Quote Originally Posted by CaptainBarnacles View Post
I'm kicking myself for not having spotted the rated current setting. I was mighty surprised that the frequency was set to 400Hz, I just assumed that it would be 50Hz. Clearly the spindle operates at much higher frequencies than the squirrel cage motor and presumably that's how it reaches 24k RPM.
I'm no expert on VFD's but sure your problem was more the Frequency than the current but yes wouldn't help. This is why I said wouldn't run much more than 100hz with a 50hz motor and why you heard the frequency change but not the speed as the motor got saturated. Suppose it's a little like when steppers get saturated at high rpm and stall.