Megging an A/C Unit

busman

busman

Senior Member
Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
All,

I don't use a Megger all that often. I have an A/C unit that is tripping a CB. To determine whether it is the OFM or the Compressor (single phase residential unit), I disconnected the compressor and the unit started and ran the fan only. Thinking it was a shorted compressor winding, I disconnected all three leads. Connected one side of the Megger to ground and then tested all three three coil wires at 100 VDC - got short to ground on all three. I know I would get that on a shorted coil, but just for grins, I did the OFM motor which I'm sure runs and got the same readings. Am I missing something. If needed, I can go back and double check the findings.

As always, thanks for any insight.

Mark
 
We were taught that megger DC test voltage should be twice the motor voltage p!us a hundred volts. On a 120 volt motor I would first test at 250 volts then move up to 500 volts. For 480 volt motors we used 1,000 volts. From 1970 to the 90' s my company only had a few old Biddle company hand cranked netters that on
Only supplied 500 volts but always picked up grounded motor coils. Had a best in class Fluke model 1587 combination VOM /megger that supplied 5 test voltage from 50 to 1,000 volts DC. I own a 35 year old Simpson battery operated megger that puts out 250 to 5,000 volts. Have picked up grounded motor windings on 2,300 & 4,160 volt motors with it. I liked to sed at least 10 megohms to grounds on motors feed from VFD'S. Guessing Fluke still has a maybe 10 page how to use a megger instructions on their internet site. I printed it out for coworkers who did not have a good understanding in the easy to use meggers. While in Vo Tech school a few of us would hk!d hands then have somebody crank the 500 foot megger. Only received a small tingle until somebody let go. Worked with a great nuclear sub trained electronic guy who seldom worked on anything over 24 volts hot. Had him crank our megger while holding oupnto.leads tightly. He could not imagine how I could hold into test leads for as long as he cranked.
 
garbo said:
We were taught that megger DC test voltage should be twice the motor voltage p!us a hundred volts. On a 120 volt motor I would first test at 250 volts then move up to 500 volts. For 480 volt motors we used 1,000 volts. From 1970 to the 90' s my company only had a few old Biddle company hand cranked netters that on
Only supplied 500 volts but always picked up grounded motor coils. Had a best in class Fluke model 1587 combination VOM /megger that supplied 5 test voltage from 50 to 1,000 volts DC. I own a 35 year old Simpson battery operated megger that puts out 250 to 5,000 volts. Have picked up grounded motor windings on 2,300 & 4,160 volt motors with it. I liked to sed at least 10 megohms to grounds on motors feed from VFD'S. Guessing Fluke still has a maybe 10 page how to use a megger instructions on their internet site. I printed it out for coworkers who did not have a good understanding in the easy to use meggers. While in Vo Tech school a few of us would hk!d hands then have somebody crank the 500 foot megger. Only received a small tingle until somebody let go. Worked with a great nuclear sub trained electronic guy who seldom worked on anything over 24 volts hot. Had him crank our megger while holding oupnto.leads tightly. He could not imagine how I could hold into test leads for as long as he cranked.
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I get that, but I started at 100 VDC so as not to stress the windings - 2X voltage is more than the peak of the 120V sine wave. Since I got short at 100 VDC I didn't see any reason to go higher.

Mark
 
I'd simply start with yout DMM in ohms. 9/10 times, that will prove the matter. There is hot debate going with rookie techs condemning good compressors due to a common field meter that says "bad". Have to read the room. Does the compressor run? If so, check amperage against name plate. If not, first suspect is usually the run capacitor.

Also make VERY sure you are megging the compressor with the main plug pulled right on the compressor.
 
busman said:
I get that, but I started at 100 VDC so as not to stress the windings - 2X voltage is more than the peak of the 120V sine wave. Since I got short at 100 VDC I didn't see any reason to go higher.

Mark
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Forgot to state to remove all electronics from motors before using a megger. On drives always removed wires at drive then meggered there. If it showed a ground then would remove wires in motor packer head and megger motor leads. Have to be careful when charging up from drive location thru load wire & motor windings especially at 1,000 volts. If you happen to touch wires right after removing them you will get a shock.
 
Thanks. I gave the very detailed 64 page megger down.load a quick read thru. Heard of the 1,5 & 10 minute test but never saw anybody conduct one. One great thing about the bullet proof reliable Biddle company hand crank megger if crank handle was hard to turn up you know that you had a dead short.
 
Here is a newer Fluke version.
 

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Just as a mention since I think I've seen a few sparkies messing with HVAC. If the compressor is suspected bad as in shorted windings, that's what we call a 'burn out', and a fast check is to pull a sample of oil off a refrigerant port. The smell and color will be obvious, and the result is acid in the system, which is very bad. I mention a DMM because that test coupled with other obvious factors usually confirms a bad compressor before a megger is needed.
 
fastline said:
Just as a mention since I think I've seen a few sparkies messing with HVAC. If the compressor is suspected bad as in shorted windings, that's what we call a 'burn out', and a fast check is to pull a sample of oil off a refrigerant port. The smell and color will be obvious, and the result is acid in the system, which is very bad. I mention a DMM because that test coupled with other obvious factors usually confirms a bad compressor before a megger is needed.
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Do some compressors still have some type of internal safety device that back in the 1980's they told us to use a digital VOM to take resistance readings on winding . Appear the cheap analog meters were burning them out. Cheap 5,000 ohms per volt sensitivity meters were burning some of them out, while the old Simpson model 260's had a 20,000 ohm /volt sensitivity were okay to use.
 
garbo said:
Do some compressors still have some type of internal safety device that back in the 1980's they told us to use a digital VOM to take resistance readings on winding . Appear the cheap analog meters were burning them out. Cheap 5,000 ohms per volt sensitivity meters were burning some of them out, while the old Simpson model 260's had a 20,000 ohm /volt sensitivity were okay to use.
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I don't know what device that would be that could be damaged with any MM. I know OEMs have many times tried to point blame elsewhere though. They've always had thermal overloads in them but those have to be robust and handle inrush currents. 80s would almost exclusively be recip compressors.
 
All. Thanks. I went back and re-did. It was my error. I neglected to remove one of the OFM wires from the contactor. Mystery solved. Sorry to make this error.

Mark
 
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