I had a couple of machines with a similar problem. The fault was found to be a bad output module. I believe there are 6 of them mounted to the back of the KTK drive. I could command the spindle speed to go up in small steps but if I shut it off or reversed direction, it would trip.
This customer was blowing them because his plant had one leg of the 3 phase power tied to ground. The drive did not like that at all. They changed the wiring to the machine and he stopped blowing those modules.
Are you on a phase converter?
Is your phasing correct?
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)