hi pot testing

[electrofelon]

We tested an old - circa 1930? - GE alternator: 3-phase, 6 leads,
250kw, 22 poles, 144 stator slots, 327 rpm, 2300v. We used a Quadtech
Sentry 20 Hipot tester. This does not directly test IR (insulation
resistance), so we derived the IR's using Ohm's law. We ran tests at
1000v and 2300v for 1 minute. All tests with the hipot were completed
to a "Pass" state. We tested phase to ground and phase to phase. Our
results were all less than 1 Meg-Ohm, say between .320 and .460
Meg-Ohms. Can anyone comment on these values and what this implies for the health of the stator?

A small side question: I have never heard of 2300V (L-L) distribution, can anyone comment on that?

 

[mkgrady]

We tested an old - circa 1930? - GE alternator: 3-phase, 6 leads,
250kw, 22 poles, 144 stator slots, 327 rpm, 2300v.

A small side question: I have never heard of 2300V (L-L) distribution, can anyone comment on that?

I remember seeing it in boston and in a Rhode Island navy base in the early 80s

 

[big john]

...All tests with the hipot were completed to a "Pass" state....

Does this simply mean the high-pot didn't trip? High-pot results can still be unacceptable without the machine tripping off.

...We tested phase to ground and phase to phase. Our results were all less than 1 Meg-Ohm, say between .320 and .460 Meg-Ohms. Can anyone comment on these values and what this implies for the health of the stator?

Yes, they those values are very poor. IEEE 43 says that your bare minimum IR should be 1M? per 1000V plus 1M?. So your minimum safe insulation resistance is 3.3M? and for a clean, dry winding should honestly be significantly higher than that.

Older winding insulation is often hygroscopic and will suck up moisture very readily and this often results in poor readings. How long had this generator been offline when it was tested? What was the winding temperature? What was the ambient humidity?

 

[GoldDigger]

Does this simply mean the high-pot didn't trip? High-pot results can still be unacceptable without the machine tripping off. Yes, they those values are very poor. IEEE 43 says that your bare minimum IR should be 1M? per 1000V plus 1M?. So your minimum safe insulation resistance is 3.3M? and for a clean, dry winding should honestly be significantly higher than that.

Older winding insulation is often hygroscopic and will suck up moisture very readily and this often results in poor readings. How long had this generator been offline when it was tested? What was the winding temperature? What was the ambient humidity?

So if you put it into service and let it heat soak for days at operating temperature you would expect the resistance to improve? Or would the change be permanent?

 

[big john]

So if you put it into service and let it heat soak for days at operating temperature you would expect the resistance to improve? Or would the change be permanent?

I could not in good conscience tell someone to apply working voltage to it, there's too much of a gamble. But it's very unlikely those values are permanent.

For small units like this our MO is to clean them, tent them, and heat them; often using a couple of large space heaters and DC welders connected to the windings. Repeat meggers over the course of several days and you should see all the winding IR values trending up uniformly.

 

[electrofelon]

Does this simply mean the high-pot didn't trip? High-pot results can still be unacceptable without the machine tripping off. Yes, they those values are very poor. IEEE 43 says that your bare minimum IR should be 1M? per 1000V plus 1M?. So your minimum safe insulation resistance is 3.3M? and for a clean, dry winding should honestly be significantly higher than that.

Older winding insulation is often hygroscopic and will suck up moisture very readily and this often results in poor readings. How long had this generator been offline when it was tested? What was the winding temperature? What was the ambient humidity?

Yeah I was having a really hard time just finding what "good" IR values should be, but then I found some figures that made these look pretty low. Good to know that formula, thanks. The unit is stored in a unheated garage with concrete floor - older, probably no vapor barrier under the slab. Humidity/moisture very likely a factor here. Test was on a hot humid day, but cooler inside, maybe around 70. Unit has been off line for decades. We were considering this unit for a hydro project. My partner is talking to a few motor shops about options.

 

[GoldDigger]

Whether you do it yourself or shop it out, the process big john described is well worth a try.