LRA CONVERSION TO TONNAGE

[cheps]

HOW TO CONVERT THE LRA TO TONNAGE (BTU/HR), BOTH FOR SINGLE PHASE COMPRESSOR AND THREE PHASE COMPRESSOR?

WHATS THE FORMULA FOR THAT...PLS EXPLAIN...

THANKING YOU...

[suemarkp]

I don't think there is one. Locked Rotor Current (LRA) or the Rated Load Amps (RLA) are directly related to the size of the motor compressor. But as compressor and system efficiencies go up, you can't convert amps to BTU's easily because the efficiency value affects it.

You could probably make some guesses on tonnage based on the age of the equipment and the RLA. OR you could try the manufacturer and see if there is a datasheet for that unit. A good data sheet will have btu/hr, MCA or RLA, LRA, and a bunch of other data.

[re2ell]

Originally posted by cheps
HOW TO CONVERT THE LRA TO TONNAGE (BTU/HR), BOTH FOR SINGLE PHASE COMPRESSOR AND THREE PHASE COMPRESSOR?

WHATS THE FORMULA FOR THAT...PLS EXPLAIN...

THANKING YOU...

I agree with Mark, there is no practical relationship between Locked Rotor Amps (LRA) or Running Load Amps (RLA) and BTU/HR capacity. LRA and RLA are maximum amperages of the motor of the compressor during startup (LRA) and the motor running after the motor reaches full speed (RLA). If on startup, the LRA is higher than its rating, usually an indication that there is a mechanical condition of the compressor, or an electrical condition, (not enough voltage, loose wiring, defective capacitor, etc.) When you experience excessive LRA, the cause needs to be found and repaired, otherwise compressor damage is likely. This also holds true for the RLA. These values are maximum allowable of the motor of the compressor.

Pretty much for 3-phase motors also, if the maximums are exceeded, the cause needs to be found and repaired.

I hope this helps you in your understanding of LRA and FLA.

[Edited by re2ell on 03-04-2006 at 02:26 PM]

[stonefly]

i posted the same question here a couple of months back.i have the formula & had used it in the past but dont now based on the opinions posted in responce.it was for 1 ph.& roughly converted to horse power as i was told in school and that was a rough comparison to tonnage.(as cope. compressors do) (460v.)fla div. by 3= tons ,(230)fla div. by 6=tons & (120v.)fla div.2 =tons. rla is 80 % of fla & fla is 20% of lra.......for what its worth.

[re2ell]

This thread brings a smile to my face, takes me back to the year 1982, the year I first enrolled in HVAC school! We did the same calculations, it was fun.

I'm presenting you with 4 condensing units' nameplate data, I'll let you do the math! It's good exercise.

All units are rated at 208/230v, 1 phase, 60hz.

Carrier 3T with Copeland Scroll compressor-90.5 LRA, 17.9 RLA.......Bryant 3T with Carrier compressor-85.0 LRA, 20.8 RLA.......American Standard Allegiance 2 1/2T with GE compressor-75 LRA, 13.1 RLA......Trane 2T with Trane compressor- 62 LRA, 11 RLA.

Mark mentioned using the manufacturers data sheet, excellent advice! But what you are doing now, the same as I did in 1982, is fun to do!

[Edited by re2ell on 03-04-2006 at 02:19 PM]

[cem-bsee]

re2ell: only one amperage for both 208 & 230V?
usually 208 relates to 3phase --
are mfgr just covering +10%, -15%?

[re2ell]

cem-bsee, yes, the values I posted are as they are printed on the nameplates of the unit. To add to the confusion, the Carrier unit with the Copeland Scroll list the maximum applied voltage @ 253v and minimum @ 187v, with only one value for both LRA and RLA. Almost makes one want to scratch his head?

[Edited by re2ell on 03-05-2006 at 10:57 AM]

[docholiday]

Unless the tech is trying to do a capacity check (of course he would have to know the actual model# for this to be any good) or is trying to decide what size unit to install as a replacment (and a proper calc should be done) I cant really think of any good reason to do all this. JMHO.

[stonefly]

another tech. school formula that recently let me down was to do with ohming out compressors. premise:start winding ohms should be between and 6 times greater than run winding ohms or compressor has failed. i condemmed an a.s. gaspack rotolock comp. that was locked up during the first yr.(not our install)& someone else got it running with soft start & a time delay relay.should have been replaced anyway since it was too new to be having such problems .i ohmed out a brand new rotolock & by my formula it was bad.

[re2ell]

Originally posted by stonefly
another tech. school formula that recently let me down was to do with ohming out compressors. premise:start winding ohms should be between and 6 times greater than run winding ohms or compressor has failed. i condemmed an a.s. gaspack rotolock comp. that was locked up during the first yr.(not our install)& someone else got it running with soft start & a time delay relay.should have been replaced anyway since it was too new to be having such problems .i ohmed out a brand new rotolock & by my formula it was bad.

Have you thought about filing that formula under '13'?, I would.

[kim]

A one HP PSC motor will draw about 1900 watts. (120V & 16A, 240V & 8A etc.) That should be about 1 ton on a 10 SEER system. Some systems are more efficient than others. It makes sense that larger motors would be more eff and small motors less eff.
3-phase is 78% of that.

LRA should be about 5 times higher than RLA. LRA depends on the start torque more than anything else. LRA are much less reliable quess on capacity.

You can use FLA to quess drive capacity too.
2 ton = 1/4 HP = about 4A
3 ton = 1/3 HP = about 5A
4 ton = 1/2 HP = about 8A
5 ton = 3/4 HP = about 12A

When you start looking at motor plates to verify this data, remember 1/3 HP is a nominal measurement. It is the closest to actual. Amps are measured and recordered properly. A 5A motor is not as powerfull as a 6A motor even though both claim to be 1/3 HP.

This little trick can also tell you about duct system capacity quickly.

[stonefly]

ok, ill bite whats 13?

[on call]

RLA stands for Rated Load Amps (not Running Load Amps). This value is recorded when the system is tested @ ARI standards.

Also, I was at a Carlyle compressor class and the instructor showed two different compressors where amps or HP doesn't correlate to BTU/H. Of course, that was on semi's, but I would think that hermatics would be the same. I would rather use ODU model numbers to determine rated system capacity than compressor RLA.