Bearing current

[Jpflex]

Hey guys. Had an HVAC tech contact me about an outside self-contained AC unit for a warehouse, a customer we both work for. They changed the blower motor out because the bearings were growling and got 2 years out of it, and now the bearings are growling again. Their supplier is suggesting we might be getting a current path through the bearings causing arc/pitting and taking out the bearings. I've only heard of this happening, never seen it though. This unit is 480v and VFD driven. I haven't looked at it yet, we are waiting for the new motor to arrive to meet up on it. Any thoughts on what I could be looking for, or to do, to remedy this? About all I can do at the moment is verify connections, and make sure our grounding system is good. Didn't know if anyone here has any experience with this? I suspect maybe something the VFD could be causing this also? I would almost consider it just bad luck but its 1 of 4 identical units, so something must be up. And it's a very clean facility, food grade storage, its kept spotless.

I read about this in a book a while back and adjusting the commutator if it had one could mitigate this.

 

[kwired]

I read about this in a book a while back and adjusting the commutator if it had one could mitigate this.

wound rotor motors aren't that commonly used with VFD's. Most are universal motors and can run on AC or DC and varying speed is often as simple as varying input voltage.

There is three phase wound rotor motors but they are not so common anymore as it is simpler and less cost to drive an induction motor with a VFD if you need to vary speed.

 

[zbang]

wound rotor motors aren't that commonly used with VFD's

Are they ever? Seems like a very specific application.

 

[Jraef]

I read about this in a book a while back and adjusting the commutator if it had one could mitigate this.

Commutators are on DC motors. These are AC motors and VFDs. There wouldn’t be a commutator.

Nobody uses Universal motors for HVAC fans either.

Wound rotors have slip rings, but not commutators.

 

[Jpflex]

Commutators are on DC motors. These are AC motors and VFDs. There wouldn’t be a commutator.

Nobody uses Universal motors for HVAC fans either.

Wound rotors have slip rings, but not commutators.

Yes I know commutators reverse lead direction so current remains dc. I was only saying what I remember about sparkling

 

[User Name]

So finally got to the job to look things over. The motors already had shaft brushes on them. No output filter on the VFD. Voltages and grounds throughout the system looked good. Bearing clearly bad on the motor, it spins nice but spin it fast and it growled. Was also told they've had motor bearing failures in 2 other units, so far 3 out of 6 have had motors replaced. This unit though is on its 3rd motor now though I think. They are using very basic Baldor gold industrial 3hp motors. Do you guys think just crappy motors? I took the front bearing out of the currently bad motor and cut it open, it looks fine to the naked eye, I don't see anything that stands out. I'm not wild about the motor shaft, a lot of aluminum slag left on it from the foundry, and the motor fan is welded on to the shaft. I'm wondering how balanced this shaft is...

 

[ptonsparky]

Has maintenance done any welding? We had a customer whose crew would attach the ground wire anywhere and use the building for one lead.

 

[User Name]

I don't think so, honestly I don't think there's a 220 outlet anywhere in the building, could have been a 110 welder if they did I guess. I'll keep it in mind to bring up.

 

[ptonsparky]

Are you these belted?

 

[bgard]

Tightening the belts to tight will cause bearing failures.

 

[User Name]

Tightening the belts to tight will cause bearing failures.

They are belted, and I'd agree to that, the hvac guys are the ones working on it so I don't know how tight they're getting them, or if they were to tight from the factory.

 

[User Name]

The motor was already out of today unit so I never got to see it function before I had to leave.

 

[ptonsparky]

Tightening the belts to tight will cause bearing failures.

For sure.