Megging questions

[mickeyrench]

We have three large sump pump motors 100 hp each 480 volt. one megged @ 2.2g.and one @ 60m. The third megged at 1meg. I did it 3x. I notice when megging and it was @ 1000vdc the voltage is usually at 1050vdc . This time when megging the voltage would go from 800-900 vdc and alittle lower and would only go to 1meg . I did it three times and changed the batteries too. Then meg the other two motors and the megger seemed to work fine.
Any ideas about this pump motor. Does the motor appear to be on the way out? It is under water when working and could the seal have failed and water enter the motor? Used a fluke 1587.

 

[richxtlc]

That sump with the 1 megohm reading at less than 1000 VDC is below the rule-of-thumb spec, 1 meg per kv of rating plus 1 meg. The minimum should be 1.48 megohms. I'm not sure what NETA says for that motor or what the manufacturer's spec say, but absent that, 1.48 megs is the minimum.

 

[mickeyrench]

We have three large sump pump motors 100 hp each 480 volt. one megged @ 2.2g.and one @ 60m. The third megged at 1meg. I did it 3x. I notice when megging and it was @ 1000vdc the voltage is usually at 1050vdc . This time when megging the voltage would go from 800-900 vdc and alittle lower and would only go to 1meg . I did it three times and changed the batteries too. Then meg the other two motors and the megger seemed to work fine.
Any ideas about this pump motor. Does the motor appear to be on the way out? It is under water when working and could the seal have failed and water enter the motor? Used a fluke 1587.

I also megged between the 3 motor leeds and the meter came up 0.0 .
I did megg motors after the bad reading and meter worked fine.

 

[Cow]

Have you pulled the pumps to test them or are you testing them while they're still down in the hole?

The second motor looks to be on the way out, I hope they keep a spare on hand. The third is toast, I'd replace it. You've verified it isn't the cable feeding them?

The last time I had a 100HP motor test around 30-50Megs, it ran 2 more weeks then blew up. I've megged quite a few motors with my Fluke 1507, it seems like they usually test really high(several hundred megs-gigs) or really low(just a few megs). So there's usually not much left to interpretation, they're either pretty good or just plain junk.

 

[zog]

We have three large sump pump motors 100 hp each 480 volt. one megged @ 2.2g.and one @ 60m. The third megged at 1meg. I did it 3x.

Ok, lets look at some real specs. In this case ANSI, NETA, NEMA, and IEEE all agree most 480V motors (There are some exceptions depending on winding types, and year of manufacture) should be >5M at 40 degrees C. I can't stress enough how important the temperature correction is. What was the winding temp at the time of the test?

That spec is for a 60 second spot reading, how long was your test voltage applied?

Did you calculate a DAR? Sometimes the DAR tells you more than the IR test but in this case it seems pretty straight forward your 3rd phase is failing and end of life is near.

I notice when megging and it was @ 1000vdc the voltage is usually at 1050vdc . This time when megging the voltage would go from 800-900 vdc and alittle lower and would only go to 1meg

Simply not enough behind a 1587 to maintain voltage with that poor of a IR reading, too much leakage current.

 

[zog]

That sump with the 1 megohm reading at less than 1000 VDC is below the rule-of-thumb spec, 1 meg per kv of rating plus 1 meg. The minimum should be 1.48 megohms. I'm not sure what NETA says for that motor or what the manufacturer's spec say, but absent that, 1.48 megs is the minimum.

That "thumbrule" was removed from any recognized standards in 1979, I suppose it is bette than nothing but dosent hold very true when you get to MV applications.

 

[zog]

I also megged between the 3 motor leeds and the meter came up 0.0 .
I did megg motors after the bad reading and meter worked fine.

The windings are all connected and should only be a few Ohms, so you would expect a megger to read close to 0. A real winding resistance reading can be valuable info.

 

[mickeyrench]

did not remove pumps . they are underwater in a well. I ran them in hand and they all pumped and drew close in amps. The only test i know how to do is the one minute test. love to learn how to do a better test. thanks again

How would a temperture correction be done when motors are under water?

 

[brian john]

That sump with the 1 megohm reading at less than 1000 VDC is below the rule-of-thumb spec, 1 meg per kv of rating plus 1 meg. The minimum should be 1.48 megohms. I'm not sure what NETA says for that motor or what the manufacturer's spec say, but absent that, 1.48 megs is the minimum.

That RULE OF THUMB will buy you a blow up.

 

[ptonsparky]

Use the low ohm scale for reading the resistance of the windings when the motor is connect as it would be to run. Not sure what you will see with that large of motor, but they should all be the same.

I am fairly new at megger use but if I have three identical motors side by side and they all read as yours have, one has a major problem that will soon become apparent. Could be just the seal and contamination or the cable but now is the time to pull it up. I agree, hope you have a spare and in this case maybe two.

Make sure to disconnect the leads from the starter and meg again. Contamination in the starter changes everything.

 

[mickeyrench]

did not remove pumps . they are underwater in a well. I ran them in hand and they all pumped and drew close in amps. The only test i know how to do is the one minute test. love to learn how to do a better test. thanks again

How would a temperture correction be done when motors are under water?

how would i megg a motor that has parrallel motor cables?

 

[Cow]

You need to pull the motor. Disconnect the parallel motor cables from the motor and isolate each of them, then meg everything.

I'd pull the other motor too and check it out.