Motor bearings

Marvo · Nov 3, 2011

I've had a small fan unit (40hp (30kW) 4-pole 380v) which has destroyed 3 sets of motor bearings from new and it's only 2 1/2 years old. The first bearings lasted less than three months and the suppliers gave us replacements under warranty.

Two months later we had the same problem and I sent them in for post-mortem testing. The results came back that the surfaces of the inner and outer races had suffered microscopic pitting leading to heat and lubricant failure. During the month and a half it took to get the results a third set of bearings were already destroyed.

I put the failure down to the Danfoss VFD causing rotor voltage to develop and the discharge current running through the bearings being the cause of the pitting. As an interim measure I fabricated a rotor earthing brush in our workshop and installed it whilst we did battle with Danfoss. To cut a long and depressing story short Danfoss conducted a barrage of logged tests on the drive and the motor. They wouldn't share their test results or any data with us but denied all responsibility. We still have pending legal action against the supplier so I can't go into details.

The motor has been running for nearly 2 years without a hic-up but failed again the other day. I went to look and noticed that the temporary earthing brush I installed had worn away and wasn't making contact with the rotor any more. Two questions, firstly is there a range of shaft earthing brush kits available that anyone can recommend as a retrofit? Secondly, has anyone else encountered similar problems and how did you remedy them?

Thanks in advance.

clicon357 · Nov 3, 2011

You can get commercial bearing protection rings. Try this lot:

Aegis Shaft Grounding Brushes: European Reps/Distributors

Engineer54 · Nov 3, 2011

This sounds very much like the Gizmo we used on all our VFD controlled motors on my last project, at the insistence of my Mechanical colleague. So, it will probably be the same again here. As Marvo says, this was after a few premature bearing failures.

Makes you wonder why they don't fit these things as standard, when they know the motor is going to be controlled by VFD controllers and the like !!!

pmac10 · Nov 3, 2011

Marvo, i think it was you who made me aware of this type of failure in the first place, anyhow, SKF do a range of electrically insulated bearings and whilst i have never used them, i always try to fit SKF bearings wherever possible in other applications.

geofff · Nov 3, 2011

I had no idea this was a problem, but will keep a lookout for this type of failure. They employ loads of VFD's at work, but very few VFD controlled motors run at speed, mostly programmed to run slowly. Many thanks for the post.

netblindpaul · Nov 3, 2011

Marvo,
This is a well known and well documented issue.
There are several accepted "papers" written on the topic.
Danfoss should be aware of the issues, well at least in the UK anyway.
There may be other issues in your neck of the woods.
Any reputable bearing maker, or test house should be able to identify the electrical pitting.
This is also identifiable with a good set of VA kit as it does show up in the vibration trace if you know what you are looking for and how to find it!

there are electrically insulated bearings as has been suggested, and there are also insulation rings that you can fit to bearings to give the same effect, however, you should address the root cause.
Find an alternative supplier, I can help if you want, that will change the opinions of Danfoss, else my supplier will help you out and give you the real deal if you want.

Knobhead · Nov 4, 2011

If you use insulated bearings it’s usual to fit just one at the drive end. You can get circulating current problems caused by the drive couplings and guarding. The reason I mention guarding I had a 2500HP motor fail twice, we changed the bearings in situe each time. The second time I hung around while the fitters put the guard on, they bolted it to the bearing pedestal therefore earthing it!

Marvo · Nov 7, 2011

Thanks for the responses, I apologize for being absent for a few days.

We've had a similar problem previously on an even smaller motor with another make of drive (emerson) but I never found a workaround for the issues, we eventually replaced the drive with an equivalent weg model which had a lower carrier frequency and that sorted it out. We later successfully re-used the emerson drive in conjunction with a different make of motor on another installation and the problems didn't reoccur, it ran fine from day one. I'm guessing we could do the same with the Danfoss drive but we don't use them often enough and this drive is somewhat larger and the cost implication of replacement is obviously higher.

I know that rotor earthing will work and I just wanted a more professional system than the one I fabricated in our workshop which was made from part of a defunkt bosch grinder. I've just replaced the brush again and made a better job of polishing the shaft to protect the new bearings as an interim measure.

The Aegis link is great, I'm going to hunt for a local agent. Insulated bearings aren't unfortunately available off the shelf here, there's a 3 week lead time so I haven't gone any further down that avenue as I don't particularly want to hold stock for the next time.