shaft &bearing currents 3

[zub]

Hello everybody,

What is shaft and bearing current? How does it occure? What are the indications? How can you measure? is there any thread about this topic before?

Thanks in advance.

zub.

 

[busbar]

Pardon me, but please have the simple courtesy to spend a few minutes in a search.

 

[zub]

Dear Busbar,

I'm actually keeping in search but i couldn't find a right source. i have found two papers on the situation. (One from ABB, one from SIEMENS) But they are global overview on the subject. I want to find something on the theoretical part. Most probably you have better resources, thats the reason why i am asking to you. And of course i don't have a good english to choose the right words while talking. But i am sure that you can tolarete this?...


Thanks again...

zub.

 

[johnspark]

Zub, if you go to the IEEE web site and look at especially industrial applications, you will find a myriad of articles including colour pics to help you. Once you are at this web site, you will only require a few minutes like busbar has suggested to see the abstracts.

cheers Johnspark

 

[busbar]

Maybe
thread237-11607
thread237-18455
thread237-19830
thread237-5626
thread237-30254
thread404-10671
thread404-22856

 

[busbar]

zub, language aside, the skill for keyword guessing for an online search seems to be as much "art" as it is "science." It’s trying to think on the same terms as the author, and that can be difficult in any language.

 

[jbartos]

Suggestion: The following site lists several good references:

http://www.ab.com/drives/techpapers/ieee/ieee.html

 

[jbartos]

Suggestion: It is a good idea to make a distinction between the motor shaft and bearing currents. Some are coming from fast IGBT (and similar fast electronic switching devices) switching, others are coming from the rotation of a long shaft, e.g. the ship shaft with a propeller that is actually a peculiar generator of the ship shaft currents.

 

[powerjunx]

hi, Zub.
shaft and bearing current of rotating machines; just a hint, a keyword "electrical knittings"

 

There has been a lot of research done in this area. This problem is common to all varible frequency and DC drives that use power electronics. Look up on the web for a bearing fluting and you can get a lot of answers.

 

[zub]

Thank you all for your responses. in my situation there is neither AC nor DC drive. it is a slipring asynchronous 6,3 kV, 2,5 MW. motor and some indications on the bearings. in my understanding this bearing current situation looks like the eddy current! is that right or am i totally wrong. (i know it is a different thing from the eddy current but i mean the same reasons the same effects...)

 

[IEEMT]

You stated that you are using a motor with a slipring !

1-On what type of maching are using this motor ?
2- Are you taking about cluch & Brake slipring to control a milling machine ? This type is used to control speed and braking and is mounted to the end of shaft of the motor.
??????

 

[snpyon]

from the looks of this thread, i believe we're referring to a generator/motor set.. am i correct?

 

[GusD]

The information related to bearing damage caused from circulating motor shaft currents is as extensive as the causes for circulating currents.Or so it appears.
You checked ABB websight and they do have some info,most of it very generic.
The problem of circulating shaft currents are not limited to the use of VFDs (Hi-switching IGBTs),they may be present on any rotating machinery that has the capability of creating Static Electricity discharges.
The most common cause of CCurrents may be an Assymetrical magnegtic field.Most of our motors on VFDs, have either insulated NDEnd bearing housings, or we have shafts that have a brush rigging to Bypass the NDEnd bearing.
In some cases ,people have done all of these things and the bearing problems still persists.In other cases, people have changed greases to conductive types, which may help .On very stuborn cases of very critical machinery people have opted for bearings with porcelane elements.Very expensive i would guess.
I guess I am fortunate in the fact that over 30 years in the Electric motor business,I have seen very few (less than a handful) bearings failed due to flutting.
In addition to the above,I read some info to the fact that a rotating bearing at times behaves much like a capacitor. Its races being one plate and balls or rollers the other ,with a dialectric separator grease.
If you have a problem ,it can be very complex to solve.

Thank you
GusD

 

[jbartos]

Suggestion: There are bearing currents originated from fast switching (ringing) of ac motor speed controller fast switching devices, and others are caused by common mode currents that originate from harmonics that are caused by switching that may be slow, however, it is still nonlinear causing harmonics of lower frequencies.

 

[GusD]

More info regarding motor shaft currents.
This past week I had the opportunity to test 4 new 600 hp /600 volts motors to be used on VFDs.
I noticed that all the NDEnd bearings (ball)had a bearing "outer race" that was coated with a ceramic material.We purchased similar motors 6 years ago that were equiped with shaft ground brushes.I like this new idea better than the ground brush.
I had not seen one of these bearings before, much less have one in my hands until last week.
Of all the methods that are used to mitigate shaft currents,this one may very well be the best.
I do not know if these bearings are a lot more expansive than standard bearings. GusD