I have a 1996 Mori Seiki cl 20 with a MSC 521/fanuc 21 tb control. The spindle tries to run in reverse but its movement is really erratic. It will run at speeds under 100 rpm, but the load meter fluctuates greatly. Anything over 100 rpm and the spindle runs forward for about one revolution and then reverse for about one revolution and continues this until you stop it. Forward runs fine MOST of the time. Sometimes it acts up in forward, but not nearly as severely. The problem in forward is usually high load on load meter and noise coming from spindle. It did it today and after running in forward for a couple of minutes it was ok, but the problem with reverse is always there. Oh by the way, I'm running this off a rotary phase converter with a 240 - 208 transformer that is tapped to balance voltages between 215 - 220. I have also replaced two of the three transistor modules in the spindle amp due to failure. When each of the two stopped working it stopped the spindle amp from working. I was thinking it may be that third transistor module, but so far it hasn't knocked out the drive completely. If anyone has any ideas i'd sure like to hear them. Thank you.
That machine is just a straight fanuc 21 with a alpha spindle drive. No built in sensors or anything that can cause that other than a bad spindle drive. You need to Meg your motor if you keep blowing transistors.