The problem is that high voltage spikes are arcing across the air gap in the bearings and leaving a little burn each time. It gradually builds up and leads to bearing failure. Two areas of solution would be to prevent the voltage spikes or to better ground the motor shaft. An electronic choke might be used to prevent the spikes. A grounding strap or brushes might be used to ground the motor shaft. I've heard of some motor manufacturers doing the opposite. Instead of making a better ground for the shaft, they better insulate it from ground by using ceramic balls in the bearings. However, that's not a solution a user can easily do himself in the field, so I'd stick with the choke or ground strap.

Terry, many thanks for your informations.
Another question: do you know if the currents circution into the bearings can produce overheating in high speed spindle drives?

I have specified the use of Inpro Seal shaft current protection for any large horespower motor. www.inpro-seal.com

I usually recommend an insulated bearing (SKF makes one with a ceramic coating to accomplish the same thing) or an Aegis grounding ring on motors that will be used with VFD's. You could also implement a line reactor to filter out noise.

I haven't heard of current creating an overheating in the bearings. Typically what happens is the current causes the ball bearing to spall and create pits. The results is that tiny metal bits act like sandpaper inside the bearing. That could be what is creating the overheating. http://www.vfds.in

I've also heard good things about Aegis.

Other questions:
What is the switching frequency?
Is there a load reactor between the drive and motor?
Is the panel grounded properly?
Are you using a motor cable and properly landing the cable shield and motor ground, or, if using 3 conductors + ground, is the ground landing on the drive, rather than on the panel ground bus bar?

Also, since you mention "Spindle" motor. Tell us more about the machine and it's history. Is it new? Did it run OK and now there is a problem? Was it recently moved or rebuilt?

It seems like spindles in modern machine tools tend to be a well controlled environment built by folks who know what they are doing. The other questions might uncover a "why" or "what changed"....

Good luck.