Rotor Winding IR Test

<...the alternator is bad and must be rewound/replaced....Any views on this?...>

Yes.

1. The <...Megger...> has fried some insulation somewhere inside the <...alternator...>.
2. Get on the phone to <...Leroy Somer...> and discuss it with them before doing anything further.
3. Time is money. Spend it on getting to the end of this matter rather with them rather than waiting and hoping someone on CR4 will provide a valuable answer to the problem. "A watched pot never boils" _{- Anonymous Poster #0}

Hi there,

To quote you "The <...Megger...> has fried some insulation somewhere inside the <...alternator...>"

The megger cannot fry anything in the alternator. During an IR test is the megger's job to introduce rising voltage (from 0 to 1000VAC) to the winding in question to test for breakdown to earth. It cannot fry anything, least of all a massive winding such as the rotor winding.

I guess my question was misleading. What I should have asked is for confirmation as to whether my test showed that the rotor is down to earth.

My megger indicates that there is breakdown to earth already occurring at 11VAC, just after the test is started, test voltage is introcuced at 0 VAC and starting to rise, and upon conclusion of the test after 60 seconds the IR resistance does not even register on the MegOhm scale on the meter.

Operational voltage to the rotor winding is between 0VDC and 100VDC after passing from the exciter and diodes to the rotor. IR test voltage is 1000VAC.

Actually I'm answering my own question here. I just wanted confirmation from someone in the know.

Well, if it wasn’t the <...Megger...> then it was something else that fried the insulation. So what? The solutions are the same.

It’s time to get on the phone.

The winding probably just needs to be re-coated with epoxy...

https://www.lasertechnologiesinc.com/lamination-refurbishing-core-plate-coating

Hi,

Thanks. Yes, we will strip the 1000KVA Leroy Somer alternator and send it away for refurbishment and/or replacement. I am on a major Chevron production site in Nigeria and way out in the swamp, so these jobs aren't easy.

I still want someone with knowledge about this to varify my initial question about my IR test.

You got it....refer to manual linked below or locate proper reference material for details...

http://www.genpowerusa.com/content/files/MANUAL-LEROY-SOMER-GENERATOR-LSA-49-1-M75-3-PHASE.pdf

https://www.genpowerusa.com/800-kw-LSA-49-1-M75-stamford-generator-alternator-1000-kva-3-phase/

Yes, it will require surgery, unless the brush rigging/ slip ring insulation has failed in a non-spectacular manner.

Unless it was subjected to a violent overspeed, it sounds like a warranty failure if it turns out the winding went to ground. If it was wet, you should get more than 11 volts.

Did you try reversing the Megger polarity? How about a regular Fluke on ohms?

What is wrong with the item, in the first place?

I explained it in the first post

Thanks all!

Conclusion: after using 3 separate Meggers to prove the same fault we now have conclusive evidence that the insulation resistance of the rotor breaks down to earth when load is introduced.

The alternator will be removed and replaced.

No you didn't. I expected you to say the alternator has no output, or it is getting very hot or the chassis is live.
If you get an ordinary multimeter, what is the resistance?
Get 6 x 9v batteries and 4k7 resistor and a LED and see if it illuminates.

And the next thing you do is get a 40 watt globe, preferably on a 100 watt isolating transformer and connect it to the transformer and then cut one lead and with the two probes, touch the two points you say have a low resistance and see if the globe illuminates.
The first law of electrical wiring is: NEVER TEST A FAULTY CIRCUIT WITH A MEGGER. It gives you a false reading. No circuit breaks down at 11v.