Bearing insulation on shaft

[jimmy2times]

Reading a past thread now closed "bearing insulation resistance test limits for INSTALLED motors"

Does anyone have the paper NETA paper that was given as a link. Is there any better guidance since this time on the subject.

We have marine electrical propulsion application where POD drive has been overhauled in dry dock. Bearings on bench are greater than 9gohm, installed back on shaft together with mechanical seals (could not be tested off shaft) resistance of shaft to ships hull is less than 500kohm at 100vdc test megger, multi suggests 300kohm. Pass criteria is 1mohm. Motor compartment assembly has been forced ventilated (normal compartment cooling not possible until pod housing reinstated) for few days on thought it was humidity but no change. Now at point where ship needs to come out of dry dock and great expense to return.

Any thoughts.

 

[electricpete]

Here's a link to the referenced thread:
thread237-208967

(the NetaWorld Link within that thread doesn't work)

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(2B)+(2B)' ?

 

[electricpete]

Here is another thread on same subject from another board

http://www.maintenance.org/topic/bearing-insulation-ressitance-limits-for-installed-motors

Just a data point to add, applies to newly-refurbished motors (may not be particularly helpful if your goal is to understand the minimum reasonable for in-service motors

https://www.easa.com/sites/files/resource_library_public/EASA_AR100-2010_1010.pdf

4.2.8 Bearing insulation should be tested with a 500V meg-ohmmeter. Insulation resistance should be 1 meg-ohm or greater

We have one large non-vfd motor family where the OEM specifies 3,000 ohms, minimum. (i.e. 3kohms). That's the lowest I've heard. We have in the past accepted even lower readings on non-vfd large motors for specific circumstances and limited duration. I don't know whether special considerations apply to your application.






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(2B)+(2B)' ?

 

[ScottyUK]

Slight humidity plus salt = terrible IR values. If you're in a port environment there could easily be enough atmospheric deposition to cause a problem.

As an example, I'm responsible for a legacy switchboard which has suffered heavy internal contamination from saline air and which is now disturbingly susceptible to atmospheric moisture. A maintenance team working all day exhaled and sweated enough moisture into the switch house to collapse the IR readings of the busbars, which then recovered overnight. We can't get the salt contamination out, so we're nursing this switchboard with permanent dehumidification until it is replaced.

 

[EdStainless]

Scotty, I feel for you. We had an incident years ago and ended up pressure washing the inside of an enclosure (and all hardware) with high purity water.
It was the only way that we could get leakage low enough.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[ScottyUK]

It is a maintenance engineer's worst nightmare - business-critical switchboard, a historical lack of maintenance because it was too difficult to release for maintenance, and an extremely aggressive atmosphere (wastewater processing from a crude oil facility) which has rotted the equipment from the inside outward. When we took it out of service it shocked my maintenance team, and most of us have been around the patch for a few years - I submitted a report which effectively condemned the board last June, and we've been working flat out figuring out how to replace it without shutting the facility down and then preparing to execute that plan.

Those who previously thought it was 'too difficult to release for maintenance' have radically shifted position!