hi can anyone help me

[dean_thomas]

ive got a question im not 100% sure on

Q why should you not use a megohm meter on a hermetic copmressor motor windings ??

is it something that could damage the winding or the compressor its self ??

any help would be grate thanks


dean

[chemi-cool]

1. It will not damage the windings.
2. Meger gives a 1000V between the windings and the ground and yes, you can use it with no problem on hermetics.

[NoNickName]

CAVEAT: the compressor must be under nitrogen or other gas charge. The megaohmmeter may damage the windings only if the compressor is under vacuum, because of possible discharge arcing between naked conductors.

[Emmett]

NoNickName,
I am a little confused by your response, we use meggars on all kinds of motors including compressor windings, with a charge and without. Why would a compressor in a vacuum be an issue, if the conductors are naked this is what you are looking for, any exposure in the insulation is a bad thing and should not occur whether the compressor has a charge or is in a vacuum. Please help me to understand your answer.

[benijoseph]

I Think, avoid using meg till it is a must, for a new unit for QC it can be used for insulation test but for a used system foriegn matters in between conductors can cause damage to the system.
regds,

[Emmett]

I Think, avoid using meg till it is a must, for a new unit for QC it can be used for insulation test but for a used system foriegn matters in between conductors can cause damage to the system.
regds,
This dose not make any sense to me is this just an old wifes tale or is there some science behind it? The purpose of using a meggar is to identify weakness in the insulation surrounding the conductor, the voltage from the meggar cannot create damage it only identifies if there is damage to the insulaion. Why would a vacumm have any effect on the operation of a meggar? why would any foriegn material have any effect on a meggar? if the insulation is good then it is good if it is not then you will find that out by using a meggar. Please can someone explain or provide an opposing view with anecdotal or scientific evidence?

[Grizzly]

The below is extracted from one of Tecumseh's manuals.
I am confident that other manufacturers state the same.
Grizzly

Vaccum Pumping

Never switch on a compressor when under vacuum: an arcing may occur (corona effect) between the terminals or a terminal
and earth. This arcing causes deposits of conductive carbon that can affect the insulation of the terminal. It may even cause it
to disintegrate with the accompanying risk of gas and oil leakage. Always check that the terminal cover has been properly fitted.

Never carry out an electrical safety test when the compressor is under vacuum. The same phenomenon as described before
may occur.

[stufus]

This is coming up in another thread also !
Good man Grizzly The Corona effect certainly ring's a bell !
I'd add a few rep points as that has been bugging me from the other thread but alas no facility for rep points at the moment,
I'll owe you one:D
Cheers
Stu

[james10]

NoNickName,
I am a little confused by your response, we use meggars on all kinds of motors including compressor windings, with a charge and without. Why would a compressor in a vacuum be an issue, if the conductors are naked this is what you are looking for, any exposure in the insulation is a bad thing and should not occur whether the compressor has a charge or is in a vacuum. Please help me to understand your answer.
With or without refrigerant in the system is not the point, if you have air or refrigerant in the system it will create insulation accross the windings when you pull a vaccum you are removing the air/refrigerant, in doing so you are removing the insulation.

[Quality]

The below is extracted from one of Tecumseh's manuals.
I am confident that other manufacturers state the same.
Grizzly

Vaccum Pumping

Never switch on a compressor when under vacuum: an arcing may occur (corona effect) between the terminals or a terminal
and earth. This arcing causes deposits of conductive carbon that can affect the insulation of the terminal. It may even cause it
to disintegrate with the accompanying risk of gas and oil leakage. Always check that the terminal cover has been properly fitted.

Never carry out an electrical safety test when the compressor is under vacuum. The same phenomenon as described before
may occur.


Thats what folk need to understand

Well done Steve

[install monkey]

the only time ur gonna get ur megger out is when u get a call to system not working and blowing fuses/tripping circuit breakers-then ur going to test it and usually the system has refrigerant in and any degradation of the windings would already be done.ideally setting ur megger at 500v helps.-meggers can zap pcbs if they are not removed from circuit.

[Quality]

all connections from any motor must be disconnected before carrying out any resistance measurements on the motor, otherwise damage and false readings will result

[martinw58]

you need to make sure you do not megga on the thermistor only on the motor circuit

[Emmett]

The below is extracted from one of Tecumseh's manuals.
I am confident that other manufacturers state the same.
Grizzly

Vaccum Pumping

Never switch on a compressor when under vacuum: an arcing may occur (corona effect) between the terminals or a terminal
and earth. This arcing causes deposits of conductive carbon that can affect the insulation of the terminal. It may even cause it
to disintegrate with the accompanying risk of gas and oil leakage. Always check that the terminal cover has been properly fitted.

Never carry out an electrical safety test when the compressor is under vacuum. The same phenomenon as described before
may occur.


Thank you Grizzly, that certainly takes some of the fight out of me and provides some serious recomendations to those of us in the field. I do not doubt tecumseh's engineering. I still dont thinkI understand the science though. if there is a physical barrior to the electrical current (insulation) what effect does a vacumm have on that insulation?
whether or not I ever fully understand it I do appreciate your input and the education I get from this site every day.

[mikeref]

i've never used a megga on a compressor under vacuum, but i was under the impression that one does not apply power to a compressor when in that state as the windings cannot dissipate heat and burnout.. Mike.

[Grizzly]

From a layman's point of view and someone else will probably correct me.
But if you have something at atmospheric pressure 1 Bar and you remove that force holding ever thing together.
Is it not easier, for atoms / energy to leak out.
If you understand were I am coming from?
So a cable say that has 1 bar pressure all around it which is maintaining that cables integrity.
Remove that balanced pressure and the mass becomes unstable.
Or something like that.
This may be complete bo**ocks but the pressures on everything within a vacuum change do they not?
Grizzly

[Brian_UK]

I don't normally like quoted Wikipedia but needs must....

Vacuum Arcing

http://en.wikipedia.org/wiki/Vacuum_arc

[Grizzly]

Thanks for the info Brian.
I would give rep points, sadly cannot.
Grizzly

[stufus]

Good man Brian
That clears that up!:D
Cheers
Stu

[SeanB]

A motor is not all wire covered with insulation, where there are connections to the power feed the wiring is open, insulation being provided by the spacing of the wiring. If you reduce the pressure you can cause any sharp point ( and anything with a diameter less than around 30mm would look like a sharp point at some voltage in a vacuum with something grounded or at a lower potential nearby) to emit electrons, depending on the metal and how hot it is relative to absolute zero. Thus you can get a glow discharge, and if there is something there that will break down at this point, and enough energy to make a spark, then it will damage either the wire or the area it lands. This can then lead to a carbon film forming on the surrounding area, and when you apply mains power it will heat up, char and become more conductive, until either the breaker trips or the wire burns out or the terminal burns though the case.

[benijoseph]

Mr cnb,
A very good point for the friends to understand.

regards,

[AUScooler:-)]

A megger will only produce 1MA DC when shorted to earth. The low current will not damage windings or terminals like a short in a power supply will.

[Brian_UK]

A megger will only produce 1MA DC when shorted to earth. The low current will not damage windings or terminals like a short in a power supply will.Maybe not when under pressure but in a vacuum it can happen.

[devlin maguire]

What I still do not understand is why anyone would wish to insulation test ( megger) any windings, this is complete suicide, what you are advocating is stuffing up to 500 V dc down the line to N or E why ? IR is done to on electrical installations IE a ring final circuit to test that it can withstand the result of a FAULT and to test the witstand capabilities of a circuit, to test windings this can only and I repeat only be done if the resistance of the windings under test is already known either from the manufacturers data or by enquiry to the manufacturer and tested with a meter capable of reading OHMS to that known value, if you wish a rule of thumb method of on site testing use the 123 method IE connection or pin 1 to 2 and 1 to 3 should should equal 2+3 for hermetics three phase motors should have equal values on all three windings and do not need to use the add up method described and it make no differance whether the 3 phase is star or delta each phase should be equal
DM

[monkey spanners]

What I still do not understand is why anyone would wish to insulation test ( megger) any windings, this is complete suicide, what you are advocating is stuffing up to 500 V dc down the line to N or E why ? IR is done to on electrical installations IE a ring final circuit to test that it can withstand the result of a FAULT and to test the witstand capabilities of a circuit, to test windings this can only and I repeat only be done if the resistance of the windings under test is already known either from the manufacturers data or by enquiry to the manufacturer and tested with a meter capable of reading OHMS to that known value, if you wish a rule of thumb method of on site testing use the 123 method IE connection or pin 1 to 2 and 1 to 3 should should equal 2+3 for hermetics three phase motors should have equal values on all three windings and do not need to use the add up method described and it make no differance whether the 3 phase is star or delta each phase should be equal
DM

I find testing with a megger very helpful in finding earth leakage problems, ones that will trip an rcd. I don't think that measuring the motor winding resistances will show up an earth fault.
You should be able to test to about twice the name plate voltage, so 500v on 240v motors and 1000v on 415v stuff. Using just a normal multi meter with a 9v battery is like looking for a puncture with no air in the tyre.

If i am wrong, i am happy to learn :D

Jon :)

[AUScooler:-)]

Maybe not when under pressure but in a vacuum it can happen.

I know that arcing may happen if using a megger in vacuum.

The point only that im trying to make is a megger is not capable of causing damage if arcing does occur.

If you take a megger and a off cut of copper and short the sharp probe of the megger to the copper as you tap the surface you will see an arc even at atmoshpheric pressure however this arc will not leave a carbon deposits or damage the surface of the copper in fact it wont even polish the metalic surface even if you do this for 10 minutes.

[Peter_1]

@benijoseph ...Every new compressor passes in factory an isolationtest

[Peter_1]

With or without refrigerant in the system is not the point, if you have air or refrigerant in the system it will create insulation accross the windings when you pull a vaccum you are removing the air/refrigerant, in doing so you are removing the insulation.

Isn't vacuum then not the perfect isolator? Far better than air?

[Peter_1]

What I still do not understand is why anyone would wish to insulation test ( megger) any windings, this is complete suicide, what you are advocating is stuffing up to 500 V dc ....
DM
I'm using my megger at least twice a month and I then always need it.
A tech without a megger in his van can never do proper repairs and needs to guess many times..or use unorthodox methods.

[devlin maguire]

An RCD will trip at 30mA trying to find an earth leakage of that value with a normal multi meter can be done if it reads mA, however most comp start wingings with a capacitor start will cause all sorts of problems on a board thats why it should be fed from its own breaker and not through an RCD, an RCD is only required where the cables suppyling the the compressor are buried less than 50mm below the surface and /or not protected by mechanical means IE SWA or other types of armoured cable, most of the time I surface mount cables and contrary to popular belief T&E can be used outside as long as its glanded at its entry points or use the loop down and up method to allow drip off. and one other point if the only reason you use a megger to test or find out why a condensing set is tripping a rcd or a breaker you would need to know the current draw on start up So remember this an RCD is an OVERLOAD and a earth fault device, an MCB is an overload device, an over load (short between L&N) is a fault in an otherwise healthy circuit, an Earth fault is a short to earth from Live so you should be checking lots of stuff as well as the comp windings I have been called out lots of times where a guy has condemned the comp, changed it and the damn thing still trips the mains or even worse kill the new comp
DM

[AUScooler:-)]

Devlin

Theres no doubt you know youre way around a electrical circuit but i cannot use a DMM on ohms to detect a short from active through water to earth, a DMM reads open circuit yet a megger will read dead short. Two very differant readings would you agree?

[SeanB]

The megger is also useful to test capacitors ( not by charging them and using the unsuspecting as a discharge resistor) but if you suspect a capacitor is breaking down intermittently inside, by charging it up to a high voltage and listening to hear if it has internal arcing. I have had a few capacitors that have the right capacitance, but break down after a few seconds at high voltage. This would give intermittent tripping on compressor starting, but only every so often.

Use a resistor to discharge the capacitor after the test, shorting it can fail a working unit due to the high current.