Compressors start under vacuum

[narkom]

Hi everyone!
In every application guideline for compressor we can read something like "The compressor must neither be started, nor may high-potential testing be carried out under vacuum"
Please, anybody, explain me WHY???
________
Airsoft 177 (http://airsoft-shop.info/tag/177)

[nike123]

Refrigerant is good electrical isolator and cools windings. By starting compressor without refrigerant you could damage it, if winding isolation is not perfect, and you also could overheat windings what again damages winding isolation. In this way you are putting compressor in great risk.

[Springbok]

Make sure you got no leaks in your system either.If you suck in some moisture,you're in for a long night!

[Argus]

..

Not exactly.

The question about starting or carrying out high potential insulation tests should refer to the degree of the vacuum - in other words, a complete vacuum or the absence of any gases, air or refrigerant.
It s a complete vacuum that is referred to when cautioning care using high potential tests.

Many hermetic systems run normally at mains potential under a partial vacuum containing refrigerant between zero and 1 bar absolute.

Common refrigerants CFCs, HCFCs and HFCs (and air for that matter) have a high dielectric strength or insulation value, that is absent in a complete vacuum.


.

[nike123]

From what I know, perfect vacuum is best insulator.
But 500 microns is far from perfect vacuum.


· Gas/vacuum as Insulator

Air at atmospheric pressure is the most common gaseous insulation. The breakdown of air is of considerable practical importance to the design engineers of power transmission lines and power apparatus. Breakdown occurs in gases due to the process of collisional ionization.

Electrons get multiplied in an exponential manner, and if the applied voltage is sufficiently large, breakdown occurs. In some gases, free electrons are removed by attachment to neutral gas molecules; the breakdown strength of such gases is substantially large. An example of such a gas, with larger dielectric strength, is sulphur hexafluoride (SF6).


High pressure gas, provides a flexible and reliable medium for high voltage insulation using gases at high pressures, field gradients up to 25 MV/m have been realized. Nitrogen (N2) was the gas first used at high pressures because of its inertness and chemical stability, but its dielectric strength is the same as that of air. Other important practical insulating gases are carbon dioxide (CO2), dichlorodifluor9methane (CC12F2) (popularly known as *****), and sulphur hexafluoride (SF6). The breakdown voltage at higher pressures in gases shows an increasing dependence on the nature and smoothness of the electrode material. It is relevant to point out that, of the gases examined to-date, SF6 has probably the most attractive overall dielectric and arc quenching properties for gas insulated high voltage systems.


However, in recent years pure SF6 gas has been found to be a green house gas causing environmental hazards and therefore research efforts are presently focussed on finding a replacement gas or gas mixture which is environmentally friendly. Pure nitrogen, air and SF6/N2 mixtures show good potential to replace SF6 gas in high voltage apparatus. In the next few years, SF6/N2, SF6 gas has to be replaced by a new gas and lot of research is being done to find such a gas.
Ideally, vacuum is the best insulator with field strengths up to 107 V/cm, limited only by emissions from the electrode surfaces. This decreases to less than
105 V/cm for gaps of several centimeters. Under high vacuum conditions, where the pressures are below 10-4 torr, the breakdown cannot occur due to collisional processes like in gases, and hence the breakdown strength is quite high. Vacuum insulation is used in particle accelerators, x-ray and field emission tubes, electron microscopes, capacitors, and Circuit Breakers.

http://tinyurl.com/2ydhus

By removing ***** and not achieving vacuum below 0,0001 torr, we creating worse insulation then ***** at normal pressure.

[US Iceman]

I think Argus is right. The dielectric strength of the insulation on the motor windings also becomes weaker in a vacuum. So, as you high pot the motor testing for insulation resistance you have an increased chance to short the motor with the high voltage of the megger.

[nike123]

I think Argus is right. The dielectric strength of the insulation on the motor windings also becomes weaker in a vacuum. So, as you high pot the motor testing for insulation resistance you have an increased chance to short the motor with the high voltage of the megger.

He did not said that. He said:

Common refrigerants CFCs, HCFCs and HFCs (and air for that matter) have a high dielectric strength or insulation value, that is absent in a complete vacuum.what is not the case! If it is complete vacuum, there is no refrigerants. Complete (perfect) vacuum is best insulator because there is no mater to conduct electricity. Current is flow of electrons, and if there is no mater, there is no electrons to flow.

[US Iceman]

OK, but we are talking about dielectric strength of the motor insulation also.

Here are some examples from Copeland application manuals:

" Do not start the compressor while

the system is in a deep vacuum. Internal arcing may occur when a scroll compressor is started in a vacuum."

" Under no circumstances should the Hipot test be performed while the compressor is under a vacuum."


" Copeland subjects all motor compressors to a high voltage test after final assembly. This is carried
out according to EN 0530 or VDE 0530 part 1.
Since high voltage tests lead to premature ageing of the winding insulation, we do not recommend
additional tests of that nature. They may also be carried out with new machines only.
If it has to be done for any reason disconnect all electronic devices (e.g. motor protection module,
fan speed control, etc.) prior to testing. The test voltage of 1000 V plus twice the nominal voltage
is applied for 1 - 4 seconds between motor winding (each one of the phases) and the compressor
shell: The maximum leak current limit is approximately 10 mA. Repeated tests have to be performed
at lower voltages.


Caution: Do not carry out high voltage or insulation tests if the compressor housing is under vacuum.
Compliant Scroll compressors are configured with the motor down and the pumping components
at the top of the shell. As a result, the motor can be immersed in refrigerant to a greater extent
than hermetic reciprocating compressors when liquid refrigerant is present in the shell. In this
respect, the scroll is more like semi-hermetics (which have horizontal motors partially submerged
in oil and refrigerant). When Compliant Scroll compressors are high potential tested with liquid refrigerant in the shell they can show higher levels of leakage current than compressors with the
motor on top because of the higher electrical conductivity of liquid refrigerant than refrigerant vapour and oil. However, this they can show higher levels of leakage current than compressors with
the motor on top because of the higher electrical conductivity of liquid refrigerant than refrigerant
vapour and oil. However, this phenomenon can occur with any compressor when the motor is immersed
in refrigerant. The levels of current leakage do not present any safety issue. To lower the
current leakage reading the system should be operated for a brief period of time to redistribute
the refrigerant to a more normal configuration and the system high potential tested again."

I have not been able to find a on-line statement about the use of hi-pot testing of hermetic motors in a vacuum, but from the above you can see it is NOT recommended.

[monkey spanners]

Also where the wires for the windings connect to the terminals that pass through the compressor body will have no insulation and they are only a few mm apart. All new l'unite compressors have warning stickers on them regarding hearing arcing from the terminal area.

[nike123]

I don't negate that in my posts. Also, I don't say anywhere that we should do starting of compressor or insulation test under "vacuum".
What is my observation is, that perfect vacuum and commonly accepted term "vacuum" (say 300-500 microns of vacuum) in refrigeration industry are not the same thing, neither insulation characteristics of residual refrigerant under "vacuum" and real vacuum are same.
First of all, absolute vacuum is perfect insulator and in that conditions arching is not possible.
Second, insulation strength of refrigerant rises as his density rise, and that is main reason for these recommendations that motor shouldn't be started or tested for insulation strength under field achieved "vacuum". As we all know, there is no perfect vacuum, and that achieved with vacuum pump on site is far from perfect.

We need here to distinguish two terms; real perfect vacuum and "vacuum" commonly accepted in our trade.
Please, read that text on link supplied by me.

If I saying something wrong here, please, inform me.

[US Iceman]

Instead of trying to define what a vacuum is perhaps we should be trying to find out why the (semi-hermetic or hermetic) compressor motors should not be started in a vacuum or hi-pot tested when in a vacuum. That was the original question.

The reason I have always heard was the dielectric strength of the motor insulation was reduced which could cause the motor to short out between windings or to ground.

With this said, since we are talking about refrigeration service requirements I would conclude the discussion be limited to the relative range of vacuum we encounter rather than absolute vacuum ranges possible. So, I think we agree.

My thoughts related to the manufacturer information and recommendations I posted would be directed to those situations or conditions found in normal refrigeration service techniques. Otherwise, if they were talking about a "perfect" vacuum the discussion becomes minimal because we cannot achieve a perfect vacuum with available vacuum pumps (or by the compressors if this were tried :rolleyes:).

I think this is similar to medium or high voltage switchgear where vacuum contactors, etc are used to provide motor starting. By having the contacts in a vacuum the high potential voltage cannot arc to the air. Even in low voltage switchgear there is a possibility of arc flashing creating safety issues for personnel.

Therefore in a commonly found vacuum created by a vaccum pump used in refrigeration service the relative lack of a vacuum may allow a situation to create a short between the motor windings or to ground/earth.

Along a similar thought you would not want to hi-pot test a motor with wet windings either.

[nike123]

Totaly agree!:)

[US Iceman]

I thought we could reach a middle ground to agree on.:D

I hope the question was answered OK for the original poster. And, I hope we can find the technical reason behind these statements rather than the generic information we so far.

It's been a long time since I had to think about this topic....

[nike123]

I thought we could reach a middle ground to agree on.:D

Of course we could, since nobody has infringe any low in physics.:D

[Tesla]

Hi Guys
Thought I'd put my 5 cents worth in here. When I was an apprentice I was taught that an arc can be drawn under a vacume of 28" 250mm long with a 300 volt potential.

[narkom]

Thanks a lot for answers!



First of all, absolute vacuum is perfect insulator and in that conditions arching is not possible.
Second, insulation strength of refrigerant rises as his density rise, and that is main reason for these recommendations that motor shouldn't be started or tested for insulation strength under field achieved "vacuum". As we all know, there is no perfect vacuum, and that achieved with vacuum pump on site is far from perfect.

You are quite right!
Let us look at the picture.
One can see that the breakdown voltage decreases while the gas pressure decreases. So if we cant achieve the perfect vacuum we cant carry out high potential insulation tests.
The breakdown cannot occur than the pressure is below 10-4 torr (0.1 micron) but vacuum created by a vaccum pump used in refrigeration service is no less than 15 micron.

Thanks a lot once again!
________
VAPORIZER WHOLESALER (http://vaporizerwholesaler.com)

[Brian_UK]

One could also go to the extreme and suggest that when under a vacuum there are no 'gas' molecules left to get between the windings. The windings therefore sit closer together increasing the chance of an arc.