Now that Im not on my phone and I can see the site properly,when does the rcd trip? Is it at power up, starting the spindle or after it has been running for a while? Does the rcd trip at any other time? Is this the main rcd on the distribution board or do you have a separate one for the vfd circuit? This could also be an accumulation of leakage from various devices in the workshop and the vfd is just enough to add to it to trip the rcd.