Hi Sir,

We have one bearing from VFD AC motor inspected and noticed with electrical fluting. We already have grounding strip on shaft, why we still have this kind of defect? See Attached for details.

Thanks,Kevin Ku.

electrical_fluting.pdf (186 KB, 146 downloads) pictures

Kevin,

Grounding of the newer inverters with high switching frequencies is pretty involved.

I would suggest that you search the vibration analysis forum for comments on the subject by Ron Brook.

BTW. That is definitely fluting but you probably already know that. I'll bet that one was a real screamer, too.

Looking at the picture of the grounding brush, I see a lot of build-up on it. I have seen times where dust, grease, debris, etc build up on the interface at the shaft will act as an insulator. This in effect eliminates the path of least resistance through the brush and puts it back through the bearings. The brushes must be kept clean to work correctly.

Gary B

Yes, it is definitely fluting from stray electrical currents. Nice picture! If you can't remove the current leakage by design, there are bearings available with an electrical barrier for this purposes. This is accomplished either with a coated bore mount surface on the bearing OR OD (the word "Insocoat" comes to mind), or with silicon nitride rollers. Both cost a premium over standard bearings.

Disclaimer: I work for a bearing company and thus have a financial interest in the bearings mentioned above.

NEMA and EASA both recommend an insulated bearing/housing on both motor bearings and one shaft grounding brush when running on a VFD.
It appears the motor is quite old? Most common "source" of the electrical fluting is dis-symmetry in the rotor, stator or air gap between the two.
Older motors were not designed to run on VFD's, the insulation system, cooling ability, rotor design are just a few examples that old motors do not have...

Grounding brushes are ineffective on motors driven by modern VFD's. What is needed is proper cable installation and high frequency grounding. Insulated bearings help as well.