Of the generally available pump and HVAC type VFDs sold from major manufacturers, generally 6 pulse, would you suggest that an overwhelming percentage of them are non-regenerative? That would be my guess.This has been the subject of debate, but yes, that is the general accepted concept at the moment, ASSuming the drive is not line regenerative. Technically, because there is a generic statement in IEEE standard 141-1993 (Redbook), section 4.2.5 that says "adjustable speed drives can contribute current from the motors to a short circuit...", most calculation software packages such as SKM Power Tools and ETAP will assume the worst (that he drive MIGHT be regenerative) and go ahead and leave the motor contribution in. But in reality if it is a simple VSI drive with a diode front-end rectifier, it cannot contribute fault current backward from the motor unless it completely melts down and becomes it's own bolted fault, which at that point becomes its own new problem.
As to the earlier concept of "just put a diode to block it" (sic), remember it's AC, so you would need TWO diodes to block anything, and then if it is blocked in both directions, nothing works! The VFD does because the diodes are configured as a RECTIFIER to DC, effectively blocking anything from going the other way. So technically it COULD be done if VERY motor were on a VFD (non-regen of course), something we VFD manufacturers would highly promote...:thumbsup::thumbsup::thumbsup::thumbsup: