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Superfridge
22-11-2016, 08:58 AM
Hi all,

I have an unusual starting issue that maybe someone can give some advise on.

I have an SZ24 mini screw that sounds terribly labourous and draws excessive current on the star start but runs normally on delta. The star contactor has been replaced in recent years and is in good condition. The voltages over the windings on star start are normal and the amps are ballanced. Electricly the starters and conductors cannot be faulted. The discharge pressure on start is normal. Both discharge and suction check valves are holding and operate. The load slide is at minimum at starting. This skid is on R22 and works as a stand alone system with 3 others that are the same on a cool store. No issues with the others. This system has no oil pump. The motor starts in the correct direction. The system is about 30 years old and I think this motor is orriginal.
There is a economiser side load connected but is isolated.

We have to disconnect the compressor and lift it to open the coupling to be able to run the motor with nothing connected so this has not been done.......yet.

Where I'm heading is either a issue with the electric drive motor, something strange with the poles maybe but you would think that if it was a motor issue then the delta run would be effected.

Or is it possible to oil log a compressor to this extent? The compressor sits about 5ft off the ground, way above the vertical oil separator oil level. I think the compressor has some sort of oil resivour as it has a low oil level float internally.

Any bright ideas?

Thanks

cricri
22-11-2016, 06:47 PM
hi superfride,
before starting the compressor, turn it by hand during many turns to drain any oil from the suction header, if the next start is better, check the suction check valve. may be passing for oil when compressor stops.

hookster
22-11-2016, 07:18 PM
Hi superfridge
Throttle in your suction valve till its fully closed, then open it a couple of turns. Start compressor and when it changes over to delta start opening up your suction valve.
It takes a bit of practice to not trip it on LP & Oil pressure but when you get the knack of it :)

RANGER1
22-11-2016, 07:51 PM
Mechanically easiest thing to check is oil filter for metal or debris.
Also if there is some type of cover on machine that could expose rotors or part of (not common).
Once running compare amps at different loads & oil & discharge temperatures.

Motor wise it could be damaged rotor bars giving poor torque on start up!

http://www.jee.ro/covers/art.php?issue=WW1331553546W4f5de50a22454

http://blog.jmtestsystems.com/index.php/ac-rotor-surge-tester/

Magoo
22-11-2016, 08:09 PM
Hi superfridge,
some of the S24's had a service bypass pipe across the suction check stop valve. It should be closed as will allow compressor to equalize when stopped and lift oil back into rotor case from separator.

Superfridge
23-11-2016, 09:19 AM
Thanks for the replys.
Check valves are good, no bleeding back to suction. The compressor does back spin on shut down but not excessive and the same as others on site.

My 3 phase motor theroy has gone out the window since being an Industrial Fridgy, sites have there own sparkys who sort out most things except this one. Is it possible to have a single star winding connected wrong? Like backwards? I'm going to have to get my books out as I cant remember if it is possible or would make a difference.

Ranger, the rotor bar damage is a real potental thank you. We will get that checked as now I have done a bit of "interogation" it sounds like it may have been an issue for some time but has gotten worse.

Magoo
24-11-2016, 12:51 AM
I agree with Ranger1
the noise / groaning sounds like broken rotor bars, particularly two pole motors star delta from vector drag between coils.

hookster
24-11-2016, 07:58 AM
Could it not be your end clearances for your rotor thrust bearings? At lower start speed you are hearing more contact. Chances of it being motor issues and disappearing in Delta seem fairly slim.

mbc
28-11-2016, 08:03 AM
hi
do you have vibration in running time
if yes test the bearing and rotors
if not check oil reg valve and ECO valve ( should open after delta ) and heater in oil should be work on off time very important ,

Superfridge
21-12-2016, 08:16 AM
Just an up date,
the sparky and some motor experts put an oscilloscope on the motor and have found no issues. Bit of head scratching going on now.........

Tycho
12-01-2017, 07:02 PM
No experience with this compressor.

I was going with oil log in the compressor, but then I ran it by an electrician friend who also knows refrigeration.
(I call him the Oracle, because he knows everything there is about electricity)

The first thing he reacted to was that the contactors had been replaced and he said that the star/delta delay relay might have been reversed, so that the compressor starts delta/star.

I didn't think it would be possible to start a screw compressor delta/star, but he said that it is very possible, and that the only difference you would notice is that it would "sound like it was dying" during the initial delta start and pull a very high current, and then it would pull slightly less amps when running normally, but that it would "choke" (pull higher amps) when put under load if it was running on the star winding.

That was his expert input after I sent him your post, I don't know if you have watched the contactors during start to make sure they engage in the correct order...

if they engage in correct order, I'm sticking with oil log :D

Superfridge
19-01-2017, 01:13 AM
Hi Tycho,

The starters are right beside the compressor so the sequence has been proven to be true. Personally I think oil logging is unlikely as I can increase the star timer and there is no change for the better as you would expect as the oil is removed, if anything the motor appears to struggle more the longer it is in star. I have my suspicions with the "motor experts".............

torque
18-03-2017, 11:47 AM
Hi Superfridge.

Are you sure this machine is at zero capacity when trying to start. I have known these compressors to sound laboured and pull excessive current in star but once up to speed and changed over to delta they run ok if they are trying to start partially loaded.
What is the capacity control on this unit ?

I suspect looking at the date of the last post you probably have sorted this issue by now.

Superfridge
21-03-2017, 07:47 AM
Hi Superfridge.

Are you sure this machine is at zero capacity when trying to start. I have known these compressors to sound laboured and pull excessive current in star but once up to speed and changed over to delta they run ok if they are trying to start partially loaded.
What is the capacity control on this unit ?

I suspect looking at the date of the last post you probably have sorted this issue by now.

thanks for your interest torque,
No, the issue is not sorted. Its a different world these days, seems like you cant just go and fix it unless its causing a problem. The compressor runs 24/7 so the starting is not causing a problem. We will take a look when we are allowed too.
The compressor has a slide valve and its movement is visible so I can see it is starting at minimum.
Watch this space.