Kendrick, I do not think your spindle is DC, changing the frequency on your vfd will change the spindle speed, so it must be a 3 phase spindle motor. Remember what I told you last night, spindle speed is pairs of poles x frequency,
ie vfd frequency say 400hz (cycles per second) x 60 (seconds to minutes) = 24k RPM. This is a little higher than the actual speed due to slip, but is near enough for what you need. Ihope that makes sense. G.