Potential Bearing Problems With VFDs

[Hawkeye]

I was doing a bit of research on VFDs this morning when I found this site: https://www.pumpsandsystems.com/preventi...r-bearings

They're talking about voltages being induced in the shaft of a motor being run from a VFD. I recall from my training as an electrician that different frequencies of AC require different 'amounts' of iron and copper to accomplish the same tasks. For example, AC systems on aircraft may run at 400 Hz, rather than 60 Hz, because the transformers and other inductive equipment can be made smaller and lighter.

The site was pointing out that voltages up to 70 VAC can be induced in the shafts of motors when powered by VFDs. I got curious and measured the voltage between the output shaft of the motor on my RF-30 and it's mount. When it was running, I measured up to 2.4 VAC at around 44 Hz. The readings were lower at other frequencies and lowest at 60 Hz. On my big lathe (2 HP motor) the highest reading was just over 3 VAC at 10 Hz, and decreasing as the frequency rose.

Any on this forum who have played with EDM (Electrical Discharge Machining) know that we use fairly low DC voltages to remove bits of metal. For that matter, arc welders operate at relatively low AC or DC voltages. The point of the site is that the races on the bearings can be damaged over time when running at various frequencies. 

Just to be sure, and because I needed something to do to stay out of our current heat wave, I made up a couple of wipers to provide a path to ground , bypassing the bearings on my motors.

I had a bag of flat springs I got at Restore, so I used those for the contacts. The ball bearing further reduces wear. The yellow wire connects to the motor mount.


On the lathe, I had to sand off some rust from the side of the pulley, to improve the contact. You have to look closely to see the strap spring.


I don't know if this will make the slightest difference in the long run. I just thought it was interesting, and it did fill the need for something to do in the cool of the shop.

[Dr Stan]

When I was in the Navy one of the issues related to motor and pump shafts was called high frequency fretting. It looked a lot like oxidation on the bearing surface and when we would repair them with electroplating we would do a final flash of tin to fill in the microscopic hills and valleys of the bearing surface and the inside of the bearing race when they were assembled. Now I wonder if you haven't identified the true culprit.

[EdK]

We had that problem at work and used wire jumpers to provide a path for the current to go instead of through the bearings. It sure is an interesting subject.

Ed

[Vinny]

When I got the new motor for my mill the manufacturer listed which ones could be used with a VFD - fortunately this one was on that list.

[Hawkeye]

It's amazing what a little stray electricity can accomplish.

[awemawson]

(07-31-2020, 11:39 PM)Vinny Wrote: When I got the new motor for my mill the manufacturer listed which ones could be used with a VFD - fortunately this one was on that list.

Usually if a motor manufacturer says an item is suitable for a VFD it’s because the insulation is up to the fast rise times of the wave form, and the fan is good for a wider range of speeds and still give adequate cooling. I very much doubt this intriguing bearing issue is even thought about. 

Back in the day big disk drives, I’m talking 20” platters, often had an earthing bush pressing on the end of the shaft, but the maintenance books referred to it as a static discharge device not an induced current device.

[Hawkeye]

I suspect that they were dealing more with static. Those big multi-disk hard drives would love to act like an electrostatic generator. Were the disk drives run by DC motors? That would eliminate induced voltages.