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I notice that most small power transformers (single-phase, 100-240V, 50hz/60hz, laminated core) are shell-type, with an EI-shaped core. But as I understand they have relatively low efficiency due to much of their surface area being exposed; not wound with wire. There is already much information on the web about the advantages and disadvantages that toroidal cores have over EI; being entirely covered by windings improves efficiency, but they are intolerant to DC offset, harder to wind because they must be wound when the core is fully constructed (as opposed to wrapped around a bobbin before the core is built around it), and mounting is less straight-forward.

But I want to know why EI transformers are often chosen over core-type core constructions such as U-core, C-core, or L-core? These are presumably more efficient, due to having more surface area covered in winding (even if less than toroidal). They seem to have the same construction advantages of EI transformers, such as being wound on bobbins before the core is assembled, mounting methods which are almost as convenient as EI, and laminations for some designs can be cut from a rectangular sheet without significant waste (like EI-cores and toroidal cores). Also, the air gaps reduce the effects of saturation.

So what are the disadvantages of these core-type transformers that leads to them being less common than EI-cores?

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Two reasons. The EI core needs only one bobbin, and it needs less iron. See the attached pic of an EI core and an U core made of its parts.

Double U core made of EI core

See how the EI core only needs 28 iron units while the U core needs 36? (Of course this is a simplification but even when you round the edges of the U core you are still missing 4 units.)

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  • \$\begingroup\$ I don't get it. What's with those two extra squares on the U-core? What's with all those extra pieces that you don't actually see in a transformer core? Shouldn't the U-core be have equal thickness at the top and bottom as the sides? \$\endgroup\$ – Electric-Gecko Feb 6 '17 at 18:05
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    \$\begingroup\$ The flux inside the core needs a certain cross section to flow. In the EI core picture above, that cross section is set by the width of the inner leg. It's 2 units wide. The flux then parts into two equal legs which each have a cross section of 1 unit. So there's 2 units cross section at all places. — Now count the top view area of the EI core. You can see it's 28 units. Okay? — Now take these 28 units and try to arrange them into a U core which also has 2 units cross section at all places. It's not possible. You need 36 units. If you round the edges of the U core you still need 32 units. \$\endgroup\$ – Janka Feb 6 '17 at 18:56

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