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AFAIK the typical transformer design includes the primary being wrapped closer to the core and the secondary being wrapped atop of primary and so further from the core. Why this way and not the other way around?

This question presumably mentions the design where the secondary is closer to the core and made of thick wide aluminum tape. It's a last mile distribution grid transformer with primary being fed with something like 6 kilovolts and secondary producing something like 110-230 volts (consumer voltage), so the secondary has 30-60 times higher current than the primary and with that current I think it's reasonable to place the secondary closer to the center so that each turn is shorter and the secondary itself is shorter and therefore has lower resistance and lower losses.

The same reasoning should apply to all transformers which lower the voltage - the secondary voltage is usually 10-20 times lower than the primary voltage (110-230 volts primary vs 12 volts secondary is typical) - it's reasonable to keep the secondary wire as short as possible to lower the losses (and save some thick wire).

Yet I've never seen a transformer with a secondary located closer to the core (except in the linked to question).

Why is primary winding usually closer to the transformer center and has smaller coil loops?

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  • \$\begingroup\$ I can't answer, but I can recommend two good books: The "J&P Transformer Book" and the "ABB Transformer Handbook". Both go into great detail about transformer design principles. \$\endgroup\$ – Li-aung Yip May 28 '15 at 15:12
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For "concentric" wound transformers, I find both ways of them to be used, and the factors influencing it are most likely (may not all apply to your case):

  • Cost of the material involved. Higher current needs thicker wires, but the same number of turns (and even thicker if its on the outside because it is longer there). Just calculate what is needed for the number of turns required in either inner or outer layer, and then do a comparison. It seems to me the higher the step down ratio, the more beneficial it is to keep the high current ones inside.
  • Tapping. A lot of transformers have multiple tappings. Precise positioning and space for them is a lot easier on the outside than it is on the inside.
  • Serviceability and failure modes. Bigger transformers are actually so expensive that it might be feasible to repair them. Depending on the expected failure modes putting one or the other winding on the outside is more useful.
  • Rectangular cores might need different material strenghts and properties since internal forces are higher than in circular cores. Aluminum might be better suited here than the softer copper. This applies of course only at rather high currents (that are distributed unevenly in the conductor). Metal fatigue considerations however might drive you away from aluminum here.

Note that I don't call them secondary/primary but low/high voltage sides, I think those factors are more influential than the direction they are used in.

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There are probably several reasons.

First off the manufacturer may have primary winding coils wound up for a whole family of transformers that use the same turns count and wire size. Secondaries are added as needed according to output specs.

Secondly it is probably always a simple calculation of wire total resistance versus total length. Primaries of the type of step-down transformers that you mention have many more turns of smaller gauge wire. Putting it near the core has a greater advantage for lesser total length and thus less total winding resistance. So it comes down to a simple trade off computation between secondary and primary.

I could imagine that there is a computation that involves wire size loss versus total winding length that resolves to the lowest physical size. The reason we see most step down transformers with the primary on the inside is because the computation most often wins out that way.

I look back to my early teen age years when I was very glad that old TV power transformers had secondary windings that I could easily remove and replace with my own to achieve the voltage and current ratings I wanted without having to touch the primary windings.

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I googled "power transformer construction" and copied various images: -

I think most small - medium power transformers have primary and secondary stacked side-by-side: -

enter image description here enter image description here

Here's one where the primary (HV side) is on the outside: -

enter image description here

And another: -

enter image description here

Considering winding losses and turns ratio, it make sense to wind the low-voltage coil on the inside. Broadly speaking, the average length of 1 "inside" turn might be half of that when wound on the "outside". This means the outer winding is probably double the resistance of the inner winding for the same number of turns.

The low voltage winding resistance is "seen" on the high voltage winding by the ratio of the turns ratio squared and, given that the current drawn from the secondary is reduced by the turns ratio (not squared) onto the primary it makes sense to keep the low voltage winding as short as possible.

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Two reasons : Safety and efficiency.

Safety : if the primary winding insulation breaks down, it is better protected from the external environment (your hands!) and closer to components that are likely to be earthed, namely the inter-winding screen (where a screen is employed). Thus a primary breakdown should connect live to earth, tripping breakers or fuses.

Efficiency. (1) The winding closest to the core is most tightly coupled to it, producing the magnetising field with lower losses. (2) As more power is dissipated in the primary, it makes sense to wind this with the shortest turns to minimise its resistance and I^2*R losses.

Both of these are relatively minor advantages in the overall scheme.

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