The 100 volt drive (Viper 100) has the same power mosfets as the Viper 200 which are rated at 250 volts @ 56 amps, so the back emf isn't a problem. The difference between the 2 drives is the voltage regulator range (25-100V or 50-170V) and the main capacitor (680uF 100V or 390uF 200V)
Also i can build the V100 with smaller connectors (10 amp vs 20amp) if requested, for space and cost savings.
What PS voltage do you plan on running ?
Larry K
http://www.viperservo.com/V200.htm