Thanks for all the replies.
To clarify, the motor in question is NOT on a VFD, it is direct-on-line from the main 480v board. The 480v board is supplied from a transformer off the 690v generator board (the 690v board also supplies the VFD rectifiers via two transformers)
I noticed the current variation on a panel meter, then confirmed it with a current clampmeter.
The rig is only a few years old, and this is all original equipment.
I will take some more measurements tomorrow, I have a Fluke scopemeter with 300v Cat III inputs, and 10:1 probes, but am not comfortable connecting this to a 480v bus...
The rig's power factor is quite shit, around .6, but there is no power factor correction.
But surely each load determines it's own PF, it's not possible for the system to "force" reactive currents through this motor.