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Thread: RLA verses FLA

  1. #14
    Join Date
    Sep 2008
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    Western PA
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    If you REALLY want to know what the amperage should be on a given compressor at a given set of operating conditions, the manufacturer publishes compressor performance curves or publishes software to predict the amperage given suction, discharge, and other conditions.

    It has been quite accurate in my experience.

  2. #15
    Join Date
    Jun 2013
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    jeddah, saudi arabia
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    I googled what is the difference between RLA and LRA, awhile ago and was linked to the forum. Im taking notes on this topic and thank you to all the contributors who explained and have given their ideas and links, again , additional knowledge gained on this forum.

  3. #16
    Join Date
    Oct 2013
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    207
    Quote Originally Posted by snewman24 View Post
    Here's my take on it. FLA is a rating for motors. RLA is a calculated rating for hermetic compressors and is only useful for sizing the circuit wiring, not for the technician to use for running characteristics. Here are is an excerpt from an article on RLA written by Joe Marchese of Coldtronics of Pittsburgh.

    "Why Not RLA?

    Most compressor manufacturers will stamp an amperage rating on their compressors. They will usually stamp the Rated Load Amp (RLA) of the compressor. However, the technician cannot use this value to determine the correct operating amperage.

    Also, trying to determine if a compressor is good or bad using RLA is not correct. It has nothing to do with what the correct amperage draw should be under its various load conditions.

    RLA is a mathematical calculation required to meet Underwriters Laboratories (UL) approval. The compressor manufacturer must run a series of tests to determine the Maximum Continuous Amps (MCA) before the overload trips. Once that has been determined, UL says to divide the MCA by 1.56 to determine the RLA. Some compressor manufacturers, such as Copeland and Carlyle, use a different factor. They divide the MCA by 1.44.

    If the RLA has any value, it is to determine at what amperage draw the compressor overload will trip, and to determine the fuse/circuit breaker size and the wire size."
    Looks like no one is disputing your comment, I'm thankfull.. I REALLY LIKE THIS ANSWER !!!! Best I have ever hear(read). NOW I can put this to rest.. Thank you... thankyou...

  4. #17
    Join Date
    Aug 2010
    Posts
    1,751
    Hi there,
    I'm sure we got the book definitions, but let me hash out what they mean.

    FLA/RLA are identical. I believe its a generational/cultural difference. A properly designed and working compressor must not exceed this value under stabilized conditions. You can't make it match real life conditions unless you're pumping between almost non-variable temperatures such as making ice from near freezing water and dumping heat into a condenser with near infinite heat sink such as free flowing sea water aboard a ship.

    If you're exceeding FLA with normal line voltage, its a malfunction or a mismatch. Overload on mechanical side(mismatch, air in system, excess drag in compressor, etc) : wattage ^, amperage ^ or power quality issues.

    Single phase comp with failed cap: wattage about same, amperage ^, power factor: down. 3ph don't use caps, so caps are not applicable.

    http://baen.tamu.edu/users/stark/AGS...andout.325.pdf

    There ya go. Not exactly from CRC Press or Springer, but coming from TAMU page should give you a bit more confidence.

    Inrush is a bit more complex. It's useful for evaluating if the power source can handle it. The ability of power source and the inrush affects the affect on voltage. Example: A den on end of the circuit in an older home can may suffer enough voltage dip that a computer in the room will reboot or drop out once in a while when the window A/C kicks in.

    As you know, AC power is a constantly varying power. Inrush current depends on when the motor catch the wave and the residual magnetization in iron. Consistent, repeatable tests need specialized testing. If you're feeling like more reading, check this out: http://support.fluke.com/find-sales/...15_ENG_B_W.PDF

    LRA is the steady current motor draws at rated voltage with the rotor locked up and its used to size protective devices matched to protect the motor. Motor's torque is directly related to current. So, if the line can't provide enough torque to break through or the load is excessive for starting, it stays stalled.

    Imagine you're towing an overloaded trailer. If you're already at cruising speed, you will make it through, but you stop on a steep hill and try to get it going with gas to the floor and letting out the brakes slowly hoping that your car will over come it. Fuses/breakers are time delayed to give the motor enough time to give you some time under these conditions. If it takes too long or you don't get moving at all, the transmission fluid will eventually catch on fire. The LRA is used to size the limiter, not to protect the shafts(analogous to wires), but to prevent the transmission from catching on fire.

  5. #18
    Join Date
    Oct 2013
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    207
    It's like my birthday, and I just received my toy I've been waiting for. Same answer, longer version I am twice as happy and thank full. Can I be your friend

  6. #19
    Join Date
    Oct 2013
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    Please please please... Have a look at my profile and look at my issue (photos) I have not received an answer yet..

  7. #20
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    Aug 2002
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    Quote Originally Posted by the_4rs View Post
    Please please please... Have a look at my profile and look at my issue (photos) I have not received an answer yet..
    And you’re not going to get one.

    You've applied for pro status. Once approved you can ask this question in the Pro area where more detailed answers can be given.

  8. #21
    Join Date
    Oct 2013
    Posts
    207
    Understood

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