bearing insulation resistance test limits for INSTALLED motors 2

[electricpete]

Industry standard documents specify an insulation resistance of 10 megaohms (EASA AR100) or 1 megaohms (EPRI 1000897) minimum when tested at 500vdc. These limits apply for motor refurbishment. What limit would you consider appropriate for motors tested while installed (not recently refurbished).

This specific motors in question is aa group of large vertical motors (non-vfd) that drive a vertical pump through a rigid coupling. Both top/bottom bearings of the motor are insulated.

We routinely remove these motors to get access for pump work (the pump bearings have had a lot of problems, although none has been specifically attributed to shaft current).

When the motor is removed, we do an insulation resistance test. Typically the insulation is < 1 megaohms. Sometimes we can improve it by cleaning / inspecting easily accessible areas. Sometimes we can't get it above 30 k-ohms without disassembling the motor.

Are there any standards that address this situation?

Does anyone know typical ranges of shaft voltages and how much current is required to cause damage? (might use this to guesstimate min acceptable resistance)?

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[Zogzog]

You can refer to NETA standards, but they are higher than 1 Meg. Depends on the type of motor. Are you temp correcting your readings to 40 Degrees C?

 

[rovineye]

I can't cite standards, as we don't really have any applicable to my industry (shipbuilding)in this case. I can tell you what our OEM tests to, and considers acceptable.

On our installed but new 20 mW motors (VFD, 2000 V) the criteria is the same as we are given for our 11mW 6.6 kV generators:
100 volt megger
> 50 k ohms new
> 1 k ohms in service

If we are not able to uncouple to test (and we usually can't), we check for voltage while running, from the shaft to the bearing pedestal (other side of the insulation). If any voltage is present, the insulation is OK. A lack of voltage does not mean the insulation is bad, only that it is then necessary to uncouple and test.

I have tested a few of our motors uncoupled, and then for voltage while running. When the insulation measured 500+ megohms the measured voltage was very low, only a volt or so, but was measurable. But that was under a very light load.

At the factory tests we use 500 volt meggers on the generators and 100 volt on the motors. Min acceptable on generator is 20 megs, and I don't recall on motor. Every test at the factory has been between a low of 558 megs and a high of 1.4 gigs, and that covers a population of about 40 machines.

 

[electricpete]

zogzog - thanks. I have of course heard of temperature correction for winding insulation resistance, but I have never heard of applying temperature correction to bearing insulation resistance. It is not mentioned in EASA AR100 or EPRI 1000897. What reference tells you to temperature correct bearing insulation resistance?

rovineye - thanks. Which OEM had the 1k / 50 k ohm limits?

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[Clyde38]

With regards to the bearings, you may consider posting on the "Bearing design, manufacture, and maintenance forum".

http://www.eng-tips.com/threadminder.cfm?pid=821

 

[Zogzog]

Not sure what standard it came from but you need to campare apples to apples, I leanred this the hard way in the Navy working on MG sets, didnt know about temp correction and was pulling my hair out because my reading were below the spec, after temp correction all was good.

 

[rovineye]

electric Pete, the mfr is Alstom/CONVERTEAM.

The NETA link below briefly discusses a lack of standard and how high and low some organizations set the bar.

http://www.netaworld.org/files/ItemFileA340.pdf

This other link is from US Bureau of Reclamation. Their thrust bearings are checked with DMM ohmmeter and only need to be 10k.

http://www.usbr.gov/power/data/fist/fist3_11/vol3-11.pdf

 

[electricpete]

Thanks guys that is some great info. I especially appreciate the very helpful links from zogzog.

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[electricpete]

I'm sorry. I meant the links from rovineye. Thanks.

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