Connect with us

why 3-phase power?

Discussion in 'Electronic Basics' started by Alan Horowitz, Dec 30, 2003.

Scroll to continue with content
  1. what is the attraction of three-phase power? Why not 9 0r 317
    phases? Why not plain ole hot & neutral?
     
  2. Zak

    Zak Guest

    It is an efficient way to do the transmission, I understand - maybe the
    most efficient?

    It also needs just 3 wires. 90 degrees would need 4 wires or 3 wires
    with 1 wire that is thicker than the other two.


    Thomas
     
  3. It better uses transformer iron and makes better/cheaper induction
    motors. For making DC it requires less in the way of filtering. It
    would be nice if we had something like 6 phase power, we could get rid
    of the input filter caps in most equipment, the only part that we
    can't make smaller by increasing efficiency and frequency of SM power
    supplies.

    Best regards,
    Spehro Pefhany
     
  4. cpemma

    cpemma Guest

    The motor is effectively running on a much higher voltage than single phase,
    so more power/weight, and to change direction just swap any 2 wires over.
     
  5. Guest

    Because the power lost in the transmission line resistance is proportional
    to the square of the current, you want the current as small as possible.

    Ways to do that are to increase the voltage and to increase the number
    of phases.

    While there is no theoretical limit to the number of phases you can have,
    3 is the generally accepted trade off between efficiency and complexity.

    And in most places, "plain ole hot & neutral" comes in as 2 phase, also
    called "220".
     
  6. Brenda Ann

    Brenda Ann Guest

    Actually, this is normally called "split phase" rather than 2 phase because
    the the neutral 'splits' the single phase 240 volt drop.
     
  7. 9 phases.... 317 phases, have you ever wired a 317 prong plug?
     
  8. Guest

    Actually, what it's called is dependant on where and who you are.

    Engineers and electricians, for example, have different terms for the same
    things.
     
  9. I read in sci.electronics.design that Brenda Ann <>
    wrote (in <bssmvp$e07$>) about 'why 3-phase power?',
    The question of whether 120-0-120 is split-phase or two phase is not
    just a fighting issue in the US, it's a TOTAL ANNIHILATION issue. (;-)

    Wholly incredible to this poor Brit.
     
  10. Actually, what it's called is dependant on where and who you are.

    Engineers and electricians, for example, have different terms for the same
    things.[/QUOTE]

    Perhaps so, but "2 phase" is wrong.

    Best regards,
    Spehro Pefhany
     
  11. I read in sci.electronics.design that Frank Bemelman
    318; you forgot the ground prong.
     
  12. Guest

    How big would the sparks be if you crossed a British engineer with an
    American electrician?
     
  13. Frank Warner

    Frank Warner Guest

    320? Neutral...
     
  14. In England a 230 V single phase is just that, one wire is nutral, nominaly
    earth but seperate from the true earth, and the other caries 230V ac.

    I belive the American system is 2 x 110 V ac lines with respect to earth,
    but at 180 degree phase angle, to give 220 V ac potential between them.

    I would say it's fair to call this 2 phase.

    --
    Jonathan

    Barnes's theorem; for every foolproof device
    there is a fool greater than the proof.

    To reply remove AT
     
  15. How big would the sparks be if you crossed a British engineer with an
    American electrician?[/QUOTE]

    Here's a picture of a suitable American electrician, let's see how the
    Brits respond..

    http://www.networkwomen.com//images/NOVEMBER_2000_Story_Photo.jpg


    Best regards,
    Spehro Pefhany
     
  16. Yes (or is it now 240 nominal there)?
    120V, but close enough.
    Not fair, just WRONG, though I can see that electricians who think of
    "phase" as equivalent to "hot wire" might find it okay. True two phase
    could be converted to 3, 6 or whatever phase using Scott-T
    transformers. Not true for this, which is just a center-tapped 240V
    line. You can go right or left on a one-dimensional line from zero,
    but it's still in one dimension..

    Best regards,
    Spehro Pefhany
     
  17. Yes, and even while being extremely careful, I swapped
    phases 127 and 131, 177 and 178, 202 and 212, 214 and 215,
    and 301 and 311.

    I think a need phase tester too ;)
     
  18. With 3 phases you never run out of juice, even without a cap,
    so why do you want 6 ?
     
  19. BFoelsch

    BFoelsch Guest

    In many large solid-state rectifiers, (> 100kw) phase shifting transformers
    are used to generate 12, 18, 24 or even 30 pulses of current per cycle.
    Aside from simplifying the filtering, it simplifies the harmonic current
    problem on the supply line.

    But, no matter how hard you try, you can't do this with a single phase
    source, 2 or 3 or 10 wire. If a transformer has a single primary winding, it
    delivers single phase energy, no matter how the secondary is tapped.

    Not sure that anybody mentioned that a single phase motor has a 4x/cycle
    torque zero (2 voltage zeroes & 2 current zeroes). 3 phase eliminates these
    zeroes.
     
  20. Well... the Europeans used 220V, the British used 240V, and we are all now
    harmonising on a nominal 230V.
    Ah... thats why some american equipment I had did not like running on a 110V
    safty transformer I had for testing
    3 phase supplies have a 120 degree phase angle between them.

    Not true for this, which is just a center-tapped 240V
    I would hate to have you navigate for me if you don't see a diference
    between left and right...

    I would sugest you would learn the difference between a single and two phase
    system in a hurry in Russia, they earth the nutral leg to the frame on all
    machines and then have a seperate earth bond. ( don't try to use an RCD
    :) ).
    --
    Jonathan

    Barnes's theorem; for every foolproof device
    there is a fool greater than the proof.

    To reply remove AT
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-