Trending Motor Meggar Readings??? 1

[RickDerkach]

I would like to take a moment of your time and ask your professional opinion in regards to motor meggaring.

Currently, our facility uses off line testing approach - meggar for the condition of our motors under a specific hp range, environment and criticality. We do use a Baker testing approach - resistance to milliohms on a few assets, and partial discharge for our 13.8kV+ motors. We are looking into MCA, but are not quite there.

Presently, I am developing an asset care program, using RCM methodology's, and software that allows us to track and trend field reading and trigger alarms if deviation from normal occurs.

In the event of motor meggaring, have anyone seen meggaring of motors trended before? I have read a few white papers showing how meggar reading have trended and allowed for a better understanding and condition, using a condition based monitoring approach on a set frequency or even a failure finding approach used after a period of shutdown.

Also, if we don't presently have a greater line of defense other than meggaring until new PdM technologies come into play at our mill, would this process of trending assist and be affective? I am really scratching my head!?

I thank you greatly for your time and hope to clarify my understanding and better my knowledge in regards to motor testing.

Yours sincerely,
Rick

 

[Zogzog]

There are 2 reasons to do a IR test (Megger).
1. Ensure the insulation resistance is above acceptable values
2. Trending

What does a 800M reading tell you, really nothing by itself, but if it was 2G last year, you have a problem coming soon.

Make sure to temp correct your readings or your trending will be useless.

 

[RickDerkach]

Thanks Zoozog. Is there any white papers that are out there that you know expressing the reasons to IR? It would be nice to see so direction and clarity in regards to the trending.

Your comment was greatly appreciated.
Rick

 

[flexoprinting]

Hi Rick

I have uploaded a file from Avo, or you can find good information at Fluke web site.

Best regards

 

[catserveng]

It seems like a pretty old standard, but Meggers "A Stitch in Time" is still one of the best references for IR testing, here's a link,

http://www.omnicontrols.com/articles/Megger-A_Stich_in_Time.pdf

Hope that helps

 

[Zogzog]

Those would be the 2 references I would look at also.

 

[oldfieldguy]

The key to trending of test data is a consistent and accurate test regimen. The old days of cranking the handle on the megger and taking down the first reading where the needle stops swinging isn't good for trending, and in too many electrical departments, that's the best you'll get.

A good carefully constructed procedure and a training program to make sure the data gatherers understand how and why to apply it can give useful data. However, I've seen few facilities that go this step. Some of my (former) clients had pretty decent programs but they did two things: 1) Applied the program to large motors only and 2) used an outside testing company to do the data gathering in lieu of training their own staff.

old field guy

 

[rovineye]

True, you need to temp correct for trending, but you can't correct for humidity. A polarization index doesn't take much longer and eliminates the correction problems.

 

[electricpete]

You might take a look at item 3-1 and possibly 3-4 at the link below. 3-1 has a lot of good info not just on insulation resistance testing, but other types of insulation testing:

http://www.usbr.gov/power/data/fist_pub.html

We trend insulation resistance on large critical motors, including thirty 13.8kv motors and around sixty 4kv motors and thirty 460 v motors (we have lot more 460v motors, but not considered critical). Over the course of 8 years and around 6 motor failures, there is only one motor failure where an advanced warning showed up in the trend, and the change was not significant enough that we paid attention to it. (post-failure inspection showed the motor had significant oil contamination that eventually caused a failure in the endwinding).

We have had several of our outdoor motors that meggered bad because the space heaters had been off for some reason. So we dried the motors until we could pass the test. That was probably a useful exercize that we might have overlooked if we didn't test.

One big decision you have to make is whetehr you will determinate for test. If you don't, sometimes the cable insulation resistance will be much lower than the motor and you are trending nothing but the cable. Also it can be work, particularly if the terminations need to be taped with Raychem. If you do determinate, realize that your quest to monitor improvement to improve reliability has the potential to introduce a failure if the termination is not reassembled properly. (we have never had that happen).

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