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Reasons for Using 3 Phase System Compared to 4 or 2 Phase Configurations

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  • #1

    Reasons for Using 3 Phase System Compared to 4 or 2 Phase Configurations

    In this topic I will prove to you why we use 3-phase system and not single, 2, 4, 5, etc phases.
    There are technical and economic factors for this reason.
    I will start by showing you the technical issues when we use 3-phase system.
    Then I will explain the technical reasons.
    Finally I will present PowerFactory software simulation to help explain the idea.
    Tags: 3-phase, power system, powerfactory
    • Likes 5
  • #2
    Generation :

    It can be shown that higher voltage output from an alternator having 3-phase winding rather than a single phase winding for a given frame size.
    If P = number of phases
    π/P = angle subtended by each section of winding
    E = Total emf available.
    Therefore, machine volts. Amps = E.I sin π/2P
    It can be seen from table that 3 phase machines is 50% better than single phase.
    P=3 is the best compromise choice.
    P 1 2 3 4 5
    volt.amps/E.I 1 1.414 1.50 1.54 1.57
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    • #3
      Thank you Mark
      What are the other advantages. please explain.

      I think one of the advantages in 3-phase system is continuous power flow to load

      Click image for larger version Name: 575px-3-phase-voltage.svg.png Views: 95 Size: 25.1 KB ID: 37
      Last edited by Nasser; 12 August 2021, 07:01 PM.
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      Comment

      • Mark
        #3.1
        Mark commented
        14 August 2021, 09:13 AM
        Editing a comment
        Thank you for your post.
        I'm still working on this subject and will discuss other issues.
      • admin
        #3.2
        admin commented
        08 September 2021, 07:48 AM
        Editing a comment
        It is really concise and useful.
    • #4
      Transmission :

      More economical in use of copper to transmit a given power by 3-phase AC rather than single phase AC.
      It can be proved that power loss in single phase = 2/sqrt 3 x volume in three phase.
      It can be proved that volume of copper in single phase = 4/3 x volume in three phase.
      Last edited by Mark; 18 August 2021, 08:54 AM.
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      • #5
        Utilisation:

        Use of AC induction motors single phase version of machine is not inherently self-starting.
        By using 3-phase winding a rotating field is produced rather than pulsating field produced in single phase machine.
        • Likes 4

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        • #6
          Thank you for this interesting post

          Three phase system has many more advantages as compared to single phase system. The reasons mentioned by Mark are showing superiority of 3 phase compared to other number of phases, not only single phase systems.
          1. Power to weight ratio of 3-phase generator is high as compared to 1-phase generator. Therefore, for generating same poser , the generator size is smaller, and hence weight and cost.
          2. Three phase system is more economical in transmission and distribution of same amount of power, because the requirement of conductor material is less in 3-phase system as compare to 1-phase system.
          3. The instantaneous power produced by 3-phase supply at unity power factor is almost constant over the cycle results in smoother operation of the machine. Whereas in 1-phase system the instantaneous power is pulsating hence change over the cycle, which leads to vibrations in machines.
          4. Power to weight ratio of 3-phase motors is high as compare to single phase induction motor. This means for the same amount of Mechanical Power, the size of three phase induction motor is small as compared to single phase induction motor.
          5. The 3-phase induction motor is self-started as the magnetic flux produced by 3-phase supply is rotating in nature with a constant magnitude. However, in single phase induction motor is not self-started as the magnetic flux is pulsating in nature.
          6. Power to weight ratio of 3-phase transformer is high as compare to single phase transformer. This means for the same amount of Electric Power, the size of 3-phase transformer is small as compared to single phase transformer.
          7. If a fault occurs in any winding of a 3-phase transformer, the rest of two winding can be used in the open delta to serve the 3-phase load. The same is not possible in single phase transformer.
          8. The 3-phase system can be used to feed a single phase load, whereas vice-versa is not possible.
          9. DC rectified from 3-phase supply is having the ripple factor 4% and DC rectified from single supply is having the ripple factor 48.2%. Hence the requirement of the filter is reduced for DC rectified from a 3-phase supply.
          • Likes 3

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          • #7
            Thank you for this post.
            I think you confused people by comparing 3-phase to single phase .
            The original topic was about reasons for choosing 3 phase instead of adding more phases to get higher power transfer.
            • Likes 2

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            • #8
              Originally posted by Mark View Post
              Transmission :

              More economical in use of copper to transmit a given power by 3-phase AC rather than single phase AC.
              It can be proved that power loss in single phase = 2/sqrt 3 x volume in three phase.
              It can be proved that volume of copper in single phase = 4/3 x volume in three phase.
              If you can use mathematical equation would be great to explain your idea

              Comment

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