HI Graham,
I didn't read your post very well ( it was late), VFD is usually associated with AC motors. My baldor vfd won't do anything for you. I think it is going to be hard to find what you want. I did a little digging and most of the DC drives i can find in that voltage and hp rating are intended to run motors with field coils. I haven't personally dealt with these but from what i have been reading it looks like the filed coils are basically replacing the magnets in a permanent magnet brush motor. I don't think these will work for your motor.
Can i ask what you are using these on? My guess is is a lathe. So based on that guess i am going assume you want to do some CSS , perhaps some ridgid tapping, or maybe just get better results with thread cutting. Again just guessing.
If you are primarily going after CSS then i would suggest you find a VFD with pwm input, open loop vector control and single to three phase converter built in and replace your dc motor with a different 3 phase AC motor.
Tell me a little more about what this motor is on. How many wires coming off this motor , is there a variable speed control on it now?
Is it an Asian mini mill or lathe with a speed control? if so this might work for you ..
http://www.homanndesigns.com/DigiSpeedXLDeal.htmlChad