HI Pader
your motor does not have an earth pin on that 3 pole connector as its a 3 phase motor that should be powered by a VFD
you will find other spindle motors for sale with a 4 pin connector that very often only has 3 pins connected !!!!
a few posts on CNC Zone shows you how to wire the unused pin to the spindle motors case
from one post I save this -
John