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3 phase system queries

  1. Nov 20, 2015 #1
    They say 3 phase system requires less conductor then single phase why that is so if we consider diagram below?
    Snapshotpf.jpg
    Also why here current going out when neutral is there in form of earthing
    Snapshot2pf.jpg
     
  2. jcsd
  3. Nov 20, 2015 #2

    anorlunda

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    A single phase line 1km long and rated for 1MW of power, requires 2km of wire. 1km going out plus 1km return.

    A three phase line 1km long and rated 3MW of power p, requires 3km of wire. Three times the power but only 1.5 times as much wire.
     
  4. Nov 20, 2015 #3

    russ_watters

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    More wires, but the amperage is lower.
     
  5. Nov 20, 2015 #4
    Why current should return?
    If the amperage is lower power would be less as P=i2r
     
  6. Nov 20, 2015 #5

    russ_watters

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    You showed it properly in your diagram...
    That's power loss due to resistance. It doesn't have anything to do with the power requirement of the load.
     
  7. Nov 20, 2015 #6
    So we should have in general more wires which 3 phase system has to lower the amperage?
     
  8. Nov 20, 2015 #7

    russ_watters

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    That is one benefit, yes. A 100 kW load at 480V is 208 Amps in single phase and 120 amps in 3-phase. So you can use much thinner wires, costing less overall even though you have an extra wire.
     
  9. Nov 20, 2015 #8
    How you have calculated that?
    P= V I
    So I =P/V, here I= 100kW/480 in both cases?
     
  10. Nov 20, 2015 #9

    russ_watters

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    Three phase power is P= √3 * V* I
     
  11. Nov 20, 2015 #10
    Okay now only this query in my first post remains.
     
  12. Nov 20, 2015 #11

    russ_watters

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    The neutral and earth may be connected, but they are not the same: the neutral carries current, but the earth should not be.
     
  13. Nov 20, 2015 #12
    I am not able to understand this. What I mean is why the flow of current is not in opposite direction in diagram?
    Earth will eventually pull all of current toward it.
     
  14. Nov 22, 2015 #13

    NascentOxygen

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    A big advantage of 3 phase power is that you don't need any "return" wire. With a balanced load, the 3 currents cancel at the load's star-point; so even if you did have a neutral wire connected, there will be only a small current (ideally zero) in the neutral.

    3 currents equal in magnitude but phased 120° apart, sum to zero.
     
  15. Nov 22, 2015 #14
    So in case of unbalanced loads this three phase is not good as compared to single phase?
    Why then we are connecting neutral wire in balanced load three phase supply?
    Can we have neutral wire in case of single phase also?

    What my main query is that when current is generated from three phase supply, why it is not pulled back by neutral wire (earthing) instead of going to loads.?
    Snapshot2pf2.jpg
     
  16. Nov 22, 2015 #15

    anorlunda

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    Your beliefs about earthing are incorrect. It does not suck down the power. All the earthing does is to establish a reference voltage, and to provide some safety in short-circuit conditions. In normal operation, nothing changes earthed or not-earthed.
     
  17. Nov 22, 2015 #16

    NascentOxygen

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    If a 3φ load is unbalanced, the star point will not be at zero volts; this may or may not be tolerable, it depends on the load/s and/or the degree of unbalance. The neutral wire serves to take the imbalance current, and fixes the star-point at zero. Even for a perfectly balanced load, the neutral wire may still be used for safety: should one of the loads fault then with no neutral the resultant voltage change could cause damage to the other loads.
     
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