open delta waveforms

Discussion in 'General Electronics Chat' started by alim, Aug 11, 2009.

  1. alim

    Thread Starter Senior Member

    Dec 27, 2005
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    1
    Hi to all Electrical engineers, I have searched this site and was unable to find what I am looking for, that is, it is stated that an open delta provides three phase power , do anyone has a diagram of the waveforms, showing the three currents or voltages displaced from the two coils on each side of the transformer.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    782
    I have tried some simulations of the open delta circuit - I took a simple resistive balanced 3-phase load case.

    Attached diagrams show the simulation schematic and current probe waveforms. You'll note the primary line currents are asymmetrical but balanced at any instant. The load side line & phase currents are symmetrical balanced. I'm not sure this is correct but if so this is an interesting observation.
     
  3. alim

    Thread Starter Senior Member

    Dec 27, 2005
    113
    1
    Hi t n k I will D/L and take alook, I would like to express my appreciation for your effort and the interest you displayed. Thanks again. alim.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Hi Alim,

    Glad to be of help if possible.

    I actually checked my simulation again and found the asymmetry in the primary waveforms was due to assumptions about the transformer magnetizing inductance [Lm] - i.e. too low a value used in the simulation. By increasing the Lm value, the waveforms on the primary side converged to a typically symmetrical balanced 3-phase set. With the primary to secondary ratio set as 1:1 for the transformers, the primary and secondary line current waveforms were identical in magnitude and phase. The only difference was the relative phase relationship between the primary abc phases and the "assumed" secondary A'B'C' phases.

    So the open delta behaves as a true 3-phase system and the only (ideal) difference is that the transformers used must be derated proportionally in relation to their true (closed) delta configuration equivalent rating. For the same physical transformer design, the normal capacity of the open delta arrangement is reduced to 57.7% of its delta rating. There are apparently other issues with real transformers including reduced voltage regulation performance in the open case.

    Cheers.
     
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