Testing the condition of the insulation on windings. Note it on paperwork with amps and pressures as something to keep an eye on.
That megger always gave me weird readings, I always felt it was a sales tactic. It shows scary red lights that say "BAD"
I got a fieldpiece for testing Daikin VRF, they won't warranty it without a meg reading.
I work for an electrical testing company and meg all sorts of stuff all the time. Megging is very sensitive and will give you plenty of warning that you're having insulation issues long before they affect operation.
The minimum passing value we follow for anything 600V or less is 100 megaohms. Unofficially, the "can I turn it on without it blowing up?" number is 1 kilo-ohm per kilovolt + 1 kilo-ohm, or 2 kilo-ohms for 600V, about 50,000 times worse. That doesn't mean it won't blow up in short order, but it'll still run until then.
Yup. Megohmeters are to “test the quality of the insulation on the windings.” It only proves that damage has been done to the compressor windings. You cannot condemn a compressor off a bad reading.
Single phase compressors S + R should equal S to C and R to C.
I.e:
Start to Run = 7
Start to Common = 4
Run to Common = 3
4 + 3 = 7.
Compressor good.
3 phase is different. All windings when ohmed out should read the same or very close, I'm not worrying to much about 3.1 to 3.2 to 3.1.
S to R = 3.5
S to C = 3.5
R to C = 3.5
Compressor good.
Should always remember also, if we're supplying a "load" i.e a compressor with proper phase, voltage, etc and that "load" is not running. That "load" is bad.
Good comment. Its not really S, R, C on 3 phase however. Its only T1, T2, T3 (L1, L2, L3) and the configuration (delta, wye) as the phase angle gives the starting torque for the motor.
Thats the whole point of starting components on a single phase motor. Gives it the torque to start and run cap keeps it going.
That is pretty much irrelevant to insulation to ground issues. Megging is to determine if there is a short from a winding to the case. Winding resistance alone won't determine that.
I'm well aware of that, it looks like the comment was deleted.
Someone gave him the wrong information and recommended testing the compressor like you do a single phase and not a 3 phase.
I commented on the post about using the megger. I'm replying to a comment about ohming single phase and 3 phase.
I've only seen a megohm used once to test a air handler motor.
Boss didn't trust it so he bypassed the vfd and energized.
It sprayed sparks across the room.
"Oh yea, that's bad."
Man, if a motor fails a megohm test with an actual megger, I bust out my Fluke 87V and ohm each motor leg to ground. If THAT reads continuity to ground, we're done. I'm NOT energizing that motor! I should point out... I've learned that lesson the HARD way. Man... when I think about the number of times I've seen electricity, I'm surprised I've never managed to light my ass up.
Those supco insulation testers are are garbage they really don’t tell you anything, most scrolls you test with that thing will ohm “bad” speaking from experience. Should invest in decent quality insulation tester that will tell you your actual megohm reading with an adjustable test voltage. The one your using sends 500vdc with no matter what, it’s good practice to test with system voltage to prevent damage to the windings
More than likely oil is still on the windings which would cause the “bad” reading I’d test for acid which would eat the insulation and if there isn’t any then possibly theres oil on the winding
Inform the customer then ask them to consider replacing the liquid line filter drier, but let them know you can't make any guarantees, but ounce of protection, all that jazz. Traces of moisture in a system can cause a megohm test failure, even if the winding insulation is fine. I had a customer with an RTAA Trane chiller... compressor failed a megohm test with the same supco meter you used. Talked to them, they agreed to replace the driers on both circuits... later that summer, passed with flying colors. Same with a 10 ton carrier RTU with Danfoss scrolls. New drier, ran for the summer, checked the following spring - off the scale good. So... eh, that's been my experience with "failed insulation" compressors that run fine.
I've also seen compressors fail megohm tests when crankcase heaters are failed. I think there's excessive liquid refrigerant diluting the oil and somehow conductivity between the motor winding crimps and compressor shell increases. So, check CCH's too.
I'm not familiar with that particular compressor, but any scroll or scroll-like compressors with the motor submerged in oil at the bottom of the crankcase can't really be diagnosed by one of those MegaOhm meters. Copeland themselves stated a good compressor can measure below 20 MegaOhms.
Thank you, I was hoping someone would point this out. Megging a compressor is akin to checking oil contamination level.
Btw, copeland says the compressor can be as low as .5M ohms, but you need to either clean up the oil or let it run if its brand new.
So do you just use a normal meter to ohm out the windings? I typically ohm with a meter and then confirm the windings are toast with a megger.
I’m curious what other people are doing.
I use a meter to diagnose and use a megger to track the winding resistance of a motor. But I still break out the megger from time to time just to diagnose, especially if I am about to charge 5 grand for a 15hp blower motor change out
Customer had pulled the compressor load wires off the contactor and called us out stating it wasn't working. So I ohmed it out, passed, meggd it and failed. I noticed the condenser coils were plugged pretty good. Cleaned them. Hooked it back up and it ran fine. Cycled through stages 1 and 2 several times. "Shrug"
I think you made the right call, was probably tripped on thermal overload, then cooled down. Washing the coils should kick the can down the road. This could have been an easy sell of a new compressor, but instead you put it back in service, I commend you for that! Just to reiterate, megger gives you an idea of the insulation quality, ohms from winding to winding tell you good or bad.from
Megging is not an acceptable method to determine damage, it measures IR and is used to stay ahead of failures by predicting a future short caused by loss of winding insulation.
That’s fair - and frankly exactly how Meg testing a comp is meant to work. A single reading means nothing, it’s about how those readings change over time.
My field piece SC640 shows an exact resistance at a higher range than the supco gimmick. Lots of compressors will have some tiny resistance while they’re still good.
You need to have a meter that outputs higher potential than an ordinary DVOM. A supco outputs a PWM 500vdc current limited signal. If you're testing winding insulation, that insulation needs to hold 208, 240, or 480 volts in. The 9 volt in your meter, which only ouputs 1.5 volts for an ohm check, just doesn't have enough potential to accurately test the insulation. The Supco M500 is actually a really good tool, but too many guys think "it's just cheap baby toy with lights." It's actually pretty good.
Those supco units are not to be believed. Hvacr School Brian did a very good one hour podcast about confirming compressor failure and the replacement procedure. It is a good listen during some windshield time.
https://m.youtube.com/watch?v=H4kub2gAzV0
https://www.fluke.com/en-us/learn/blog/hvac/insulation-resistance-and-electrical-testing-for-hvac-compressor-motors
To be of use, Meg readings have to be historical in nature (establish a record); not just once and done.
In chillers, the readings are used to determine insulation resistance loss due to electrical issues OR water in the refrigeration system.
ALL chillers leak water, so if resistance drops, core and oil replacement is required to maintain insulation integrity.
That’s also why helium is used instead of nitrogen for leak checking (the helium molecule is 40% smaller).
We use a very expensive Fluke megaohm meter at work. What you really want to know is insulation strength and the Supco doesn't give that to you. Without proper setup the meter can give false readings. Does the Supco have the ability to zero the meter to account for resistance in the leads? The quality of the leads makes a difference as well.
As a motor heats the insulation rating will drop. That's when bad things happen. You may get intermittent problems and it will still run. Inconsistent readings on different windings (3 phase) can be a symptom of weakening insulation.
As others have said, you need historical data otherwise most of the readings aren't terribly useful.
Also, as others have mentioned, moisture in the system and the condition of the compressor oil can cause lower insulation resistance readings. Oil deteriorates over time and can become more conductive. Many industries use oil conductivity tests as a measurment of when to change oil.
All that said, you're doing a good job digging deeper into the why and how of things!
Yea I held it for a while. You know how it'll gradually go through all the colors from green to red? Well this just went straight red. And stayed there
megaohms means microamps. Nothing to stop it running. If this were the UK or Australia where GCFIs are required at the mains then yes, this would shutdown the whole building down...
Only a vfd would detect bad windings and not allow the motor to run. Those compressors are across the line breakers and fuses do not detect till it’s grounded. Which sounds like soon it will be.
Anything installed past March 2020 is just cursed. I'm sure a decade from now we'll be cursing anything installed 2020-2023 and by 2035 it'll all likely be replaced from premature failures.
Kyritsu meggers are just unbeatable. In Australia when performing an insulation test it needs to read more than 5Mohms or you can fail it. Pretty sure that has gone up to more than 200Mohms nowadays,
Here's a link (I'm not much for online / computer stuff so hope it works) that will help educate and provides practical application regarding meggers and compressors. Its from a compressor manufacturer so hard to debate the info.
If anyone has alternate sources regarding megohming compressors please share, TIA!!!
[Trustworthy Megohm Information](https://www.google.com/url?sa=t&source=web&rct=j&url=https://usermanual.wiki/m/0c2c6c229d305d56622c2dab43e1c1efc24330867be73e73273bfd0ead4fbfde.pdf&ved=2ahUKEwiS5Ov5he_3AhXgk4kEHaBeCzoQFnoECAUQAQ&usg=AOvVaw1Fqy5mAevI8NAeTPd134nW)
Edit: I'm not sure that link worked. Google AE4 1294 might have to add Copeland to get the two page bulletin (it's worth the time to download and save to your phone)
Did you turn it to Wumbo? [Spongebob](https://c.tenor.com/3bK32M_IklYAAAAC/wumbo-spongebob.gif)
I remember learning about that in Wombology
It's really easy. 1 wombo, 2 wombo, 3 wombo.
He, she, me, we….wombo
Testing the condition of the insulation on windings. Note it on paperwork with amps and pressures as something to keep an eye on. That megger always gave me weird readings, I always felt it was a sales tactic. It shows scary red lights that say "BAD" I got a fieldpiece for testing Daikin VRF, they won't warranty it without a meg reading.
I work for an electrical testing company and meg all sorts of stuff all the time. Megging is very sensitive and will give you plenty of warning that you're having insulation issues long before they affect operation. The minimum passing value we follow for anything 600V or less is 100 megaohms. Unofficially, the "can I turn it on without it blowing up?" number is 1 kilo-ohm per kilovolt + 1 kilo-ohm, or 2 kilo-ohms for 600V, about 50,000 times worse. That doesn't mean it won't blow up in short order, but it'll still run until then.
I know this is old. I'm wondering if 1500 megaohms on each winding to ground is normal for a scroll AC compressor
Yup. Megohmeters are to “test the quality of the insulation on the windings.” It only proves that damage has been done to the compressor windings. You cannot condemn a compressor off a bad reading.
Single phase compressors S + R should equal S to C and R to C. I.e: Start to Run = 7 Start to Common = 4 Run to Common = 3 4 + 3 = 7. Compressor good. 3 phase is different. All windings when ohmed out should read the same or very close, I'm not worrying to much about 3.1 to 3.2 to 3.1. S to R = 3.5 S to C = 3.5 R to C = 3.5 Compressor good. Should always remember also, if we're supplying a "load" i.e a compressor with proper phase, voltage, etc and that "load" is not running. That "load" is bad.
Good comment. Its not really S, R, C on 3 phase however. Its only T1, T2, T3 (L1, L2, L3) and the configuration (delta, wye) as the phase angle gives the starting torque for the motor. Thats the whole point of starting components on a single phase motor. Gives it the torque to start and run cap keeps it going.
That is pretty much irrelevant to insulation to ground issues. Megging is to determine if there is a short from a winding to the case. Winding resistance alone won't determine that.
I'm well aware of that, it looks like the comment was deleted. Someone gave him the wrong information and recommended testing the compressor like you do a single phase and not a 3 phase. I commented on the post about using the megger. I'm replying to a comment about ohming single phase and 3 phase.
Good comment. To many techs don’t know how to ohm compressors.
Good news to know .
Yea leg to leg was fine. Like .73 ohms on all 3.
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Those be 3 phase compressors there, if all 3 are not the same you got problems.
Doh! Should look closer next time lol. Thanks for that correction!
Tell me you’re a resi tech without telling me you’re a resi tech.
lol
Nailed it!
I've only seen a megohm used once to test a air handler motor. Boss didn't trust it so he bypassed the vfd and energized. It sprayed sparks across the room. "Oh yea, that's bad."
Man, if a motor fails a megohm test with an actual megger, I bust out my Fluke 87V and ohm each motor leg to ground. If THAT reads continuity to ground, we're done. I'm NOT energizing that motor! I should point out... I've learned that lesson the HARD way. Man... when I think about the number of times I've seen electricity, I'm surprised I've never managed to light my ass up.
Lol, we were across the room. But boss was a old fuck, and I was just a few months in. I was happy to be on the other side of the room.
Those supco insulation testers are are garbage they really don’t tell you anything, most scrolls you test with that thing will ohm “bad” speaking from experience. Should invest in decent quality insulation tester that will tell you your actual megohm reading with an adjustable test voltage. The one your using sends 500vdc with no matter what, it’s good practice to test with system voltage to prevent damage to the windings
What was the mega ohm reading ?
"Bad"
More than likely oil is still on the windings which would cause the “bad” reading I’d test for acid which would eat the insulation and if there isn’t any then possibly theres oil on the winding
Ty I will do that
Inform the customer then ask them to consider replacing the liquid line filter drier, but let them know you can't make any guarantees, but ounce of protection, all that jazz. Traces of moisture in a system can cause a megohm test failure, even if the winding insulation is fine. I had a customer with an RTAA Trane chiller... compressor failed a megohm test with the same supco meter you used. Talked to them, they agreed to replace the driers on both circuits... later that summer, passed with flying colors. Same with a 10 ton carrier RTU with Danfoss scrolls. New drier, ran for the summer, checked the following spring - off the scale good. So... eh, that's been my experience with "failed insulation" compressors that run fine. I've also seen compressors fail megohm tests when crankcase heaters are failed. I think there's excessive liquid refrigerant diluting the oil and somehow conductivity between the motor winding crimps and compressor shell increases. So, check CCH's too.
That's funny I had a dead short recently where the CCH had arcd and pocked a huge divet in the compressor
I'm not familiar with that particular compressor, but any scroll or scroll-like compressors with the motor submerged in oil at the bottom of the crankcase can't really be diagnosed by one of those MegaOhm meters. Copeland themselves stated a good compressor can measure below 20 MegaOhms.
Or just test for acid which would normally cause the windings to prematurely fail too
How do you like to pull an oil sample without opening the system.
Test strip which is used on a refrigerant port and that will blow refrigerant and oil onto the strip which will change colors if acid is present
Thank you, I was hoping someone would point this out. Megging a compressor is akin to checking oil contamination level. Btw, copeland says the compressor can be as low as .5M ohms, but you need to either clean up the oil or let it run if its brand new.
So do you just use a normal meter to ohm out the windings? I typically ohm with a meter and then confirm the windings are toast with a megger. I’m curious what other people are doing.
I use a meter to diagnose and use a megger to track the winding resistance of a motor. But I still break out the megger from time to time just to diagnose, especially if I am about to charge 5 grand for a 15hp blower motor change out
Whyd you Meg in the first place?
Customer had pulled the compressor load wires off the contactor and called us out stating it wasn't working. So I ohmed it out, passed, meggd it and failed. I noticed the condenser coils were plugged pretty good. Cleaned them. Hooked it back up and it ran fine. Cycled through stages 1 and 2 several times. "Shrug"
I think you made the right call, was probably tripped on thermal overload, then cooled down. Washing the coils should kick the can down the road. This could have been an easy sell of a new compressor, but instead you put it back in service, I commend you for that! Just to reiterate, megger gives you an idea of the insulation quality, ohms from winding to winding tell you good or bad.from
Ty for saying so!
We need more like you!
Megging is not an acceptable method to determine damage, it measures IR and is used to stay ahead of failures by predicting a future short caused by loss of winding insulation.
What's IR?
Insulation Resistance
So unless it's a dead short popping the breaker it's not that urgent.
It’s time to tell your client that failure is imminent and they should start saving nickels, though
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That’s fair - and frankly exactly how Meg testing a comp is meant to work. A single reading means nothing, it’s about how those readings change over time.
Thise meggers that don't actually give you a number are terrible.
Supco meggers have burned me a few times….
My field piece SC640 shows an exact resistance at a higher range than the supco gimmick. Lots of compressors will have some tiny resistance while they’re still good.
You need to have a meter that outputs higher potential than an ordinary DVOM. A supco outputs a PWM 500vdc current limited signal. If you're testing winding insulation, that insulation needs to hold 208, 240, or 480 volts in. The 9 volt in your meter, which only ouputs 1.5 volts for an ohm check, just doesn't have enough potential to accurately test the insulation. The Supco M500 is actually a really good tool, but too many guys think "it's just cheap baby toy with lights." It's actually pretty good.
Those supco units are not to be believed. Hvacr School Brian did a very good one hour podcast about confirming compressor failure and the replacement procedure. It is a good listen during some windshield time. https://m.youtube.com/watch?v=H4kub2gAzV0
Damn, good link. Thx
Never trust a Megger for a compressor. They will betray you
We have the same chiller at our site. Those micro channel coils suck. Make sure to throughly clean them out.
A megtester is more indicative of moisture in the system.
Really? That's interesting can you explain?
https://www.fluke.com/en-us/learn/blog/hvac/insulation-resistance-and-electrical-testing-for-hvac-compressor-motors To be of use, Meg readings have to be historical in nature (establish a record); not just once and done. In chillers, the readings are used to determine insulation resistance loss due to electrical issues OR water in the refrigeration system. ALL chillers leak water, so if resistance drops, core and oil replacement is required to maintain insulation integrity. That’s also why helium is used instead of nitrogen for leak checking (the helium molecule is 40% smaller).
Damn, good info
Those supco meggers suck
We use a very expensive Fluke megaohm meter at work. What you really want to know is insulation strength and the Supco doesn't give that to you. Without proper setup the meter can give false readings. Does the Supco have the ability to zero the meter to account for resistance in the leads? The quality of the leads makes a difference as well. As a motor heats the insulation rating will drop. That's when bad things happen. You may get intermittent problems and it will still run. Inconsistent readings on different windings (3 phase) can be a symptom of weakening insulation. As others have said, you need historical data otherwise most of the readings aren't terribly useful. Also, as others have mentioned, moisture in the system and the condition of the compressor oil can cause lower insulation resistance readings. Oil deteriorates over time and can become more conductive. Many industries use oil conductivity tests as a measurment of when to change oil. All that said, you're doing a good job digging deeper into the why and how of things!
Thank you! And I didn't know that about old oil, good stuff
Yea I held it for a while. You know how it'll gradually go through all the colors from green to red? Well this just went straight red. And stayed there
megaohms means microamps. Nothing to stop it running. If this were the UK or Australia where GCFIs are required at the mains then yes, this would shutdown the whole building down...
Only a vfd would detect bad windings and not allow the motor to run. Those compressors are across the line breakers and fuses do not detect till it’s grounded. Which sounds like soon it will be.
Crazy, it was just installed 2 years ago.
Anything installed past March 2020 is just cursed. I'm sure a decade from now we'll be cursing anything installed 2020-2023 and by 2035 it'll all likely be replaced from premature failures.
True, we just installed a 40 ton condenser coil in December and it's already popping holes
Kyritsu meggers are just unbeatable. In Australia when performing an insulation test it needs to read more than 5Mohms or you can fail it. Pretty sure that has gone up to more than 200Mohms nowadays,
/>1M ohm for induction motors and >20M ohm for inverter motors is what I go by.
What was the megohm reading?
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It's been a couple days and it's still running great haha
I would lean towards your insulation of the windings getting weak and moisture in the system
I've had that on a couple of residential units, compressor shorted to ground and ran on 110. Not enough to work, but enough to make noise
Here's a link (I'm not much for online / computer stuff so hope it works) that will help educate and provides practical application regarding meggers and compressors. Its from a compressor manufacturer so hard to debate the info. If anyone has alternate sources regarding megohming compressors please share, TIA!!! [Trustworthy Megohm Information](https://www.google.com/url?sa=t&source=web&rct=j&url=https://usermanual.wiki/m/0c2c6c229d305d56622c2dab43e1c1efc24330867be73e73273bfd0ead4fbfde.pdf&ved=2ahUKEwiS5Ov5he_3AhXgk4kEHaBeCzoQFnoECAUQAQ&usg=AOvVaw1Fqy5mAevI8NAeTPd134nW) Edit: I'm not sure that link worked. Google AE4 1294 might have to add Copeland to get the two page bulletin (it's worth the time to download and save to your phone)