a section as the "Phase Shift Circuit." What exactly does that do, and is there a part of the WIAD that serves the same purpose?