Grounding brushes for DV shaft currents.

[oldspark]

If you have shaft currents caused by a convertor and are installing two insulated bearings is it considered all right to put grounding brushes on both ends of the shaft. We seem to have different views on this subject in our company, my opinion is that the current wants to seek it's own source so there will be no short circuiting by having brushes on both ends of the shaft as the current wants to return to the convertor.

 

[GusD]

Hi oldspark

You already have motors with insulated housings,this should eliminate your shaft current problems.One insulated bearing housing is enough to interrupt circulating currents.If your motor has a double extended shaft to connect to other equipment at both ends ,than you should have both bearings insulated ,or other means such as shaft brushes.We currently have 3 different methods of bearing protection in our motors.Insulated housings,insulated bearings and shaft brushes.But we don't have anymore than one method in any single motor.
Shaft Brushes offer some protection, but their possible High resistance contact to shaft may not eliminate discharges through bearings.Whichever happens to be the path of least resistance.
I personally like the method where the bearings themselves are coated with a ceramic material on the Outer race.If cost is not a factor,that is probably the way to go.
Did you experience some bearing failures due to the passage of Electric Current?Just curious.
Thank you

GusD

 

[oldspark]

Thanks Gus for replying to my post and yes I could write a book on shaft current bearing failures. The company I work for is a little cautious about airing it's problems but I will try and be diplomatic. The magnitude of the problem caught everyone off guard and we have been playing "ketchup" ever since. We have went to two insulcote bearings and bristle grounding brushes on one end (two brushes), I am one of the people that has rebuilt over 90 percent of these units and believe me I have seen a little bit of everything. The grounding brushes are a backup to the insulated bearings as they are not perfect in operation. I want to put brushes on both ends of the shaft for extra continuity and longevity as we has some with this setup and seemed to work well. My company does not want it.

 

[GusD]

Hello oldspark.

Obviously you people have had enough grief with bearing problems due to circulating shaft currents.It's interesting to note that some plants having similar installations , have a terrible time with this problem,while others hardly heard about it.You guess you people fall on the "former"class.
Your original question about having multiple "ground brushes".I believe that having a greater resistance at the barings ,rather than the shaft/ground continuity, will make sure that current will flow through the brushes,not the bearings.Probably the more the marrier.(to a limit I guess).This is a problem, or should I say"phenomenon",that at times defies common knowledge.I have heard of people that did all of the right things,insulating housings,
insulated bearings,ground brushes and the bearings still failed due to Circ currents(fluting of races).It is also a problem during plant construction or new installations when "Welding" ground electrodes are clamped to Motors /Gboxes and they proceed to do welding.This too does a lot of damage to bearings,gears,couplings or
anything else that doesn't like to carry ><250 amps of zillions of electrons.This however don't cause typical fluting of the races.On stuburn problems,people have gone to Conductive greases ( to mitigate bearing acting as a Cap)and in extreme cases ceramic ball/roller bearings.It is expensive but if it corrects a problem it is worth it.

bye now
Good luck


GusD