Missing LRA on compressor - backup system

jaggedben

Senior Member
So I'm involved in sizing a battery backup system where the customer wants to run the air-conditioner on the backup. (Don't criticize please. ;))

Attached is the label for the compressor unit. The LRA is missing, and I want to check my sanity that the proposed battery system can deal with the inrush current of this unit. The battery system has a peak power (surge) rating of 11.4kVA for 10 seconds. I think this is sufficient? We could propose higher if needed.

My understanding is that the LRA is locked rotor amps, and that this is for practical purposes equivalent to motor-starting current for a fraction of a second. Correct? Also is the LRA listed on the unit because it's (apparently) a DC motor ?
My understanding is also that the motor-starting current is always higher than the FLA.
Is there any kind of rule of thumb on ratio of LRA to FLA such that I can make a conservative guess on the requirement here? Alternatively, if I use the max OCPD rating is that not conservative enough?

Finally, if this compressor handles linesets for multiple airhandlers (zones) in the house, how do you think the compressor amps would vary if only a portion of them are turned on?

Your thoughts appreciated.
 

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PaulMmn

Senior Member
Location
Union, KY, USA
Try to convince him to have a generator to handle the A/C load. Our computer room has batteries to handle line glitches and outages. They handle power supplies and fans in the computers. But they don't handle the A/C. The A/C is handled by the honking big (at least for us) generator in the back parking lot. And, once the generator kicks in, the computers run from the generator, not the batteries. So, theoretically, the batteries only handle the load for about 5 seconds until the generator kicks in.

Theoretically, as well, the batteries can keep the computers running for about 1/2 hour if the generator fails or doesn't start. Just enough time to shut them down gently.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So I'm involved in sizing a battery backup system where the customer wants to run the air-conditioner on the backup. (Don't criticize please. ;))
Tesla recommends that a soft start component be added to any A/C units connected to a PowerWall.
 

oldsparky52

Senior Member
Try to convince him to have a generator to handle the A/C load. Our computer room has batteries to handle line glitches and outages. They handle power supplies and fans in the computers. But they don't handle the A/C. The A/C is handled by the honking big (at least for us) generator in the back parking lot. And, once the generator kicks in, the computers run from the generator, not the batteries. So, theoretically, the batteries only handle the load for about 5 seconds until the generator kicks in.

Theoretically, as well, the batteries can keep the computers running for about 1/2 hour if the generator fails or doesn't start. Just enough time to shut them down gently.
Are your batteries always supplying the computers (and the building power charges the batteries) or does the load switch to the batteries on the loss of power?
 

PaulMmn

Senior Member
Location
Union, KY, USA
Are your batteries always supplying the computers (and the building power charges the batteries) or does the load switch to the batteries on the loss of power?
I'm reasonably sure we're constantly on the batteries-- there's zero switching time during a 'power event.' The computers never notice any gl!tches.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
A variable speed unit will not have a large starting current as stated. However, the input rectifiers will conduct current in narrow pulses that have higher peak values than the normal √2 = 1.414... times RMS current that applies when the waveforms are sinusoidal. In other words it's a nonlinear load. So some extra margin may be needed for the battery powered inverter to accommodate this.
 

StarCat

Senior Member
Location
Moab, UT USA
The last 2 entries are golden. This type of unit did not exist until recently. On the load question, this kind of rig should unload if its supplying multiple metering devices to keep the saturated evaporator condition " in range " in theory. I have not laid hands on this new generation of Tech.
I would throw a Fluke 87 or peak recording meter on it and look at actual inrush AMPS under different operating conditions. Also for LRA you generally have tp go directly to the " compressor " nameplate. Many MFGs also have historically witheld Compressor RLA because there was once this fear that Techs were attempting to charge systems by forcing the running amps into the RLA range. This has made it hard to locate RLA, especially for some fractional refrigeration gear, when it is really needed. The newer fractionals, being designed right on the wire and highly intolerant to the slightest over charge on a Fixed Restrictor. Stuff is still getting cheap, and thinner every year.
 

winnie

Senior Member
Agreeing with the folk above.

Based on the dataplate my strong guess is that the compressor and fan are both operated by variable speed drives, and thus the supply circuit will never see the LRA or starting current of the motors. Look at the wide voltage range and the 'frequency' spec for the fan and compressor. Based on this I think that you can conservatively say that the unit will draw the MCA, and will likely draw less.

IMHO you can use the MCA for your initial sizing of the system, but I would not actually finalize this without input from the manufacturer.

-Jon
 
Agreeing with the folk above.

Based on the dataplate my strong guess is that the compressor and fan are both operated by variable speed drives, and thus the supply circuit will never see the LRA or starting current of the motors. Look at the wide voltage range and the 'frequency' spec for the fan and compressor. Based on this I think that you can conservatively say that the unit will draw the MCA, and will likely draw less.

IMHO you can use the MCA for your initial sizing of the system, but I would not actually finalize this without input from the manufacturer.

-Jon
Along the same lines, I'm doing a service upgrade and this POCO is obsessed with LRA. I did another service in their territory last year and they sized the transformers to the LRA of the 30 hp motor. So this one was just a residence with a mini split. I told the guy it has a vfd and has no inrush. He said he "still needs the LRA" :mad:
 
I don't think the Tesla system likes jolts. A slowly building load can be absorbed but a jolt, like a welder striking an arc, for example, will sometimes shut down the Tesla for a few minutes. Mine did this once, even though the load was within spec, so we reset the house to the grid to use the welder, and the Tesla restarted soon thereafter. After we were done welding we reset the house back to the Tesla system.
 
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