Why insulated bearings fail? 4

[THooper]

I have a customer that has a new 200hp 449T,4-pole, 950vac
belt drive motor. The customer said he has sent this motor back to the oem for premature bearing failure again (failed twice) in three months. The failure results of the bearing is evidence of electrical pitting in the bearing. (confirmed by bearing mfg.

My question is: The bearings that have been failing are insulated bearings, is it possible for current to flow through the shaft and bearing with one end insulated? I was under the understanding that is why mfg installed a insulated bearing in one end to prevent this problem?

Thanks in advance for your input.

 

[electricpete]

I agree with your thought process. It sounds like maybe the insulation is not effective for some reason.

 

[BJC]

Maybe that the current is not coming from the inside the motor. It could be passing trough from the belt via the chain or gearbox and out the motor frame.
Belt systems can generate static charge and it has to go somewhere. The amount of charge the belt generates depends on things like the material being moved, the belt material, weather etc. Some belts particularly where moving dry things that can blow up use grounding brushes that continually rub the belt. Since they don't appearing to be doing anything useful they don't get replaced when they should.
You did not say if you had a VFD on the belt. Shaft currents are most common in VFD drive motors.

 

[THooper]

electricpete, insulation was tested to be good with a megohmmeter.

BJC, Yes this application is operated off a vfd.

Check this article report I just found on the subject, out of EA, Let's solve your problems: It states, This problem apparently isn't common. But recent research indicates that some vfd's produce high-frequency harmonic voltages within the motor that can cause bearing currents to flow via capacitive coupling. Insulating the bearing doesn't help, because that insulation just adds to the dielectric component of the capacitance involved-the bearing assembly itself acts as a capacitor.
Solutions: include filtering on the line between inverter and motor, to mitigate the harmonics, or so called "Faraday shield" of copper foil, wrapped inside the stator bore and connected to ground (this is still in the experimental stage). The user's best bet is to consult with the vfd supplier.

 

[Lewish]

OK, my $0.02 worth.

First, you article is wrong. It is a common problem. It exists with any PWM generated voltage to any motor, whether that motor is AC or DC. As a designer of motor controllers, I am constantly being asked about this by customers.

The only solution that has proven mostly effective is to ground the motor shaft at the pulley end. I am still trying to come up with an electrical model as to why it matters which end you ground, but it does.

There are 2 popular methods of accomplishing the grounding. The first is to mount a carbon brush, like those used in a DC motor, in a holder with a spring to keep it against the shaft. Just like the brushes in a DC motor, or the sliprings in an AC motor. The second method is to do a 180 degree wrap of the shaft with a piece of copper foil of 10 to 15 thousandths of an inch thickness and about a quarter of an inch wide. One end of the foil is attached to the motor base and grounded, and the other end goes to a spring which keeps the foil in contact with the shaft.

A third method exists for very small horsepower motors, and that is to completely isolate the motor and motor drive voltage from earth ground. You use a virtual ground in the output of the VFD.

 

[jbartos]

Suggestion: Visit

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

etc. for more info

 

[BJC]

Shaft Grounding Systems make a good shaft grounding system. There website has gone away but there phone number is 541 926 2544. Lots of there kits on motors over at the big I (Intel) here.

 

[jbartos]

Suggestion: Visit

http://www.ab.com/drives/techpapers/ieee/APEC96 Mech.pdf

from the previously posted site for electrical equivalent circuit of shaft bearing and associated phenomena caused by VFD and higher frequency currents flowing through bearing to ground. Some dv/dt current values are given in Table 1.
There are also other related papers listed and useful references.

 

[THooper]

Appreciate all the great input and help! Now I have somewhere to start with this customer and application.

Thank's again
motorhead

 

[redtrumpet]

We must all be losing our memory cells to old age This topic was discussed thoroughly in thread237-11607, a thread in which motorhead 1 contributed several posts.

 

[THooper]

Thanks redtrumpet, I know I am getting old, I call them "brain farts" I should of tried the search first, but I sure like Lewish "new input" he has on the subject.

 

[BJC]

Back to static electricity from the belts.
I found lots of stuff on google about the topic. One is here. the artical address "conductive grease" for bearings.

http://www.belting.cc/beltdata/static-electricity.htm

There is a German company Kluber that makes conductive grease. the bearings most likely to be damaged by static electricy are head rollers and idler roller bearings.
I got into this subject once ( before the internet and google) and it's amazing the research you can do just sitting at you desk.

 

[micjk]

We are using ceramic bearings (very expensive) in some VFD motors that we have had terrible problems with electical arcing, not to mention even worse problems with oil from gearbox getting into the motors (hopefully we have found a cure for that also using a magnetic mechanical seal.
Regards,
MICJK

 

[THooper]

Interesting link BJC, This is an underground coal mine belt line and according to the link, allowed static electricity is 300 megohms. More issues on top of the other topics to inspect. One thing on the conductive grease that would be essential in the bearing, as stated in the article, Jbartos helpful link posted say's, conductive grease is formed by suspending metallic particles in the grease. Laboratory test data indicates conductive grease accelerates mechanical wear by 60% and would shorten the life of the bearing. Very good article on the subject.

 

[jbartos]

Question to redtrumpet (Electrical) Nov 6, 2002 marked ///\\\.
We must all be losing our memory cells to old age This topic was discussed thoroughly in Thread237-11607, a thread in which motorhead 1 contributed several posts.
///Please, what is your experience with the keyword search? Does it work for you accurately? I tried it several times and often I am getting such postings that are not relevant to the keywords I input. Should not this problem be posted under some computer wizards Forum sections?\\\

 

[redtrumpet]

jbartos - there was a problem with the search function that was brought up in the Round Table a while ago. Apparently it wasn't working properly for a while but it seems to be functioning okay now.

FWIW, the default parameter is to search for "any of the words", which usually returns too much garbage, so I always have to remember to click "all of the words" to get relevant results. I searched on "bearing currents" and the thread link I posted was the first thresd returned by the search engine.

 

[jbartos]

Thank you very much, redtrumpet.