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  • I design single phase transformers, around 400W - 8000W, but now im lost because i have to design a three phase transformer ( delta - delta connection ).

  • I searched, trying to find a page that can provide me the formulas, but I dont find anything.

  • For what I know, if its delta-delta:

1. Phase Voltage = Line Voltage
2. Phase Voltage = ( 1/sqrt(3) ) * Line Voltage

  • When designing, I will have 3 cores, so the voltage in the three of them will be the same, and the current it will be 1/3 in each one? or it will be 1/sqrt(3) in each one?

  • In single phase transformers, a good and easy way to know the maximum power of the core its, measure the interior part, lets say side A, and B. So it will be A * B = Area ( cm*cm), and then the Pmax will be Pmax= Area * Area. How can I do this in the three phase transformers? Its the same? I Can choose the core between some that are in this page: http://www.pryde.com.ar/subcategorias.php?c=1&s=2

-The transformer I want to design its: Voltage input: 380V, Voltage Output: 51V, Secondary Current: 100A, Connection: Delta-Delta. So if im not wrong it will be:

  1. Pmax = 51v * 100A * sqrt(3) = 8833W
  2. Each Core Pmax = 51v * ( 100A / sqrt(3) )
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  • \$\begingroup\$ @Transistor Yes, im sorry, i didn't know how it was named in english... \$\endgroup\$
    – samuel mattio
    Commented Jun 12, 2023 at 19:17
  • \$\begingroup\$ Have a look at 3-phase core laminations(?) (for potentially unbalanced loads there used to be cores with 4 or 3 2/2 limbs). As to power/current, it is complicated unless load is balanced. (They managed to screw up the image of UI core sheets?!) \$\endgroup\$
    – greybeard
    Commented Jun 12, 2023 at 20:03

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When designing, I will have 3 cores, so the voltage in the three of them will be the same, and the current it will be 1/3 in each one? or it will be 1/sqrt(3) in each one?

Delta-in to delta-out means basically three individual transformers with no core sharing. Treat the design just like three single phase transformers with each handling one-third of the power. Based on this you don't need to worry about \$\sqrt3\$ or anything like that.

enter image description here

Image from Three Phase Transformers

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  • \$\begingroup\$ So if i want to design a Three phase Transformer, Vin=380v, Vout=37V, I=100A. The power of each core will be (37v * 100A)/3 = Around 1233W per core. If i use a core 5.2cm x 7.2cm = 37.44 cm^2, it will be OK, because the max power of the core will be around 1400W. Thats ok? \$\endgroup\$
    – samuel mattio
    Commented Jun 12, 2023 at 23:23
  • \$\begingroup\$ @samuelmattio your first sentence starts like a question and ends like a statement with a period so I don't know what to make of it. What is important is the final load power and you don't mention it other than by inferring it from voltage and current and that's the wrong way round compared to normal circumstances. I cannot tell you whether certain core dimensions imply that the power handling will be OK. I always calculate from 1st principles and this takes into account number of turns and core mean length. \$\endgroup\$
    – Andy aka
    Commented Jun 13, 2023 at 7:38

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