CNC mill DC motor field wiring

I finally got around to installing a 5HP 4 quadrant regenerative
drive (from KB) on my shizuoka B3V mill. I didn't really know the
specs on the motor, except that it is a 5HP DC motor, so I figured it
was probably 180V armature, 200V field. The old broken spindle
driver was labled 3.7KW DC motor speed control model 1-4064 by Sanyo
Denki Co.

Anyway, it works just fine, except I don't know if it is rotating as
fast as it was designed. The field resistance is about 116 ohms, so
I figured it had to be 200V. Would it hurt things to run it at
100V? I guess it is going to give reduced torque and double? the
speed, which might be too much for the bearings.

Any idea what the designed max rpm on this mill is? The old fanuc
control panel had an analog spindle speed meter that has about
3500rpm and 740rpm as the max meter movement. That makes sense as
the mill has a geared head with a 5 or 6 to 1 ratio (haven't actually
bothered to measure the ratio yet). I would estimate the motor is
spinning much slower than 3500...perhaps 1800 or so.

I just got a 240VAC 9.5 amp autotransformer at an auction, so I
guess I could easily build a variable field voltage generator with
the addition of a small bridge rectifier....

Thanks for any ideas or suggestions,

Richard