Hi,

The problem is with OUR New BAR ROD Mill built by Danieli. After few months of running different problems were arised. Due to current leakage at both Mill Stand gearbox and Motor bearings are getting damaged( Electrical fluting) at a period of 6-8months.. Can anyone suggest some ideas.. FIND THE ATTACHED PHOTOS... How to avoid current leakage??

Sany

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First time on this blog and I see these postings are a couple years old, but here goes it anyway:

I reviewed the pictures and agree you are experiencing electrically induced bearing failure.  Incidence rates of bearing failure on VFD motors is at least 60% by 2 years and only continues to climb from there.  It is a well-accepted and pervasive problem that particularly plagues operations that run 24/7.  The following information is based on long-term field and lab studies over 30 years.

1) Capacitive discharge through the bearing is a local event caused by the VFD gated mechanism switching on and off.   VFD brands, altering the carrier frequency, installing induction absorbers on the wiring, shunts, conductive grease, etc. are not a fix for this phenomenon.  A shaft grounding device is the only known way to economically eliminate capacitive discharge through the bearing.

2)  Circulating currents can be present on larger framed motors and are inherent to the motor.  This is different than capacitive coupling as described above under #1.  Our field research has shown that motors 100-200hp and with rpm <=1200 or motors >=200hp regardless of rpm are susceptible to circulating currents.  Incident rates of circulating currents being present is 25-35% for the motor parameters described above.  Insulated bearings will not take care of circulating currents as the current will find a parallel pathway and take out bearings in connected equipment.  A typical set-up to deal with circulating currents is to either provide shaft grounding on both ends of the motor or use an insulated bearing on the non-drive end and a drive end shaft grounding system to break the current loop.

3)  Not all shaft grounding devices are equal.  It is certainly confusing with the different available products on the market.   The following are key principals that cannot be ignored while researching shaft grounding products.  Always have these concepts and questions at the forefront of your mind when researching shaft grounding options:

The grounding system MUST be a better competitor to the bearing for grounding the voltage/current away from the bearing.  While most shaft grounding systems lower shaft voltages right out of the box, not all shaft grounding systems lower voltages below the threshold to prevent current flow through the dielectric (bearing grease) then onto the bearing.  Moreover, most shaft grounding systems cannot maintain for the long-term voltages below the known threshold.  One major manufacturer of shaft grounding rings (SGRs) was asked the question during a webinar "how do you know the grounding system is working?" and their response was if the shaft voltage readings are below 10v, then the grounding system is working.  Problem with that response is shaft voltages needs to be much lower than 10v to prevent breakover through the bearing grease.  While 10v will certainly slow down electrically induced bearing damage, it will not eliminate the problem.  We've extensively lab tested the above referenced shaft grounding products and found that out of the box, the shaft grounding ring drops shaft voltages from ~30v peak-to-peak (no shaft grounding) down to 5-7v peak-to-peak.  Even the 5-7v peak-to-peak isn't low enough to prevent dielectric breakover.  As our test motors ran 24/7, the 5-7v creeped up to ~15-17v after just over a year of continuous operation.  These results are better than no shaft grounding, however, the test motors' bearings fluted after 3 years of operation; far less than the motors service life.  

In short, don't be afraid to asked the following questions.  They will cut to the core of how a shaft grounding system performs: 1) what shaft voltages does the shaft grounding system achieve?  2) how does the system maintain the shaft voltages below that threshold known to cause breakover through the bearing grease? 3)  If the environment is dirty, then how does the shaft grounding system ensure a path of least resistance for current to flow away from the bearing?  It doesn't take a lot of contamination to negatively impact shaft grounding performance.  

I hope this information is help.  Feel free to let me know if you have any follow-up questions. 

Sam P.

www.shaftgroundingsystems.com

 

 

 

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