Bearing Fluting & Rotor Bonding

[peebee]

I just finished reading two very interesting articles in this month's IEEE Industry Applications (Sep/Oct 2002, Vol. 8 No. 5, "Bearing Fluting," p. 53, & "Motor Bearing Systems," p. 58).

Both articles mentioned bonding the stator to the rotor as a method to reduce or eliminate bearing wear due to high frequency voltages on the rotor due to variable speed drive use on both AC and DC systems. The first article specifically recommends against using carbon brushes for grounding for several reasons, including the high-resistance carbon film that will eventually build up on the shaft. (The first article also recommends against insulating the bearings as this would simply transfer the bearing wear problem to downstream mechanical equipment).

However, neither article gives any specifics regarding other recommended ways to bond the stator and rotor. Any ideas?

 

[wbd]

I have seen shafts grounded using braided copper straps that rub on the shaft. I don't remember anymore details than that.

 

[BJC]

Try this. Very good paper on this site. They are very helpful if called.

http://www.shaftgrounding.com/index_1.html

 

[Lewish]

One of my customers uses insulated bearings and a copper braid pickup on the shaft. Their motors are all PWM DC drives. Fluting seems to be predominant on the load end of the shaft.
On another system I have seen, a flat copper tape which was spring loaded against the shaft. The tape went half-way around the shaft. One end of the tape was anchored to the frame, and the other end went to a spring which held tension on the tape.

 

[Petri]

Unfortunately with the gaining popularity of variable speed AC inverter drives and DC converter drives, the occurrence of EDM will become a significant issue in any industry.
Most solutions offered do not address the root cause and only provide high maintenance band aid solutions.
We have tried earth brushes on DC motors where EDM was picked up during post-mortem examination. We'll see if EDM reoccurs or presents it self as another problem.
The solution must be to provide a smooth/filtered output from the drive to the prime mover.
What you may want to do is detect or measure the presence of the offending cause whist the drive is operating. This will at least give you an indication of the size of your problem.
CSI have long marketed a shaft voltage probe for detecting the stray, unwanted potential voltage.
Measure the motor/prime mover/machine shaft voltage during operation ( Using safe practices - the hazards are obvious ). Then you may know which machines are potential victims of this phenomena.
Now, I'm no expert and I don't have any experience with this technique but I'm investigating its merits as a fault-finding tool.
CSI ( Computational Systems Incorporated ) would gladly be able to assist if you contact them.
Have a read of this interesting technical paper from CSI.
Can't get enough of these, can we ?

http://www.compsys.com/drknow/aplpa...2f085b6fad75d290852566aa0062720e!OpenDocument

Good luck
Petri