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  1. #1
    I am having an argument with some people at school about what locked rotor amps are. Even though I have read in two different books that locked rotor amps are the amps that a motor draws at start up, the other students and teacher disagree with me. The teacher said the only time a motor will have locked rotor amps is when the motor is actually locked up, as in not working properly. Now I am not calling the teacher a lier and I would never pretend to know more then him but I read it in black and white and I have a hard time believing that two different books could both be wrong and I am not the type of person who will blindly believe something just because he should know more. Maybe this is one of those things where people in the field use slang terms or something. So what I am looking for is people in the field that do this for a living day in and day out to see what the general consensus is.

  2. #2
    Inrush current is what you are thinking of.

  3. #3
    Originally posted by smilies
    Inrush current is what you are thinking of.
    Book #1 refrigeration & air conditioning technology 5th edition

    "motors are rated with two different current ratings, full-load or rated-load amperes (FLA or RLA) and locked-rotor amperes (LRA). Locked-rotor amperage is often referred to as inrush current. When a motor is still, it takes a great deal of torque to get it turning, particularly if the motor has a load on it at start-up."


    Book #2 Modern refrigeration and air conditioning 18th edition

    "At the instant of starting, the curent flow will be quite high (two to six times the running current). This initial high flow of amperage is called locked rotor amperage."

  4. #4
    Join Date
    Oct 2004
    Posts
    3,400
    You've got the proof. What else do you need?

    HVAC instructors are NOT infallible.

    Are you using either of those books in class?
    Go to the instructor quietly, and in private, and show him those two passages. It would be better if you actually showed him the books, rather than quoting from it.

    Please realize that there may be just as many books quoting something else.

    Likely, he will change his stance, and make it known to the class on the next meeting.

    Don't expect a medal.



    Now go learn something new.

  5. #5
    In the field... very simple RLA good. 2am W/I freezer pump @ LRA no # 10 boot is gona revive it...bad.In the field...
    FEN

  6. #6
    Originally posted by Wild Leg
    You've got the proof. What else do you need?

    HVAC instructors are NOT infallible.

    Are you using either of those books in class?
    Go to the instructor quietly, and in private, and show him those two passages. It would be better if you actually showed him the books, rather than quoting from it.

    Please realize that there may be just as many books quoting something else.

    Likely, he will change his stance, and make it known to the class on the next meeting.

    Don't expect a medal.



    Now go learn something new.
    I don't expect a medal and it is not like I am just out to prove somebody wrong. It is just that I know that sometimes people use the terminolgy the wrong way and after a while it is accepted that way. I just don't want to be book smart that is why I am asking you guys just so I can see what is used in the real world, not just what you read written in a book by somebody who probably never worked on an air conditioner in his life.

  7. #7
    Bruce, I've got this small framed certificate in my study that reads member of the National Honor Society Alpha Beta Cappa 1985. You know what it means SQUAT. 10% books 90% field smarts. Don't stir the pot,bite your tongue.
    FEN

  8. #8
    Join Date
    Jun 2006
    Location
    Memphis
    Posts
    155
    I like mojo,s idea. There is no point in creating a disagreement with the instuctor.

    LRA are what the books say and can also be when a mechanical lockup occurs. Neither will last long. The inrush current to start the motor is extrememly short and if mechanical lockup occurs either the breaker or the motors internal overloads should take it out as the motor heats up. Reoccuring of the latter will definitly lower motor life.

  9. #9
    Rule 1: Don't argue with the instructor.
    Rule 2: If the instructor is wrong, see rule 1.

    inrush current: Is very short and best measured with a meter capable of peak-hold readings. It's also dependent on where on the AC line cycle the motor is turned on. So, for each turn-on inrush current will be different unless it's done electronically at a zero-crossing.

    LRA (locked rotor amps)Steady state current when the motor shaft is not turning.

    FLA (Full load amps) Current draw at rated HP of motor.


  10. #10
    Join Date
    Sep 2006
    Location
    Tennessee
    Posts
    399
    I think the confusion would be in the term "locked rotor", which is simply that, a locked rotor. So the term Locked Rotor Amps refers to the amperage drawn at any time during which the rotor is locked, or not moving, whether it be start up or an actual mechanical lock up. So you're BOTH right.
    UA LOCAL 614...WE DO IT RIGHT THE FIRST TIME

    Always remember, those who hate you don't win, unless you hate them. And then, you destroy yourself. -Richard Nixon-

  11. #11
    Originally posted by the mojo
    Bruce, I've got this small framed certificate in my study that reads member of the National Honor Society Alpha Beta Cappa 1985. You know what it means SQUAT. 10% books 90% field smarts. Don't stir the pot,bite your tongue.

    Never said it meant SQUAT. I just wanted to know what you guys thought. like I said, I am not out to prove somebody wrong. This is for my own information and I do not even intend to go to the instructor. Funny thing is that everybody is avoiding the question that I am asking and they are making it an issue that I am questioning the instructor. You guys are no different then the other students in my class in that you can't think for yourself. Just like your saying goes, "question everything, assume nothing."

  12. #12
    Join Date
    Nov 2004
    Posts
    2,927
    Originally posted by bruce campbell
    It is just that I know that sometimes people use the terminolgy the wrong way and after a while it is accepted that way.
    I think that's the lesson to be learned today.dont worry about it

    -----------------------------
    I could be wrong about this part, but I think the LRA is a measurement...like if you hold shaft still and apply power and start drawing high amps...then the motor will burn up at so many amps in so many seconds.

    say, 40 amps in 10 seconds would be LRA 40.
    something like that

  13. #13
    Join Date
    Oct 2004
    Posts
    3,400
    Sorry, I went to work for a while and didn't check in.

    I didn't realize I had dodged the question.

    YES, LRA occurs during startup, just briefly (inrush amps).

    It also will occur in the event of a mechanical failure, until the motor smokes or the overload/breaker/fuse/diligent operator takes it offline.

    If you were in my class, and I was spouting bogus information, I would want you to call my hand.

    Sometimes, a shorthand version is taught, and not totally accurate. (A good example is using Ohm's law with AC.)

    Sometimes, an instructor is very strong in one area, and a little weak in others. A little nudge might get him off center.

    Please remember, the instructor is not your adversary.
    He is your advocate.
    He wants you to learn, do well, make a good living, and pay lots of taxes.

    Keep reading, keep learning, keep asking questions.

    I really appreciate a good question.
    It proves to me that you are bright & interested.

    Keep up the good work.

    What Jacob said...

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