Liebert CRAC comp overloads

[dave1234]

Im working on a Liebert model# UH199A-AAM ser#32012-002. There are 2 copeland discuss compressors. Comp 1 alarming on Overload. Now this may seem like a dumb question but please bear with me here. As far as i can tell the overload is just the internal overload in the comp (underneath the connection block i believe)- there are no overload relays and other then the overlaod it is just fuse protected- so how the hell does the control board get feed back and know that the internal overload is open and to alarm when there are just the 3 lines going to the compressor?! After "reseting" the alarms by powering it down for 10+ seconds and then turning it back on it immediatley alarms "comp 1 overload". How does it know that the internal overload is open? Magic? It seems to be auto reset and is most likley defective as the comp doesnt even try to start but the question at hand still remains. Hope my question makes sense and doesnt sound dumb. Thanks.

[The Doctor]

Isn't it a two-wire thermal switch that reports through the control circuit or directly into the microprocessor?

[dave1234]

Not that I could find infact I forget the exact verbage but the advanced O&M manual in the alarm and alarm description section states something to the effect that the overload is in the electrical connection box on top of the compressor on Copeland models and can be auto or manual reset (there is no manual reset that I can find so im assuming its just a klixon underneath the connections?) -, but still there are no leads going to the comp other than the power leads and the diagram actually shows an optional overload on it (that breaks all 3 legs) that are just in series with the motor- not a control that breaks control voltage. It was 1am when I was there, but im pretty sure all my observations are at least close to be beingon point. It seems as though there has to be another overload somewhere or I dont see how the board would know.

[hands]

Did you check the overload to see if it was really open? Do the compressors have motor protection modules?

[pecmsg]

I believe there is a current sensor in the compressor circuit. If the unit calls for cooling and doesn’t read amperage after x amount of seconds then it goes out on alarm / overload.

[dave1234]

I believe there is a current sensor in the compressor circuit. If the unit calls for cooling and doesn’t read amperage after x amount of seconds then it goes out on alarm / overload.

That was my first assumption after finding no overload relay or external overlaod protection of any kind. By current sensor who mean one of those dounut deals right? I looked and found none and traced the lines from the block to the contacter and up to the compressor. There is no motor protection module.

[anytech]

The microprocessor control is looking for 24V on Plug 33-6. No voltage indicates an overload, but this can be deceptive. Verify that fuse #1 is still good to rule out a short in the circuit.

[dave1234]

The microprocessor control is looking for 24V on Plug 33-6. No voltage indicates an overload, but this can be deceptive. Verify that fuse #1 is still good to rule out a short in the circuit.

Yep! turns out that was the ticket the whole time. I was actually just hopping on to post my findings. There is no low voltage overload control to speak of like overload relay and the fuse was popped. On the units that utilize Carlysle (this has Copelands) compressors the low voltage thermal overloads in the compressor would open above said circuit causing the alarm. Thanks everyone for all the help!