Hi, We have a 800 hp VHS in the shop, that we sold new to customer 2 years ago. We sold him 2 of these motors. The one we have now would be the second time in that 2 years it has returned for top bearing failure. We are seeing bad fluting on both the rollers, and the outer race.

The customer is not running a VFD on, or near this motor. The last time it failed, he put another 800 in its place, and hasn't had any problems. When this motor returned, it went to another well site. Year later, another bearing failure.

I tested the rotor using acore tester and magnetic paper, and detected some voids and hot spots. We are thinking this is the cause of the fluting.

The factory claims they have never seen a fluting problem from a bad rotor, but are willing to send us a new one free of charge.

If they never seen this problem, why are they so willing to warrenty a motor almost 3 years old?

I'm not an engineer, just a wrench/winder, so in laymens terms, have you seen this problem before?

There have been some previous discussions on this topic and I know some have expressed an opinion that bearing fluting is unrelated to rotor bar problems.

I have never seen it before, but I believe rotor bar problems can certainly contribute to bearing currents and fluting. The rotor bar defect can cause a magnetic assymetry which could induce circulating currnets between the shaft and frame if outboard bearing is not insulated.

I have run accross some articles discussing shaft voltage measurements in presence of broken rotor bar. A relatively large shaft voltage appears with a frequency of pole pass frequency and can be a driver for bearing current imo. When/if I get a chance, I will post link or excerpt for those articles.

Dude,
You definitely have a voltage potential difference from one end of this motor to the other. Use a DC brush on a non-conducting stick and connect the pig tail to an o'scope. Measure the voltage. Then, connect the pigtail to a ground point on the motor and use a current clamp to read the pigtail lead. You will then have documented the shaft voltage and current.
Usually, when a VFD is NOT involved, the problem is circulating currents in the motor due to a potential difference between the ends of the stator. Bore the end bracket and insulate one bearing. That should resolve the issues.
Remember, if the insulated bracketed bearing has a tach attached, it must be insulated. If there is a bearing RTD, it must be insulated. Make sure there is nothing that reconnects the path once you have insulated the bearing.

One lesson I have learned long ago:
Identical machines that do not behave identical are not identical.
I guess one machine has more shaft voltage than the other because magnetic assymmetry (air-gap uniformity, rotor bar condition, etc.) is not identical for both machines.

Kerbouchar,

Whether the casting voids you suspect are the source of the shaft currents remains to be seen, however... Below is some text that I got out of EASA's Root Cause Failure Analysis handbook.

"In standard machines, any break from uniformity in the rotor or stator can cause shaft voltages. Shorted laminations, gaps in the stator laminations (as occur with large machines built with segmented laminations), variations in air gap or spacing for fields or interpoles in a DC machine--all of these conditions can result in shaft voltages in rotating equipment."

Can you post any pictures of the fluted bearings and rotor?

If you'd like I could send you more literature on the subject that I've dug up over the last couple of years.