is compressor pumping efficiently?

[SkyWalker]

hi guys, I will be testing a compressor next week to make sure its pumping efficiently(old r22 mitsi 2 pipe condenser) now the only test i am aware of is the vacuum test, ie will the pump pull down to -30" hg,

the problem i face is the system im looking at is inverter driven so as the suction pressure drops the inverter will ramp down. on this system there is also a high to low bypass valve which "apparently" activates at about 40psi to ensure the pump doesn't pump into vacuum, i will be unplugging this!

what i would like to do is bypass the inverter circuit, so put 3 phases directly onto compressor, possibly straight from the incoming power for the unit? this would only be for say 20 - 30 seconds just to make sure pump will pull down. I will be closing the suction valve so it wont take long.

now what i don't want to do is make the pump run in reverse!! but how will i make sure its not? the windings are labelled u,v,w. I have a phase rotation meter but don't know where to put which phase on the compressor, if this makes sense. Its a scroll compressor so wont like going backwards.

once a compressor has pulled down i presume a good test of the reed valve is to make sure it holds a negative pressure? presuming the suction valve isn't leaking into the comp.

any opinions or help on wiring it up?

sorry guys didnt intend on post being this long

[frank]

Reed valves on a scroll?????

3 phase direct to an inverter compressor.....

Bypass the inverter circuit....

Closing the suction valve during pump down....

Won't take long.......

[monkey spanners]

I think you will risk killing the compressor, air con comps are not designed to pump down, let alone into a vacuum, (a good piston comp will only reach 24"hg anyway, have heard of ac one dieing very quickly when being pumped down)
I won't even go into disconnecting the safety valve and running off a different voltage...

If i had a comp i suspected wasn't pumping i'd give mitsi technical a call, i wouldn't have much hope they'd have a clue mind, but at least i could look the customer in the eye and say i have done everything i could and followed the manufactures technical experts advice.

And then sell them a nice new unit on R410A

[SkyWalker]

well this is the problem, mitsi said theres no way to prove it, but surely this is rubbish, not sure what the valve is called but theres defo one on these scrolls as i have cut the exact same one apart in the past. its a none return valve between the scroll and high side, what would this valve be called?

i was of the opinion that an inverter compressor was wound the same as a direct online 3 phase only has fancy circuit which alters the frequency, the pump is still rated at 415v 50-60hz? could you enlighten me if im incorrect.

so how on this earth can i prove this pump is pumping efficiently? and other comps out there? which pumps can be taken into vacuum to test? all the comps i see are hermetic

when i said pump down i didnt mean get all the gas into the condenser i meant get the comp into vac, i understand comps are not designed to go into vac due to electrical arcing, but i (thought)i had read in several places including this forum that pulling them into vac proves it pumps efficiently?? obviously i was wrong :/

[monkey spanners]

I'd be happy to pump test a refrigeration recip comp but not a scroll and certainly not an air con one, but thats just me, feel free to do as you see fit.

If you can find the air on and air off, dry bulb and wet bulb, and air flow you can work out the duty the system is doing and compare this to what is should be doing. I have never done this in practice but have seen demo vids

Jon

[SkyWalker]

so really a vac test should only be performed on a recip comp, but not ever aircon comps......

[monkey spanners]

They are optimised for different evaporating temps, most refrig, below 0c most air con comps above 0c.
On one of the air con manufacturers training courses i went on one of their tech guys tried pumping down a scroll to see what would happen and broke it, he ended having to change the comp in his own time on the weekend as punishment, with this in mind i would not try it but i'm no expert on compressor testing!

Jon

[SkyWalker]

lol not good, cheers MS, frank wasnt impressed either lol, think i will put my idea in the bin lol, need to read up on inverter comps and testing them electrically, they are still tested as if they are 3 phase motors are they? equal ohms readings? and at what voltage setting on the megger 500v or 1000v

http://www.emersonclimate.eu/literat...mpressor_0.pdf 6.4 functional test is a good read, backs up your post MS

also 4.3 inverter operation

[Brian_UK]

Going back to your earlier questions.

Connections U,V, W are the international reference for the three phases that we call red, yellow, blue.

[SkyWalker]

I see, but as this is an inverter compressor i wont be wiring it up direct OL to 3 phase, new colours are of course brown black grey

[glenn1340]

As far as I`m aware most inverter driven motors are just bog standard. We`ve had lots of screw air compressors retrofitted with inverters to increase efficiency and any testing we`ve done just entailed removing the motor leads so I guess you could do the same.
Ingersoll Rand have a patent on a hybrid permanant magnet motor but tests on that are a pain in the butt.

[Tayters]

well this is the problem, mitsi said theres no way to prove it, but surely this is rubbish, not sure what the valve is called but theres defo one on these scrolls as i have cut the exact same one apart in the past. its a none return valve between the scroll and high side, what would this valve be called?

That is a 1 way valve but not a reed type. It's a flat bit of metal which sits over the scroll discharge by the force of gravity with a cage over it to stop it flying off.
When the scroll pumps gas out it lifts the plate and out goes the gas. When discharge pressure tries to get back in the high side pressure sits on top of the plate which blocks the way to the scroll.
Therefore if you were to perform a pumpdown test and pressure rose I suppose you could well conclude the valve is faulty.

[SkyWalker]

That is a 1 way valve but not a reed type.

thanks Tayters appreciate the explanation

As far as I`m aware most inverter driven motors are just bog standard

hi glen, by bog standard do you mean the electrical windings are the same as a 3 phase motor?

found this old post on here http://www.refrigeration-engineer.co...ad.php?t=18386, everyone seems to agree that a inverter comp ac can be tested using 3 phase as long as overload is used for protection? i have trawled google but cant find a definite answer

also i thought dc inverters went ac - dc - then back to ac? is there actually dc voltage supplied to the compressor?

[Collie]

thanks Tayters appreciate the explanation


hi glen, by bog standard do you mean the electrical windings are the same as a 3 phase motor?

found this old post on here http://www.refrigeration-engineer.co...ad.php?t=18386, everyone seems to agree that a inverter comp ac can be tested using 3 phase as long as overload is used for protection? i have trawled google but cant find a definite answer

also i thought dc inverters went ac - dc - then back to ac? is there actually dc voltage supplied to the compressor?

There are many different types of inverters but generally they will take in AC and output a DC voltage that mimics the AC sine wave form.
You will be able to test a 3 phase inverter driven motor the same as you would a DOL motor, as in, the windings should be of equal value if they are sound.
As for putting power DOL to the comp I cant dont see why you couldnt, but I wouldnt like to say for definite that you should do it

[Collie]

They are optimised for different evaporating temps, most refrig, below 0c most air con comps above 0c.
On one of the air con manufacturers training courses i went on one of their tech guys tried pumping down a scroll to see what would happen and broke it, he ended having to change the comp in his own time on the weekend as punishment, with this in mind i would not try it but i'm no expert on compressor testing!

Jon

Hi Jon, can you tell me why scrolls dont like being pumped down? It would be very helpful to know this. I thought the scroll was almighty compared to recips haha also might you be able to explain to me something Skywalker said,

"i understand comps are not designed to go into vac due to electrical arcing"

I wasnt aware of any potential arcing due to vaccum its scarey how much i DONT know sometimes about fridge!!

[monkey spanners]

Hi Jon, can you tell me why scrolls dont like being pumped down? It would be very helpful to know this. I thought the scroll was almighty compared to recips haha also might you be able to explain to me something Skywalker said,

"i understand comps are not designed to go into vac due to electrical arcing"

I wasnt aware of any potential arcing due to vaccum its scarey how much i DONT know sometimes about fridge!!

The scroll can seperate as there isn't enough pressure to hold them together, you can hear them get noisier as the pressure drops and then theres a different noise and the suction pressure jumps back up and the whole process starts again. Also i think some newer ones have a differential pressure device to bleed gas from the high side to the low side if the pressure ratio goes above 10:1.

We had a customer with a faulty lp switch on a 7.5hp scroll that left it running like it and eventually killed the compressor, this was a refrigeration comp, from the manufacturers air con courses i have been on i believe air con scroll don't cope as well with pump down situations as refrigeration ones.

[SkyWalker]

this is a direct quote from thermacom compressor guide, they do not make it clear what type of compressor you should conduct this test, im lost now lol

Pump Test This is the best check of compressor function
reasonable or not.
Whilst effecting Close in the compressor suction valve tight,
these tests keep & link out LP control switch.
head pressure low Attach a compound gauge with a coarse Vacuum
scale and operate compressor. Allow machine
to pump down to below 24” Hg.
If at least 18” Hg not achieved suspect valves
are faulty. Do not allow compressor to run in
Vacuum for more than 1 minute.
If 24” Hg achieved stop compressor & monitor
vacuum rise rate. Normal criterion for a new /
re-manufactured compressor is not more than
5” Hg rise / minute. A reasonable field machine
should not rise more than 10”/ minute.

[monkey spanners]

this is a direct quote from thermacom compressor guide, they do not make it clear what type of compressor you should conduct this test, im lost now lol

Pump Test This is the best check of compressor function
reasonable or not.
Whilst effecting Close in the compressor suction valve tight,
these tests keep & link out LP control switch.
head pressure low Attach a compound gauge with a coarse Vacuum
scale and operate compressor. Allow machine
to pump down to below 24” Hg.
If at least 18” Hg not achieved suspect valves
are faulty. Do not allow compressor to run in
Vacuum for more than 1 minute.
If 24” Hg achieved stop compressor & monitor
vacuum rise rate. Normal criterion for a new /
re-manufactured compressor is not more than
5” Hg rise / minute. A reasonable field machine
should not rise more than 10”/ minute.

These will be semi hermetic compressors, most likely recips, i don't think there are many scroll semi hermetic compressors though i did see some very large copeland ones in a magazine once.


Jon

[Robin]

The motor is wound like a step motor. It is a"brushless DC motor" & runs off 380VDC. I have been advised not to connect mains 3 phase to them.

[Robin]

Sorry I was distracted I meant 380VAC!!!