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Thread: Reversing a 3 phase motor.

  1. #1
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    Reversing a 3 phase motor.

    In one of Mike Holt's exam prep books, it says to reverse a 3 phase motor you can switch any two leads. It goes on to say that the industry practice is to switch L1 and L3.

    This goes against what I was taught in my early days, which was to reverse L1 and L2 or L2 and L3, but not L1 and L3.

    Why would I have been taught not to reverse L1 and L3 if it doesn't make a difference which two leads are swapped?
    The Man Prayer: I am a man ... I can change ... If I have to ... I guess.

  2. #2
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    Quote Originally Posted by Stevareno View Post
    Why would I have been taught not to reverse L1 and L3 if it doesn't make a difference?
    That I do not know. All I can offer is that if you reverse any two, the motor will spin in the opposite direction. Perhaps it has something to do with physical positioning. If, for example, the three connections are in a straight line, then L1 and L2 are next to each other, and L2 and L3 are next to each other, but to swap L1 and L3 you have to move both sets of wires past L2. That might make the job a bit harder to do, and might make the final connections look a bit ugly. That is my best guess.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    I have always picked the two easiest. Never really cared which one was L1, L2 or L3.

  4. #4
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    If thats the only bad teachin' you get, you will be way ahead in this game.
    You've been here for 5 months. We're slipping if we have not given you more bad advice than the reversing teracher. :-)
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  5. #5
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    I look at it this way. Say you have a 480 volt motor with your leads marked brown-orange-yellow. when looking at them in the motor starter they are in sequence left to right. if you swap L1 and L3 the will be brown-orange-yellow in sequence right to left. It just makes it easier to tell at a glance that that particular motor needs to be running in the opposite direction from similar motors in your facilities.
    Some people are like slinkies. They serve absolutely no useful purpose. But still put a smile on your face when pushed down a flight of stairs.

  6. #6
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    Quote Originally Posted by charlie b View Post
    That I do not know. All I can offer is that if you reverse any two, the motor will spin in the opposite direction. Perhaps it has something to do with physical positioning. If, for example, the three connections are in a straight line, then L1 and L2 are next to each other, and L2 and L3 are next to each other, but to swap L1 and L3 you have to move both sets of wires past L2. That might make the job a bit harder to do, and might make the final connections look a bit ugly. That is my best guess.
    Another thought.
    The motor terminal box, in my experience, has a cable/conductor entry point located symmetrically. That means that L2 will be located closest to that entry and L1 and L3 equally displaced from it..
    That means that the conductors from entry to L1 and L3 would be equal to each other and longer than from entry to L2.
    Swapping two conductors of equal length just might be easier and neater than two of unequal length.

    Just a thought and not asserted with any degree of certainty but it may be what's behind the L1-L3 swap.

  7. #7
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    reversing tecniques.

    One of the best tricks to play one the new guy is to:: turn the fuses "upside" down to make the motor run backwards. Its easy -- oh no the motor is running backwards, pretend to be befuddled...... send the new guy for "parts" (a skyhook or something)...... quickly switch the leads.... when he gets back , act surpised to see that the fuses are upside down, Then, While he is watching - reverse the fuses and watch the motor spin.

  8. #8
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    Quote Originally Posted by augie47 View Post
    If thats the only bad teachin' you get, you will be way ahead in this game.
    You've been here for 5 months. We're slipping if we have not given you more bad advice than the reversing teracher. :-)
    Well, that bad(?) lesson was taught to me 20+ years ago, and I'm sure over the years I've passed the myth along.
    Instead of being taught wrongly, I was hoping that maybe it had to do with a special situation (where reversing L1 and L3 doesn't have any effect) that someone here might have heard of.
    That's the beauty of this forum. A vast wealth of experience. If no one here can think of something, I will be able to mark it off as just being mis-informed.
    BTW, I was union schooled if that makes any difference. I've had my fair share of fetching left handed benders and wire stretchers.
    The Man Prayer: I am a man ... I can change ... If I have to ... I guess.

  9. #9
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    In my part of the world it is standard practice to reverse the motor at the load side of the disconnect/starter.
    It is also not unusual in my part of the world to encounter a grounded B system.
    With the grounded b system you will have a white wire on B phase, so moving that wire to A or C would be a code violation.
    Hence our standard practice of swapping A & C, 1&3.

  10. #10
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    Quote Originally Posted by Stevareno View Post
    Well, that bad
    Possibly, they let me post here....
    "Electricity is really just organized lightning." George Carlin


    Derék

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