Electrical insulated bearings on DC Motors 2

[Mendit]

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.

 

[davidbeach]

Better yet, ground your shaft to the motor frame. The high frequency switching on the DC could cause the same problems on DC motors as the high frequency noise does in VFD systems.

 

[aolalde]

I agree with Davidbeach, the high frequencies are not limited to AC motors. Insulating the bearings will not eliminate capacitive coupling at high frequencies (the rotor and frame become a capacitor, C) and remember that the capacitive reactance Xc tends to zero when frequency (F) tends to infinite.

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.

 

[Skogsgurra]

I did some comparative measurements on DC motors with and without shaft grounding brushes last week. As aolalde says, there is a good capacitive coupling between armature winding an rotor (winding is in slots in rotor) and that makes the effect more pronounced than in an AC motor.

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

http://www.gke.org

 

[Skogsgurra]

Sorry. May need to explain that EDM = Electric Discharge Machining (electric erosion) and that PPS = Pulses Per Second (used to count number of discharges in bearing). Also, V/microsecond is the trigger level for the counter. EDM at 50 V/us is usually harmless while 500 - 1000 PPS at 200 V/us usually will destroy a bearing in less than half a year.

Gunnar Englund

http://www.gke.org

 

[Drivesrock]

Yes, I have had this too. In one case, customer wanted to blame the DC drive, mains borne interference, earthing, harmonics etc. have complicated and goofy studies performed. The real reason were the belts coupling the motor to the machine. The static electricity generated on the belts was discharging to earth thru the dc rive bearings as was not able to do so via the connected machine. Shaft grounding brushes resolved the problem. The same machines without belts never experienced the same problem.

Always go for NDE insulated bearings and shaft grounding on any motor.

 

[aolalde]

Skogsgurra:
Thank you for sharing all that valuable real life experiences. The figures you provided are amaizing.

 

[Mendit]

Thanks all of for the valuable info, we are in the process of putting a grounding brush on the rotor shaft in the center at the sheave end as this the most convienient spot right now. They can not shut down now as it is the middle of the ski season and they would strand people on the mountain.
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

 

[Skogsgurra]

It doesn't matter, really, but the main source of bearing EDM seems to be the winding/rotor capacitance and not from the belt. I have had some charging from belts, but not anything serious.

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

http://www.gke.org

 

[UKpete]

Sorry to contradict you Gunnar (I say that respectfully, believe me) but EDM is also a problem in engine-driven single bearing alternators. The larger machines say above 1000kVA are often fitted with an insulated bearing, which is of course at the non-drive end.

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://www.sea.siemens.com/motorsbu/product/Technical Documents/TS003.pdf

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...

 

[Skogsgurra]

I am used to all kinds of opinions with regard to bearing current and bearing damages. And, Pete, I know from several years of contacts in these fora that you always are a reliable and good source of information. But, in this case, I think that my field experience (where I am hired by Siemens, ABB, SKF, paper and steel mills to do EDM measurements) since more ten years has taught me more than most other guys.

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

http://www.gke.org

 

[UKpete]

Thanks Gunnar.