Why insulated bearings fail?
Why insulated bearings fail?
(OP)
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.
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.





RE: Why insulated bearings fail?
RE: Why insulated bearings fail?
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.
RE: Why insulated bearings fail?
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.
RE: Why insulated bearings fail?
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.
RE: Why insulated bearings fail?
http://www.ab.com/drives/techpapers/ieee/ieee.html
etc. for more info
RE: Why insulated bearings fail?
RE: Why insulated bearings fail?
http://www.ab.com/drives/techpapers/ieee/APEC96%20Mech.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.
RE: Why insulated bearings fail?
Thank's again
motorhead
RE: Why insulated bearings fail?
RE: Why insulated bearings fail?
RE: Why insulated bearings fail?
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.
RE: Why insulated bearings fail?
Regards,
MICJK
RE: Why insulated bearings fail?
RE: Why insulated bearings fail?
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?\\\
RE: Why insulated bearings fail?
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.
RE: Why insulated bearings fail?