Take a look at the image. I always understood that transformers have as little air gaps in them as possible, but you can easily look through this mains transformers that I found in China. Not just for the EM-field, but also for dissipation purposes I'd expect the transformer to be one huge brick of metal. Where is my thought wrong?

mains transformer

  • \$\begingroup\$ If you found it somebody may have lost it. I would bring it to the nearest police station. \$\endgroup\$ – stevenvh Sep 7 '12 at 13:23
  • \$\begingroup\$ They're common transformers, not only in China. \$\endgroup\$ – Vlad Sep 7 '12 at 19:43

Those fins you see are for cooling (they're full of oil). They have nothing to do with the electrical function; that's all inside the central casing.

  • \$\begingroup\$ But would fins instead of the thin beams not have lower thermal resistance? \$\endgroup\$ – jippie Sep 7 '12 at 13:42
  • \$\begingroup\$ Ha! You just asked another question. No, the thermal resistance from the transformer outward into the fins would go up if they were thin. The reason fins are made thin is so they can get more of them into a given space. \$\endgroup\$ – gbarry Sep 7 '12 at 14:25
  • 4
    \$\begingroup\$ The "thin beams" are hollow; as I said, there's oil circulating inside them that carries the heat from the transformer core by convection. \$\endgroup\$ – Dave Tweed Sep 7 '12 at 14:37

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