Megging!

[techreptdh]

Hi All,

I was wondering what the thoughts are on doing a megger check on your compressors as a P/M procedure. We're having mixed opinions at our shop.
What do you all feel about this?

Please send your thoughts or comments, I'm curious.

Thanks!

[Six]

I dont see any thing wrong with it. Meggers output a lot of volts but verry little amps. Whats the argument against it ?

[Six]

Im assuming your not megging anything under a vacuum.

[techreptdh]

Glad you asked.

The majority feel theres more harm than good. Windings take a beating during the process. Don't know if its true.

[Achicagoperator]

I hated when meggering pm time came around.Would almost hope to find something grounded just to prove to myself that i wasnt completely wasting my time.If everythings running properly i see no need to megger.Seen guys misplace motor leads,wires,etc.after meggering.

[Six]

I hated when meggering pm time came around.Would almost hope to find something grounded just to prove to myself that i wasnt completely wasting my time.If everythings running properly i see no need to megger.Seen guys misplace motor leads,wires,etc.after meggering.

Now thats a great argument against it. I change my mind.

[will smith]

I have to say I find it to be useful. If the compressor meggs out poor but is still running, you have a chance to recommend a replacement before it turns into a burnout. If it burns, the customer loses the gas (and the only cheap refrigerant these days is the air drifting through the double hung valves), has to do a system cleanup :recovery and disposal of the burnt gas, probable system flush-RX11 isn't cheap- suction core drier, possibly with isolation valves if you can't pump it down, the obligatory liquid line drier replacement with drier replacements after it's back up and running, along with new gas. Not to mention the fact that if the system isn't cleaned up properly with follow up acid tests/drier changes, the new compressor won't last. All of which is VERY expensive.
A meg test can also tell you if you may have a lot of moisture in the system-which might be resolved with simple drier changes.
Tag your wires and don't lose stuff. Megging out a compressor is a chance for you to make money by doing things right. I say chance because just because you show a customer a bad meg reading on a compressor doesn't mean they will approve a replacement. You have to explain exactly what the test means and the worst case scenario. Then quote them the price for a new compressor. Also quote the price of a cleanup separately.
After that-it's up to them. But when (not if) the compressor burns out-at least you tried to do the right thing-and maybe next time they'll take your advice.

[KnewYork]

Windings take a beating during the process.

Really? In what way? All you are doing is inducing DC voltage into the winding. You do want to make sure that you are inducing the correct amount of voltage. You don't want to megger a 240 V. compressor with 1000 VDC. General rule is 2.1 times the name plate voltage. One exception being on medium voltage motors (4160 VAC). They should be meggered with a more sophisticated megger that can deliver 2400 VDC for 10 minutes.

[hvacker]

A megger isn't going to hurt anything. Those that claim that have a limited understanding of electricy. If that were so there would be warnings in big red letters. Don't Use This Thing!!
Where a megger is valuable is if on p&m's a log is kept so any changes can be seen over time. I've found a number of compressors, using a megger, that had the beginnings of failure where an oil and drier change (or two) corrected the problem. I wouldn't bother with small compressors < 10 ton. Just me.

[Six]

Really? In what way? All you are doing is inducing DC voltage into the winding. You do want to make sure that you are inducing the correct amount of voltage. You don't want to megger a 240 V. compressor with 1000 VDC. General rule is 2.1 times the name plate voltage. One exception being on medium voltage motors (4160 VAC). They should be meggered with a more sophisticated megger that can deliver 2400 VDC for 10 minutes.

Most of the smaller meggers out put around 800 v. I have yet to see a megger damage a motor. Amp draw is negligible from a hand held device thats powered bx 2 1.5v C batteries.

[KnewYork]

Most of the smaller meggers out put around 800 v

The megger I have has 500 and 1000V. settings. I occasionally verify with DC voltage meter. My analog megger has eight 1.5 volt batteries (which have lasted forever).

[pillclinton209]

meggers good for motors suck on compressors to many factors refrigerant oil etc get wierd readings

[KnewYork]

meggers good for motors suck on compressors to many factors refrigerant oil etc get wierd readings

Can't say I agree with this statement. I used meggers on early York hermetic chillers as well as present day hermetic screw compressors and they always worked there. I have also meggered oil pump motors that are submersed in oil...no problems there either.

[pillclinton209]

It matters on the compressor I have megged many only to find inconsistent reading due to refrigerant and oil even called copeland to ask about readings on a job I did which a new building lost one leg entering power and a bunch of compressors were grunting for about 15 min and got so many different readings and supposed bad insulation readings but haven't lost a compressor and followed with acid test was ok. Also if you megged the compressor on its first start then megg after warmed up diff readings due to liquid / oil etc

[pillclinton209]

Don't meg a low pressure centrifugal lol

[KnewYork]

Don't meg a low pressure centrifugal

May I ask why you wouldn't? In fact, I'll tell you why you think you shouldn't...because the motor is in a vacuum. It runs in a vauum doesn't it? When people say not to megger a motor in a vacuum they should be saying in a "deep" vacuum. Meggering a motor in it's normal conditions will not hurt it. You can megger a hermetic motor at 15" Hg and it won't hurt it. The motor is designed to run in those conditions. Introducing low current DC voltage under the same as its operating conditions won't hurt it.

[pillclinton209]

The reason they say not to meg a motor in a vacuum is because vacuum there is no atmospheric insulation between the phases and from phase to ground. Arcing can occur inside the motor. Even in a low pressure machine that's in a vacuum when it's sitting idle, the refrigerant is the atmosphere that helps to insulate the phases from one another and ground.

[pillclinton209]

recommend megging low pressure centrifigul raise pressure to 1 psi

[KnewYork]

The reason they say not to meg a motor in a vacuum is because vacuum there is no atmospheric insulation between the phases and from phase to ground.

I agree with this statement IF there is no refrigerant in the machine. When a megger test is normally performed there is refrigerant in the machine which concurs with your next statement.

Even in a low pressure machine that's in a vacuum when it's sitting idle, the refrigerant is the atmosphere that helps to insulate the phases from one another and ground.

So why would you go to the expense and trouble of raising the pressure to 1 psig?

Say what you will. I worked for the manufacturer long enough to know what was acceptable. Like I said before the motors run in a vacuum which is lower than the pressure normally found when the chiller is idle. Meggering the motor in those conditions will not harm it.

[Failure2Comply]

If I get a questionable meg reading I take an acid/moisture test of the refrigerant to see if the reading is caused by contamination or winding insulation beginning to deteriorate.