Intermittent Compressor Rotor Stall (LRA) Symptoms

[JONAH]

ok so it helps if u give ur correct email ad for ur user id code!!!then u can post!!!

ok 1st time at this so bear with me!

recenly commissioned 3.75m island site freezer case,
case=arneg s.bilbao
condensing unit=artic circle a300 75x,twin fan,both active no cycling on h/p or temp,unit 3ph 380v-430v, 50hz
ref=404a,aprox 8kg
piperun aprox 10m to plantroom,7/8 3/8 pipe service
operated=pumpdown,cpr@1.5b,ast@3mins,def@30mins or +10.c term,case temp cycling@-22.c with 2.c diff.
comp lra@34a,fla@5a.voltage 396v measured at comp terms
S/HEAT 5k @-20.c case temp
on site yesterday to a breakdown
case off on comp external overload in control panel,(designated tele 10a motor rated overload),pressures equalized,reset and monitored, comp fired up and ran fine measuring current pressure etc.checked all connections,contactor,motor rated o/load (set @10a),comp terminals,voltage balance etc.
It wasnt untill i short cycled comp on LP by reducing ast to 0 min and throttling s.s.v that the fault came to life. On restarting comp went as if locked rotor by not starting (cond fans also 3ph running)and drawing upwards of 80a before overloading.
voltage balanced,correctly wired 3ph dol y.At first i thought not balancing on off cycle increased ast but unit still tripped again intermit,my manager suggest new comp (against my judgement!)fitted last nite but was back on site at 8am this morn to reset again,have temporary fitted contactor and o/load assem and set to auto reset monitor for 2hrs no problems,had similar prob year ago but with 3hp 240v unit,where after much deliberation and comp electrics we called in meb.they found supply voltage inadequate for store.im not ruling this possibility out,but just looking for a fresh approach from other engineers. sorry so long
thanx jonah.

[Dan]

Jonah, whew. You have great detail and you tell the story as if it were live. Had you a camera I would be dizzy, wondering the solution.

I bet you could ask this question better if you broke it off into paragraphs. Marc has a capacity to absorb large bites and proffer an opinion.

I need short bites. I haven't a clue where Marc's answer comes from, but I will trust his perception. Meanwhile:

It wasnt untill i short cycled comp on LP by reducing ast to 0 min
and throttling s.s.v that the fault came to life. On restarting
comp went as if locked rotor by not starting (cond fans also 3ph
running)and drawing upwards of 80a before overloading.

Ast? Time delay? s.s.v. Suction service valve?

Why would you choose to close the suction service valve and take out the time delay for a test? Is this single phase equipment?.... naahhh. I am not used to seeing external overloads, however on three phase stuff.

But I am not used to seeing a three phase something or the other running fine and failing to start the next time it is put to the test.

I am not willing to condemn the compressor just yet.

Not understanding anything all that well, I would hope that you do not close the suction service valve as a part of any test.

[Gary]

Question: How can a compressor with LRA of 34 amps draw 80 amps?

Answer: Severe drop in voltage.

I would check all of the wiring sizes, condition of the connections, contactor capacity, etc., starting from the power source.

[JONAH]

wow!thanks for response,
im off to same site on monday 25/03 to meet tech support guy from artic circle!
I checked oil dilution, foming etc but seems fine to me.For the 3-4 hours ive sat and watched comp cycle i couldnt find fault untill i simulated short cyce scenario.
I agree with u dan about not seeing 3ph comps starting cleanly it is usually a problem more associated with single phase comps,specially 3hp 240v compressors.
I also always have suction ssv throttled when either commissioning comps checkin lp settings or when replacement comp is being commission to try and prevent any imenient floodback,and comp efficentcy tests.
My initial thoughts was that of lack of low/high side equallibriam during start but after monitoring seems fine,i will if need be fit 1/4 braid hose and mag valve between low/high side on head.
one question i have got though is-
1.Is it best to pre unload comp ie have a 10-20sec with high and low sides equalizing whilst comp on delay
or-
2.to unload whilst comp is off delay
your thoughts would be much appreciated!

thanx once again jonah

[JONAH]

marc u are a star thanx for your input m8
let u know outcome
thanx again

[Robert Rumpff]

although I am a little lost on the basics of whose compressor your using etc. I can't imagine this problem being caused solely by any low side control. For a current reading exceeding locked rotor you would have to be dead headed on the high side of the compressor. I maybe don't understand the problem here but if the symptom is an over current condition I would be taking a closer look at my my high pressure controls and my condenser and at the possablity of non-condensables in the system.

[Gary]

When the rotor locks, is the load inductive or resistive?

[herefishy]

Well, I have have thought that on occassion when a rotor is "locked", it may not be the locking up of the rotor (itself) that is causing the compressor not to run. Perhaps a crank is broken, piston siezed, or some other mechanical failure (aside from the rotor in races that is preventing rotation.

Maybe it is the rotor bearings, and the rotor is actually contacting the stator causing diferent inductive characteristics of the circuit. I consider the nameplate LRA rating to be the rating of the perfectly good motor in a condition in which the rotor doesn't rotate. When there is damage, the rating is merely a guidline, because obviously.... it's no good!

When dealing with locked rotor, I don't over analyze it. Primarily, I use the LRA when troubleshooting, in the manner that if I am not near Locked rotor amps, I will head for the start components (on single pahse), or perform some other diagnostic, before condemning the compressor.

[JONAH]

Still waiting for chance to meet a/circle tech on site,my service manager has been to site to check unit over.
he cant find anything wrong with unit,
he,s putting it down to voltage drop as well(we had same prob before with a/circ unit but that was 240v 3hp comp).
Hopefully ill get chance to meet a/circ on site this week altho times running out because of bank hol friday.
Ill keep you posted!
thanxs jonah