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Primary winding and secondary winding wraped around a iron core

  1. Apr 9, 2010 #1
    I got a few questions about transformers..

    I understand there is a primary winding and secondary winding wraped around a iron core.

    Now for the primary connection - take 7.2kv 1 phase line and connect it to one side of the winding and take ground (earth) to the other winding end.

    On the secondary winding if the windings are set up right by amount of turns i can get 240 volts correct. -120v one line and +120v on the other

    If i connect a wire in the center of the secondary winding the voltage would be 0 here? and i get 120 between center tap and line? connecting the middle of the winding to ground is just for protection? ( just making sure that the connection from the middle of the winding to ground is whats making it get 120)
     
  2. jcsd
  3. Apr 9, 2010 #2

    stewartcs

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    Re: Transformers

    Give this a read: http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/hsehld.html

    It should answer your questions.

    CS
     
  4. Apr 9, 2010 #3

    sophiecentaur

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    Re: Transformers

    If there is no connection / path to ground from your secondary then the Potential Difference between the coil and ground is totally undefined. (It 'floats'; there is no DC connection via a magnetic coupling) If you choose to connect the centre tap to Earth then the extremities of coil will be in antiphase, symmetrical about Earth. The PD between the two is the same whatever part of the coil you connect to Earth. (Needless to say, you can only connect one part if you don't want a smoke making machine)
     
  5. Apr 10, 2010 #4
    Re: Transformers

    Just need one thing cleared up when you are talking about the PD between the two is the same do you mean between one phase an the center tap?

    What if i where to connect a tap 3/4 of the way in the coil to earth what would the voltages be from each phase to earth?



    And also does this connection i mention make sense at all or does it work? Now for the primary connection - take 7.2kv 1 phase line and connect it to one side of the winding and take ground (earth) to the other winding end. (from powerlines don't undersstand if they take one phase and earth or use two phases to make 240 volts in the end)

    On the secondary winding if the windings are set up right by amount of turns i can get 240 volts correct. -120v one line and +120v on the other (like showed in the link)
     
    Last edited: Apr 10, 2010
  6. Apr 11, 2010 #5

    sophiecentaur

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    Re: Transformers

    Steady on - we don't have two "phases" yet; that's a bit of shorthand which supply engineers use. All we have is the ends of two wires. The volts on end A are alternating positively and negatively relative to end B (the PD) - each turn of the secondary is providing a small PD, which adds up to the total. If you connect the centre to Earth, the same PD exits across AB as before but relative to Earth the voltages at A and B will be half the original PD and in antiphase. If you put a tap 3/4 of the way between A and B, the Volts on A will be three times that at B and still in antiphase. Again, the total PD between A and B is the original value.

    But what you are suggesting is only the equivalent of having two separate secondary windings and connecting them in series to get a higher output voltage. The Volts per turn are the same everywhere in the transformer and you can just add up all the turns (carefully connecting so that the Volts to add and not subtract).

    Whether or not there is one leg of the input connected to Earth makes no difference to the secondary volts - why should it? There is no difference in the magnetic flux change - linking to the secondary (i.e. no DC involved). If the supply is floating then you should leave it floating or your Neutral conductor is being shorted by the Earth return which may not be what the supplier wants. You can do what you want as regards Earth connections in the secondary once you are isolated with a transformer - as discussed above.

    In your last sentence, you don't really mean +120 and -120V, do you? You surely mean 120V AC and 120V(@180degrees) AC.
     
  7. Apr 11, 2010 #6
    Re: Transformers

    If you connect one lead of the primary winding to 7.2k volts and the other lead of the primary to earth isn't this a short circuit?
    Isn't it like connecting a hot wire in your house to ground what am i missing here
     
  8. Apr 12, 2010 #7

    sophiecentaur

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    Re: Transformers

    I think you are missing the Primary Inductance.
    By your argument, all transformer primaries would be short circuits to AC.
     
  9. Apr 12, 2010 #8
    Re: Transformers

    Can you explain in more detail please
     
  10. Apr 13, 2010 #9

    sophiecentaur

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    Re: Transformers

    Assuming the transformer is designed correctly (enough turns per volt) the reactance of the primary will mean that it will not constitute a short circuit to the AC. The only current that the transformer will draw will be due to current in the load placed across the secondary.
    That's how all (conventional) transformers work. Look in Wiki.
    I think the OP is describing conventional use.
     
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