# A/C compressor 3PHase issue



## Wirenuting (Sep 12, 2010)

Samjon said:


> Hi.
> I was working on a Roof top unit and checking the resistance of the compressor (3 Phase) windings.
> 
> L1/L2= 2ohm, L2/L3= 2 ohm, L1/L3= 1.2 ohm. No short to ground.
> ...


In a hermetically sealed unit or an oil emerged unit, over Work, contaminated oil and cheap units die quickest. If your lucky the unit had a motor saver inside the head that cut out the unit before compressor burn out. Hopefully someone didn’t bypass it to, just get by a little longer.


----------



## MechanicalDVR (Dec 29, 2007)

What kind of meter were you using to get your readings? Do you trust it 100%?

Was the battery in the meter new or old?

Did you take megohm readings on the windings at all?

I really wouldn't stress it, all motors have a life span, none run forever.


----------



## John Valdes (May 17, 2007)

What size is this motor? Large motor windings have very little resistance. Very hard to test with a meter.


----------



## joebanana (Dec 21, 2010)

Was the motor running before you took those readings? If not, what makes you think it's a motor problem? Did you put an amp clamp on it?


----------



## joebanana (Dec 21, 2010)

John Valdes said:


> What size is this motor? Large motor windings have very little resistance. Very hard to test with a meter.


Hey John...where's the "Like" option on your post?
Just for the record, I "Like" your reply.

Never mind, it's there now. WTF?


----------



## MechanicalDVR (Dec 29, 2007)

joebanana said:


> Hey John...where's the "Like" option on your post?
> Just for the record, I "Like" your reply.
> 
> Never mind, it's there now. WTF?


Gettin old sucks...

Didn't have your readers in hand?


----------



## joebanana (Dec 21, 2010)

MechanicalDVR said:


> Gettin old sucks...
> 
> Didn't have your readers in hand?


Yeah, I couldn't find them, then realized I was wearing them.
Now, I can't find my Depends..............Oh, wait.........Eeewwww.


----------



## MechanicalDVR (Dec 29, 2007)

joebanana said:


> Yeah, I couldn't find them, then realized I was wearing them.
> Now, I can't find my Depends..............Oh, wait.........Eeewwww.


That poor chair!


----------



## Wirenuting (Sep 12, 2010)

joebanana said:


> Now, I can't find my Depends......



Like I explained to my grandson many years ago, we call them pull up’s “Astronaut Pants”. Then I explained about the Mercury space program and Allen Shepard waiting for a delayed launch.


----------



## Samjon (Jan 19, 2017)

MechanicalDVR said:


> What kind of meter were you using to get your readings? Do you trust it 100%?
> 
> Was the battery in the meter new or old?
> 
> ...


I used a Fluke multi meter. Yes i megohmed compressor- O/L between windings and compressor shell.


----------



## Samjon (Jan 19, 2017)

John Valdes said:


> What size is this motor? Large motor windings have very little resistance. Very hard to test with a meter.


I didn’t get info. Off compressor. I’ll be heading back early next week to further investigate.


----------



## 1/2bender (Mar 30, 2018)

Is this compressor located in an industrial environment? If so perhaps the 5th ,7th harmonic causing counter tourque, overheating windings?


----------



## MechanicalDVR (Dec 29, 2007)

1/2bender said:


> Is this compressor located in an industrial environment? If so perhaps the 5th ,7th harmonic causing counter tourque, overheating windings?


Welcome aboard @1/2bender!


----------



## joebanana (Dec 21, 2010)

MechanicalDVR said:


> That poor chair!


Fortunately, I was still "living rent free" in a former members head at the time......and, it wasn't just the chair.
Then......I got the eviction notice, along with the County Health departments "red tag" for "unfit living conditions", now I gotta find a new place to _squat. :vs_laugh:_


----------



## MDShunk (Jan 7, 2007)

I'd say it's a safe bet that something remarkable has happened based on many many similar tests I've done on small hermetic compressors. I'd bet money that if you took amp draw readings on each phase while it's running they'll be different also.

As for whether it's mechanical or electrical? Well, it might have started out mechanical, but now you probably have an electrical problem. Good part is, it doesn't matter. A hermetic compressor is all one piece.


----------



## Samjon (Jan 19, 2017)

1/2bender said:


> Is this compressor located in an industrial environment? If so perhaps the 5th ,7th harmonic causing counter tourque, overheating windings?


No. Roof top unit on a commercial building.


----------



## John Valdes (May 17, 2007)

joebanana said:


> Hey John...where's the "Like" option on your post?
> Just for the record, I "Like" your reply.
> 
> Never mind, it's there now. WTF?


Joe. Its not you. There are times when I see no "Thanks" button as well.
I have no idea why, but I promise you I will find out.
Its not your glasses or your age.
I thought there might be a way to remove it from ones posts? There have been several times that button was missing.


----------



## pudge565 (Dec 8, 2007)

The windings should be fairly close not necessarily all the same. I have had motors that ran fine with readings like that and others that were dead. There isn't really a firm science to it. The readings are kind of useless unless you have a baseline to compare it to from install anyway. As long as the motor has been running fine I'd leave it alone. If it is opening OCPD I would recommend changing the motor based on those readings. However, if that is the case and you have access to a recording meter I would put that on first and see why it is opening.


----------



## JRaef (Mar 23, 2009)

Winding resistance makes very little difference in the operation of an AC induction motor. Electrically, it's the IMPEDANCE that takes the place of resistance in the motor equivalent circuit, and you can't test impedance with a 9VDC battery (which is what your Fluke is using). The difference between 2 ohms and 1.2 ohms of wire resistance is basically irrelevant. It just tells you that they are not shorted or open.


----------



## paulengr (Oct 8, 2017)

JRaef said:


> Winding resistance makes very little difference in the operation of an AC induction motor. Electrically, it's the IMPEDANCE that takes the place of resistance in the motor equivalent circuit, and you can't test impedance with a 9VDC battery (which is what your Fluke is using). The difference between 2 ohms and 1.2 ohms of wire resistance is basically irrelevant. It just tells you that they are not shorted or open.



This is partly true. If the motor is not turning or turned you can’t test the rotor. The stator is really just a bunch of coils (at least 6 for a 2 pole motor, more for more poles). So in a motor shop we usually test the insulation to ground resistance first because it’s fast and about 80% of failed motors fail this test. You can do the same test with a multimeter and it catches shorted motors about half the time or better.

The big thing though is an insulation resistance test is a safety precursor to running more advanced tests. If it fails megger, we don’t high pot or surge.

Then we test the resistance coil to coil using a low ohm bridge that typically pushes around 10 A to measure resistance down to the milliohm range. Typically contact resistance in starters and breakers is around 50 milliohms so this tool can find when coils are getting weak too. With small motors a standard multimeter does OK but it’s not as accurate. I don’t really trust it above 1 HP.

Then we come to a slew of tests. PdMA and some others use -“a high frequency signal to measure the inductance of the stator. Baker uses high voltage. The inductance is the other part of impedance


Sent from my iPhone using Tapatalk


----------



## paulengr (Oct 8, 2017)

paulengr said:


> This is partly true. If the motor is not turning or turned you can’t test the rotor. The stator is really just a bunch of coils (at least 6 for a 2 pole motor, more for more poles). So in a motor shop we usually test the insulation to ground resistance first because it’s fast and about 80% of failed motors fail this test. You can do the same test with a multimeter and it catches shorted motors about half the time or better.
> 
> The big thing though is an insulation resistance test is a safety precursor to running more advanced tests. If it fails megger, we don’t high pot or surge.
> 
> ...



They are different but typically inductance follows resistance. Taking these tests together covers about 99% of motor failures. So even though resistance isn’t importsu


Sent from my iPhone using Tapatalk


----------



## paulengr (Oct 8, 2017)

paulengr said:


> They are different but typically inductance follows resistance. Taking these tests together covers about 99% of motor failures. So even though resistance isn’t importsu
> 
> 
> Sent from my iPhone using Tapatalk




Important.

Checking for current imbalance while it runs is more definitive though than the diagnostic tests. We just do the others because it’s fast and easier. Approximating these tests with a multimeter sometimes works but it’s not as accurate.



Sent from my iPhone using Tapatalk


----------



## glen1971 (Oct 10, 2012)

Samjon said:


> Hi.
> I was working on a Roof top unit and checking the resistance of the compressor (3 Phase) windings.
> 
> L1/L2= 2ohm, L2/L3= 2 ohm, L1/L3= 1.2 ohm. No short to ground.
> ...


Is the motor running or were you there to troubleshoot an issue? Did you take current and voltage readings as well?


----------



## JRaef (Mar 23, 2009)

But a megger is not testing the insulation resistance with a 9VDC battery... that was my point. You NEED the megger to charge it up to a reasonable voltage to detect an insulation leakage issue. A simple ohmeter test from a DMM will only show you that the winding is not totally shorted or totally open. 80 milliohms is statistically insignificant, it's essentially a rounding error when compared to megaohms of resistance.


----------



## paulengr (Oct 8, 2017)

JRaef said:


> But a megger is not testing the insulation resistance with a 9VDC battery... that was my point. You NEED the megger to charge it up to a reasonable voltage to detect an insulation leakage issue. A simple ohmeter test from a DMM will only show you that the winding is not totally shorted or totally open. 80 milliohms is statistically insignificant, it's essentially a rounding error when compared to megaohms of resistance.




You’re mixing insulation resistance with coil resistance. If you have interturn shorts coil resistance drops. A surge test detects it in the first few turns. A high frequency tester detects it across the whole stator coil. And those tests penetrate out beyond the conductor and pick up weak coil insulation between turns. This is very different from measuring the varnish and other layers of slot and face insulation which is what the megger detects. A motor that ages out or has bearing problems or burns up shows up on megger tests. One that has taken a high voltage hit shows up on coil resistance / inductance. Not the same thing.


Sent from my iPhone using Tapatalk


----------



## paulengr (Oct 8, 2017)

Also megger tests if it’s not failed just measure the amount of contamination. And it’s not 89 but 800 milliohms. Absolute values aren’t used. Standards on coil resistance give 10-15% as a maximum.


Sent from my iPhone using Tapatalk


----------



## EB Electric (Feb 8, 2013)

paulengr said:


> Also megger tests if it’s not failed just measure the amount of contamination. And it’s not 89 but 800 milliohms. Absolute values aren’t used. Standards on coil resistance give 10-15% as a maximum.
> 
> 
> Sent from my iPhone using Tapatalk


I wouldn't get too bent out of shape about the exact milliohms for the OP's application. I'd be amazed if many service guys out there are pulling out a $4000 low ohm bridge meter for your standard 480v or 600v industrial 3 phase motor in rooftop equipment. Couple hundred HP motor sure it's a large frame and work to change, but I can't see the need to get in depth to the same level as testing as a 5kv 2500hp motor. Multimeter readings and a megger test is about as far as most guys are gonna go I would assume.


----------

