Megohmmeter on Hermatic compressors

[aaronh]

Hi folks, I am new here and I must say what a wonderful and very useful site this is.

I have a small dilemma which I would be extremley grateful if you could help me with.

I have heard that megohmmeters are not to be used on some hermatic compressors, I have spent severla days trying to find out why this is but still cant work it out. Any help on this matter would be very much appreciated.:D

[nike123]

Bul****! Only what is forbidden is make measurement while compressor is in vacuum.

http://www.mediafire.com/?11izldhsd3d

[monkey spanners]

Some small domestic sized compressors (Danfoss i think) have solid state starting devices in the windings that will be damage by the high voltage of an insulation test.

On bigger compressors i believe it is the thermistors that will not take high voltage, only megger the windings on these.

In all cases never megger test when the compressor is under vacuum as the high voltage can spark and damage a previously good winding.

Jon

[aaronh]

Thanks very much for the response, I knew that you couldnt carry out the test while the compressor was in a vacuum. I think it was mainly aimed at more domestic compressors. I thought that if by injecting a higher voltage using the megohmmeter it could ossibly have broken down the resistance in the windings or as you mentioned damaged the capacitors.

Or......... Could you get an incorrect reading from a megohmmeter if say the resistance was was not in the megaohms?

[monkey spanners]

Thanks very much for the response, I knew that you couldnt carry out the test while the compressor was in a vacuum. I think it was mainly aimed at more domestic compressors. I thought that if by injecting a higher voltage using the megohmmeter it could ossibly have broken down the resistance in the windings or as you mentioned damaged the capacitors.

Or......... Could you get an incorrect reading from a megohmmeter if say the resistance was was not in the megaohms?

When you test a comp for a short to ground you remove all the connections to it (make a note where they go) first or you could be finding a short in the wiring or even the fan motor for example as they often share a feed with the comp and make a mis diagnosis, as all the wires are removed there is no danger of damaging the capacitors.

I don't know to much about the small hermetic comps that are not suitable for megger testing, just remember reading about it somewhere that they had a solid state device inside the compressor that varied its resistance with temperature (or something like) so that it would disconnect the start winding at the right time when the comp starts.

[Frikkie]

I don't see any reason you can't Mega test any device using a test voltage that is approximately the same as the normal operating voltage.

[chilliwilly]

Interesting post, I never knew that you couldn't megger a pot under a vacuum. And I always understood that any associated controls were external to the pot and were disconnected when ohming or meggering.

If the controls were inside the pot you would be guessing if the windings were shorted or open wouldn't you? And you'd be replacing pots more often than you needed to.

Correct me if I'm wrong but the most common associated starting and control devices for single phase pots are relay, start capacitor, permanent split capacitor, and a klixon/thermal overload, which is always on the outside. And usually plug straight on or have spade terminals on the pot.

Although I have come across an old Russian domestic fridge that had one on the inside of the pot. And it had to be hit with a hammer to reset it when it had cooled down it was that old.

Anyway can someone explain to me why a pot can't be meggered under a vacuum? If a system has been evacuated down to say 500 microns, and gives an unsatisfactory low reading, you can outrule moisture in the system. And go for the fact its windings are shorting to earth/ground or between each other.

[Peter_1]

Danfoss small hermetics have their clixon inside the compressor, the Maneurop compressors (now Danfoss) and many hermetic 3-phase compressors have a clixon in the internal starpoint. Not to forget all the small servicefan motors.
Dielectric resistance decreases with lowering pressure. http://www.insulatorseal.com/graphics/DielectricGraph.gif

[chilliwilly]

Forgive me but it just doesn't seem or sound right. Air dielectric I'm familiar with, but how will that do any damage or show as a low reading or fault when testing with DC 500v+ to a winding to earth/ground, or between any of the other windings? Unless there's moisture or acid inside the pot. You would normally disconnect any controls or parrallel circuits to avoid any damage to them, and so you know the condition of the pot.

Even if the temp overload device is in the pot chamber and down to 500 micron, there won't be anything in there to conduct from the exposed metal of the temp overload contacts over to the pot casing. So I can't see how air would be a dielectric inside there when its the last thing you'd want anywhere inside your system.

Even the schelac on the windings wouldn't expand under that kind of vacuum for it to crack open and cause the voltage to conduct to the casing, and give a low reading. If it did the pot would be damaged before you commisioned it. Their dielectric constant or the insulation resistance wouldn't wouldn't change neither under a vaccuum.

If you think when your charging in the liquid phase dregs on the low side with a 400 series juice, on a day thats -15 deg C on a roof top package unit. You would have to simulate a pump down and shut off the high side from the receiver. Then let the compressor pull the low side down to say 800 microns briefly to flash off the excess liquid in your manifold and lines. Wouldn't you cause this dielectric weakness problem and cause the pot to cut out the o/l or blow a fuse if it was less than a 1/2 horse pot?

I have used this charging method on many occasion and never had the pot burnout or cause a problem with the protective device to trip.

So is there any chance of you or anyone else explaining this theory or fact of not meggering windings under a vacuum futher. And I appologise if I've appear to have come across as bombastic, I'm a laid back friendly kind of guy and I don't want folk to think ey up its him again. Its just a new theory or fact to me that I've never heard of and has got me thinking I've missed something out of my training and knowledge of fridge. And I also know that I don't know it all, but I would like to.