Does the barrel of the connector on the spindle (center top of the spindle in your picture) have conductivity to the metal case of the spindle? I'd bet it does so you could use a shielded cable from your VFD to get a ground connection. If you don't have a shielded cable, you might be able to get a ring terminal that goes over the the top of the barrel and use that for a ground wire run back to the VFD. They make them with up to 3/4" ID rings.