Pulling a Vacuum on a Semi Hermetic Compressor Powered by a VSD or Solid State Starte

[piewie]

Recently the Guvnor went on a York YMC2 training course. For those that don’t know the YMC2 fluid chiller fitted with a York Semi Hermetic Centrifugal Compress with a Magnetic rotor levitation system and powered by a VSD. On his return he tells me that York say that you must never pull a vacuum on the chiller while the VSD is powered up. Not running but simply powered up.He also says that the same applys to the York range of YR screws which could be either VSD powered or started with a Solid State Stater (Soft Starter). I being me asked WHY? To be shot down with a simple that’s what they say. At my age I still have a keen interest in what I do and cannot simply accept this.
Some of the reasoning can be attributed to the fact that the IGBT’s or like are not actually a switch. That there is still a current flowing albeit very small to the motor as the resistance across the IGBT is very large but not open circuit. This part I can buy.
The only reason I can think of is that somehow under vacuum there can be a transfer of electrons between the conductors. But surely there is no physical path for this conduction to occur as all the windings etc. are insulated.
Does anyone out there have a decent answer to this.
Many Thanks

[install monkey]

surely 1st step on your method statement would be isolate,lock off and prove dead prior to work commencing

[piewie]

In the case of the YR screw the unit would have been locked out by the removal of the neccessary start permissions it is not veasible to simply isolate the chiller power supply as oil migration is could be a problem.

My question was not of a health and safety nature it is a technical question if you don't have a resonable answer to the question please do not respond.:off topic:

[1mikeefc1]

Would put it down to moisture being pulled through the whole system and more likely a preventative measure for a one off problem they've had with moisture into the windings.

[install monkey]

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Re: Metering a comp

The dielectric quality of the insulation goes down in a vacuum from what I remember. Therefore when you use a megger to hi-pot (high potential) test the motor windings the insulation can be weakened resulting in an arc from winding to winding. Effectively destroying a new motor.

You can hi-pot test under pressure, but never a vacuum.

You also do not want to hi-pot test a motor where moisture may be concerned. Same effect and end result.

is this reasonable????
ymc2 are oiless so no oil migration:D
the name of the thread is different from the question:confused:




In the case of the YR screw the unit would have been locked out by the removal of the neccessary start permissions it is not veasible to simply isolate the chiller power supply as oil migration is could be a problem.

My question was not of a health and safety nature it is a technical question if you don't have a resonable answer to the question please do not respond.:off topic:

[Neddy]

I've heard the same as above. The winding can arc causing a major bang. Also being told never carry out an Insulation resistance test on a compressor under a vacumn.

[Tesla]

I was taught as an apprentice that an arc can be drawn in a vacuum of 27" with 300Vac at a length of 280mm. Plasma is not really taught all that much at trade school as they just want us to be ignorant - there are five known states of matter and fridgies are generally only taught three of them.

[HVACRsaurus]

I would have thought a vacuum would be a good insulator, just as it is in a thermal sense..

[monkey spanners]

My question was not of a health and safety nature it is a technical question if you don't have a resonable answer to the question please do not respond.:off topic:

https://i.chzbgr.com/maxW500/3889994752/hCF50248E/

[piewie]

@ NeddyThe kind of answer I was looking for thanks.

[piewie]

@ Tesla Thanks a lot one new thing learnt today and a further two things to research further.

[chilliwilly]

Just to add to this, I have pulled a vacuum down to 280 micron after a leak repair and meggered 250, 500, and a 1000 volt to the terminals. I was getting a 1.2 meg reading when the system had a leak, probably due to moisture being sucked in on the low side. When I tested it under vacuum I measured >200 meg, this was on a hermetic with just a basic current/PD relay and no solid state.

I had heard that you shouldn't carry out insulation tests under a vacuum but could never understand why. If there's is hardly any matter left inside the system, I can't see how it can conduct? In the case of a cathode ray TV tube they used to leave the tube on vac for almost a week. Obviously this is the same reason as a light bulb has a vacuum except its filled with an inhert gas, but the TV tube isn't. And the cathode ray is around 30000 volts to overcome the shortage of matter to conduct across the cathode to the anode. And to reduce oxidisation to the bare minimum, and extend the life of the tube.

I can't remember the power value of an actual cathode ray using the basic ohms law formula VxI, or even the current, there's probably more power consumed with copper losses and hysterisis. But an insulation tester applying a voltage of say 500 v between two known substance offering a resistance of say 0.5 meg (500000 ohm) using ohms law would give a current of 0.001 (1 ma), So in the case of a TV tube applying the same criteria, 300000 v/X, you would expect to find a current well down in the scale of the pico or nano scale or beyond due to the tube having no subtance and very little matter to offer a siemens, dielectric, or ohmic value?

So I would agree with piewie on this one that there must be a solid state device having a parrallel connection via dielectric or other kind of junction. That would be damaged if an insulation test was carried out.

[Tesla]

Such an interesting subject here I must add a little more. http://www.refrigeration-engineer.com/forums/showthread.php?39185-Static-sparks-from-gauge-lines&highlight from monkey spanners shows arcing on a positive pressure where velocity and rapid enthalpy change is the likely cause. Interestingly on a perfect vacuum the arc would be less than on a partial vacuum. I wonder what would be happening in the future if more flammable refrigerants become popular. And one last thing - on an old 14" CRT TV I had I noticed when I would first turn it on it would condensate on the front flat glass in front of the tube for around five minutes, I suspect this would have been some form of magnetic cooling. Enough said I'm already rambling.

[Rob White]

Such an interesting subject here I must add a little more. http://www.refrigeration-engineer.com/forums/showthread.php?39185-Static-sparks-from-gauge-lines&highlight from monkey spanners shows arcing on a positive pressure where velocity and rapid enthalpy change is the likely cause. Interestingly on a perfect vacuum the arc would be less than on a partial vacuum. I wonder what would be happening in the future if more flammable refrigerants become popular. And one last thing - on an old 14" CRT TV I had I noticed when I would first turn it on it would condensate on the front flat glass in front of the tube for around five minutes, I suspect this would have been some form of magnetic cooling. Enough said I'm already rambling.

Hopefully that should not be an issue as long as air or O2 is not inside.
If the system is charged with refrigerant then there should not be any air there
and if the system is on a vacuum then there should not be any air or refrigerant.

The issue lies in if there was a leak on the low side and air was sucked in?????? :eek:

Rob

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