1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do we use the 3phase and not the 4phase or other phase supply?

  1. Jun 23, 2012 #1
    why do we use the 3phase and not the 4phase or other phase supply???

    please answer....
     
    Last edited by a moderator: Oct 24, 2012
  2. jcsd
  3. Jun 23, 2012 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: why do we use the 3phase and not the 4phase or other phase supply???

    do you mean 3 phase instead of 4 wire systems, or do you mean 3 phase instead of more than 3 live wires ? If the first, no need for the neutral if load is balanced amongst the live phase conductors as in AC transmission, although 4 wire is common at the lower distribution voltages main lines that directly supply customers. Three phase has become the norm for AC transmission, due primarily to economics. And Tesla. Some research was done awhile ago on 6 phase systems during the EMF issues of health concerns, but nothing was ever implemented of significance.
     
    Last edited: Jun 23, 2012
  4. Jun 24, 2012 #3
    Re: why do we use the 3phase and not the 4phase or other phase supply???

    I think its due to Star-Delta transformation.
    Transmission using delta , 3 wire(no neutral) and users star formation
     
  5. Jul 3, 2012 #4
    Re: why do we use the 3phase and not the 4phase or other phase supply???

    thanx for your supportive answer bt i think you did not get my question..ok let me elaborate it further that my question was...why do we use the 3 phase supply like in generation, transmission etc,,and why these devices are not supplied 2phase or 1phase supply??
    please elaborate!
     
    Last edited by a moderator: Oct 24, 2012
  6. Jul 3, 2012 #5
    Re: why do we use the 3phase and not the 4phase or other phase supply???

    I think that the only answer to this is that Tesla decided to use three phase, and once the early infrastructure was laid down there was no point in changing to any other combination.
    Why Tesla went for three phase I don't know but would guess that it's a compromise between fears that two phase power would cause synchronous motors to run less smoothly then three phase and the extra cost of cabling that would be needed by four phase.
    As an after thought and I'm having real trouble visualising it, would a two phase motor run in the same direction every time you started it?
     
  7. Jul 3, 2012 #6

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: why do we use the 3phase and not the 4phase or other phase supply???

    In a three phase AC system, the voltages between wires are time wise 120 degrees apart , and same with the current. With some vector algebra, the power delivered can be shown to be equal to (root 3)(V)(I) , where V is the voltage difference between any 2 phase conductors. For a 115 kV phase -to - phase 3 phase line, carrying 1000 amps in each conductor, this is about 200 MW of power assuming unity power factor and no line losses. Now with a single phase line (2 wire system with neutral return), you only get 115 MW out of it for the same current....it's economics....and other factors already mentioned.
     
  8. Jul 4, 2012 #7
    Re: why do we use the 3phase and not the 4phase or other phase supply???

    There are a couple of problems here by confusing V line and V phase (VL =VP/Root 3) in the first case V is VP in the second V is VL.

    Power in a 3 phase system P= 3*VL*I (the 3 comes from the fact that that you have the same I on three conductors)

    VL=VP/(root3) so P=(root3)*VP*I

    BUT if you are taking power from a single phase P=VL*I
     
  9. Jul 4, 2012 #8

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Re: why do we use the 3phase and not the 4phase or other phase supply???

    The terminology is confusing. For a three phase AC line, the voltage between any 2 phases, A to B, A to C, or B to C, is called the line voltage, V_ l, or what I refer to as the phase to phase voltage. The voltage between any one phase and ground is called the phase voltage, V_p, or what I refer to as the phase to ground voltage, and where V_l = (root 3)V_p.
    For a 3 phase 115 kV phase to phase line voltage, V_l = 115, V_p = 66, and Power = (root 3)(V_l)(I_l) or 3 (V_p)(I_l) if you wish, same result.
     
  10. Aug 2, 2012 #9
    Re: why do we use the 3phase and not the 4phase or other phase supply???

    ok thanks phantomjay....
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why do we use the 3phase and not the 4phase or other phase supply?
  1. Phase shift using FFT (Replies: 11)

  2. Why do we find roots (Replies: 3)

Loading...