Electrical insulated bearings on DC Motors
Electrical insulated bearings on DC Motors
(OP)
We are familiar with the NDE insulated bearings on AC drives and both bearings insulated on AC drives for VFD applications.
We have a large 750HP DC motor which has a classic fluted raceway approx 120 degrees of arc on the inside of the outer race. A perfect set of frosted balls and a frosted band around the inner race way after less than 6 months service. Is it normal to see this on DC motors ? Is it acceptable to insulate only one bearing NDE, or do both bearings need to done. Bearing size is 6320. Application is Ski Lift Gondola.
We have a large 750HP DC motor which has a classic fluted raceway approx 120 degrees of arc on the inside of the outer race. A perfect set of frosted balls and a frosted band around the inner race way after less than 6 months service. Is it normal to see this on DC motors ? Is it acceptable to insulate only one bearing NDE, or do both bearings need to done. Bearing size is 6320. Application is Ski Lift Gondola.





RE: Electrical insulated bearings on DC Motors
RE: Electrical insulated bearings on DC Motors
Xc=1/(2*Pi*F*C); at high frequencies the insulated bearing is a conductor.
That is why a shaft grounding brush should be added. See the commercial of Aegis advertising at the top of this site.
RE: Electrical insulated bearings on DC Motors
It is capacitvie coupling and that makes the use of one insulated bearing ineffective since the non insulated bearing now will get all the discharges and will fail even faster. That has been demonstrated very convincingly. So, instead of getting more hours between bearing breakdown, you get less if you install one insulated bearing. Two insulated bearings would be OK, though.
A grounding brush is the solution. An interesting observation is that also an oily and dirty slip ring/brush combination reduces EDM in the bearings. We had EDM breakdowns in the thousand PPS region (at 200 V/microsecond) when the brush was lifted and only a few hundred breakdowns at 50 V/us when there was oil on the ring. Wiping the ring brought the EDM down to zero at all levels.
One should know that 50 V/us is a harmless level so the cleaning of the slip rings and brushes normally isn't needed. These motors have been running dor several years and history confirms that dirty slip rings are not a problem.
Gunnar Englund
www.gke.org
RE: Electrical insulated bearings on DC Motors
Gunnar Englund
www.gke.org
RE: Electrical insulated bearings on DC Motors
Always go for NDE insulated bearings and shaft grounding on any motor.
RE: Electrical insulated bearings on DC Motors
Thank you for sharing all that valuable real life experiences. The figures you provided are amaizing.
RE: Electrical insulated bearings on DC Motors
We don't have room to put an Aegis brush as the sheeve is almost up to the motor bracket. Do Static conductive belts work OK or are they just limiting the amount of current flow ? Thanks once again
RE: Electrical insulated bearings on DC Motors
I think that the conclusion that belts charge the shaft/rotor comes from the observation that directly coupled machines don't have EDM. The reason is that a direct coupling (not insulating) does the same job as the grounding brush in shorting the shaft to ground. Also, the belt looks a little like a van de Graaf generator - which it is not.
The charge build-up from a belt is quite slow. I have measured voltage ramps lasting for tens of milliseconds and that doesn't produce more than 10 to 100 PPS, which is a safe EDM frequency.
DC motors used to be equipped with grounding brushes. But that practice seems to be more or less be abandoned during the last ten or twenty years. The reason is probably that bearing currents in AC motors is better known and DC motors are now regarded obsolete and not having "modern" problems.
BTW, an Aegis device can be divided in two halves and fitted without a long shut down of the lift. Dividing it does not reduce its effectveness.
Gunnar Englund
www.gke.org
RE: Electrical insulated bearings on DC Motors
On a more general note and perhaps going against the flow of what is written above, is this dc motor being subjected to any high frequency voltages? If not, what you are experiencing perhaps is old-fashioned shaft voltages due to geometric assymetry within the motor, which is low frequency and can be successfully cured by insulating one bearing only. e.g. with reference to the following Siemens tech note we have cause 1 and not cause 2:
http:
One problem perhaps with shaft brushes is that their performance deteriorates with time, due to contamination. No experience of them so perhaps I'm about to get shot down in flames...
RE: Electrical insulated bearings on DC Motors
I would like to send you a presentation from 2003, where I have collected facts and analyses as well as measurement techniques that are unique to the business.
You can surely make out how to contact (kontakt in Swedish) me so that I can send you the presentation. Then we an take the discussion from there.
I also know you well enough to hope that you are not offended by what may be seen as some bragging.
Gunnar Englund
www.gke.org
RE: Electrical insulated bearings on DC Motors