Stray current causing mechanical damage to motor?

[mull982]

We have a very large 6000hp motor that we took out of service recently to do a routine inspection/maintance on the motor and gearbox. The company that was going through the motor noticed that there was several locations on the motor especially the bearings that were pitted which he is claiming is a result of stray current. Someone here has said that in the past the shaft of this motor was damaged due to stray arcing current as well.

My qustion is weather or not this stray is known to cause this type of damge in motors, or if it is just a red herring that someone is throwing out there? Can anything be done to prevent this damage if it is indeed from stray current such as grounding or something else?

 

[Bigrig]

Vfd?

Vfd?

There has been a lot of talk recently about stray current causing bearing damage in VFD-driven motors. I recall there was an artical about it in a recent trade magazine. I'll try to find it today (if someone else does not find it first).

Nathan

 

[noonan]

Probably not a red herring. Stray currents have been known to do what you're saying. They can cause fluting as a result of EDM (Electric Discharge Machining). One way I've seen to counteract this is using some kind of grounding brush against the shaft.

Here is one article.

http://www.greenheck.com/technical/tech_detail.php?display=files/Product_guide/fa117_03

 

[kingpb]

Another good article that may help

http://ecmweb.com/mag/electric_bearing_current_problems/

 

[mull982]

Thanks for the reference articles they were very informative. The one thing that I find interesting are the voltages that can be induced on the shaft due to common mode voltages. I'm not really sure what a commom mode voltage is, but it sounds like it results from magnetic aysemmetry.

Typically in a motor I know that the changing magnetic field in the stator induces a voltage current and magenetic field in the rotor. Is the rotor typically insulated from the shaft? Doe these fulx lines that normally cut between the stator and rotor not typically cut between the rotor and shaft, but sometimes because of asymettry they do and therefore induce a voltage in the shaft.

Should any of these stray current be picked up by my zero sequence ground monitoring on this motor or are they happening to quickly?

 

[charlie k.]

It was explained to me a few years back. We had some large motors (2500hp) that we were redoing the controls and instrumentation on. There were RTD's on the shaft bearings that had to be electrically isolated from the bearing due to Eddy currents forming and destroying the shaft.

Charlie

 

[Jraef]

The voltage is created by capacitive coupling between the stator and rotor. It's easier to think of it as similar to a static charge building up on the rotor. It's small, but it wants to flow to ground and the only path is through the bearing races to the frame. But the bearings are constantly on the move, with grease affecting the conductivity, so the flow to ground is discontinuous and a very very small arc is created. Over a long time and a LOT of revolutions, that small arc does incremental damage to the bearings and races, like a tiny tiny welder, or more like a tiny plasma cutter, hence the reference to EDM. Shaft grounding bushings provide a more reliable and continuous ground path for those currents.

Aegis offers some excellent papers describing the phenomenon. Look for the Application Notes and you can send for a free Technical paper.
http://www.est-aegis.com/

This outfit offers a detailed explanation as well, but it's a little long (25 pages) and technically dry, so get a cup of coffe or you will nod off...
http://www.motorshaftgrounding.com/index_1.html?gclid=CJychOPe8JUCFRoVEAod9QazfA

The thing you didn't mention is how the motors are being powered and controlled. This phenomenon was virtually unheard of before VFDs. If you are not powering those 6000HP motors with VFDs, you must have some soft of more uncommon problem that may have a different solution.

 

[benaround]

Ceramic Bearings and ground brushes to the shaft.

 

[mull982]

The thing you didn't mention is how the motors are being powered and controlled. This phenomenon was virtually unheard of before VFDs. If you are not powering those 6000HP motors with VFDs, you must have some soft of more uncommon problem that may have a different solution.

You are right I should have mentioned that this motor is a wound rotor motor that uses an electric rehostat during starting only. Once the motor is brought up to speed the rheostat is shorted out.

I'm not sure what kind of new variable this rheostat may add to the problem

 

[Jraef]

Not likely. But is the rheostat shorted out inside the brush assembly, or outside somewhere? An incomplete shorting of the rotor can cause all kinds of problems, I can imagine that building up a static charge is only one of them.

But if the bearings have lasted for years, I would not be too concerned about it; you can't prevent everything. The problematic EDM destruction of bearings happens in months.