Megger/ohm meter needed please!!!!

I haven't needed to use meggers much, but this sounds like absorption or capacitance and would be normal.

I'm pretty sure this is the problem.

Maybe @ChapmanF can give a more definitive interpretation.

 

We do not use meggers much in big phone (ish...) but I'd say Jerry is right on both counts.

I'm not even sure that I can use a megohm meter that doesn't have a crank on it.

"Here....hold these leads for a second........"

 

I couldn't get the doc in this link to download for me.
It might be that Big Bell is being anal about my browsing or something else.


THIS worked for me:
www.testequipmentdepot.com/megger/pdf/low-resistance-testing.pdf

Thanks!

 

This seems to be evidence that the ground fault is within MG2. What happens if you use a regular ohmmeter and perform the same test on MG2? Does it show infinite resistance; or does the regular ohmmeter also show 2 or 3 megohms of resistance?

 

'Twas something else. Like me putting the wrong URL there. Now fixed in the original post.

I wanted to link to that one because I had already plugged a Megger pamphlet for the high-resistance end of things.

I take it that of your two photos, the one showing 4 MΩ is MG2, and the one showing 1.8 GΩ is MG1?

The 10 MΩ go/no-go figure in the manual is a minimum. If MG1 has an insulation resistance 180 times higher, we won't hold that against it.

MG2 having less than half the minimum, now that's not looking good for MG2.

I take it you got the high readings on MG1 for each of U to ground, V to ground, and W to ground?

And the low readings on MG2 were similar for all of U to ground, V to ground, W to ground?

What test voltage were you using? (Edit: n/m, I can see in the photos the 1 kV button's pressed.)

The behavior you saw with MG1 is not unexpected, a reading that starts one place and rises quickly, then more slowly, eventually reaching a plateau. During that rise, you are storing energy into MG1, first by charging up its capacitance to match the test voltage of the meter, and then more slowly by packing some charge into the insulation. You're looking for the reading near the plateau.

(By the way, all the energy you stored into MG1, while that reading was rising, needs a way out when you're done with the measurement. Your megger should be built to automatically bleed that energy off after you let go the test button. That can take at least as long, even several times as long, as the time it took to charge, so you don't want to be in a hurry and unhook the test leads right away. Not sure how carefully that's covered in the cheapie instrument manual ... it looked a little thin.)

Sounds like MG2 behaved the other way though: initially looking like it was above the minimum (still not as far above as MG1's initial reading), but moving in the wrong direction over the duration of your test. Seems like that insulation has been compromised somehow.

 

Sounds like MG2 initially presents as a couple hundred meg, then drops under the influence of high test voltage. It's possible the low voltage from an ordinary multimeter doesn't make that happen, and the multimeter just reads overrange.

By "quickly disconnect after testing", I hope you mean "let go of test button, wait for capacitance to discharge, then move test leads for next test."

I just looked at a 2-page VC60B PDF manual from the web and it seems to skip right over that part, which seems like asking for trouble. With their $50 price point, they're going to have customers who don't already know, and they're not informing. If you unhooked the leads quickly and lucked out of a bite, it's still not a habit you want to cultivate.

There's more about that on pages 25 through 27 of Stitch. They suggest waiting roughly 30 to 60 seconds, or around four times as long as you had the test button held, if that's longer.

A megger will typically be built so that when you let go the test button, the display reverts to being an approximate voltmeter, so you can watch while it drains the charge out, and see when it's safely low.

I don't know whether the VC60B has that feature ... its manual doesn't seem to tell me.

 

I've never used one without a crank. And that was probably 10 years ago.

Sounds to me like you did. See post 21.

 

I've changed my mind ... after also reading the one ETC(SS) found (which is by the Megger people), I think it's the one I would have linked to in the first place. (Though the Cropico one does provide some interesting data tables the Megger one doesn't.)

So once more for good measure, https://www.testequipmentdepot.com/megger/pdf/low-resistance-testing.pdf

 

(munching popcorn).......

 

Yup ... we've got the physics explaining why good megger readings go up.

Your MG2 going down was not a good megger reading.

What readings did your new used part's windings end up at, when the increase slowed down?

 

What yards are you using?

Did you try HP Motors in Rancho Cordova?

They might be worth a shot as they have rack after rack of Prii there.

 

How many good transaxles does one Prius need?