Why is the EI- Core most popular for power transformers?

Note that the majority of small transformers (single-phase, 100-240V, 50hz/60hz, laminated core) are shell-type, with an EI-shaped core. Yet, as I understand it, they have comparatively poor productivity due to the visibility of much of their surface area; not wire wounds. There is also a lot of information on the web on the advantages and drawbacks that toroidal cores provide over EI; being entirely protected by winding increases performance, but they are harder to DC offset, more difficult to wind as they have to be wound when the core is fully assembled (as compared to being wrapped around a bobbin until the core is formed around it), and the installation becomes less straight-forward.

But I want to learn why EI transformers are always preferred over core constructions such as U-core, C-core, or L-core? These are presumably more effective due to the fact that more surface space is protected by winding (even though less than toroidal). They tend to have the same construction advantages as the EI transformers, such as being wrapped on the coils before the core is mounted, mounting methods that are just as simple as the EI, and laminations for certain designs can be removed from a rectangular sheet without substantial waste (such as EI cores and toroidal cores). Furthermore, air distances increased the impact of saturation.

And what are the drawbacks of these core-type transformers that make them less popular than the EI-core ones?

E-I cores are flat stampings with no bending or forming needed, so cheap to manufacture.
They interleave in a way that allows simple corner fixings hold the whole thing together - or just vacuum impregnation with lacquer is sufficient.

The usual "E" interleave also means they have very good magnetic coupling between the two ends of the core, as the E legs overlap layer to layer.

Symmetrical C or U Cores need additional clamping as there is no interleave to give a mechanical joint, as with flat laminations.

Plus, the core-to-core joint is a flat surface, which must be very precise to get good magnetic coupling - the cores have to be ground or or finished in some way after forming to get a good joint, which makes them a lot more expensive than simple stampings.

I believe L cores are quite often used, but I think they are generally in welded core type transformers, where the core can be clamped for welding after assembly into the coils.

eg. - A C core type, with metal strapping to keep the cores in place:

Winding a bobbin for an ei is fairly simple, you do so before you assemble the trans, winding a torroid is harder, search for it on youtube.
What were your intentions of using dc offset?, or was it just for discussion.

Actually with the improved machinery to do it now, the Toroidal is becoming very popular.
.Not only more efficient that the EI, they are very easy to modify or add an overwinding.
Max.

Yes I believe so.
I've never worked on a torroid winding machine, I have worked on bobbin winders & one time a multicore cable making machine, that was a work of art.

EI core steel stampings are what is known as scrap-less design. It makes very efficient use of the magnetic steel sheet and reduces cost.

Get yourself a pair of Es and a pair of Is, and assemble them together and you'll see.

schmitt trigger said:

EI core steel stampings are what is known as scrap-less design. It makes very efficient use of the magnetic steel sheet and reduces cost.

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Not quite as lossless as Toroidal though
Max.

I am not talking about loss-less.... I am talking about scrap-less. Meaning that during stamping there are no empty spaces of sheet metal which would be scrapped.
Magnetic losses is another story.

Anyone who's pulled a 100va transformer apart will know how tricky it is to get all the e's and i's back.

dr pepper said:

Anyone who's pulled a 100va transformer apart will know how tricky it is to get all the e's and i's back.

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In my early days when money was tight, I did my share of dismantling a few, rewind secondaries on the bobbin/primary and re-assemble.!
Max.

So you'll know whats its like then, hands with little red lines all over them.

I used to hand wind bobbins for a living lol They were on ferrite C cores though, there were the occational ones which just had a ferrite rod going through them too.

(I say hand wind, we used a mandrel which had a gear ratio of about 1:3, but with a 2500 turn coil, that is still a lot of turns of the handle)

Prototype said:

that is still a lot of turns of the handle

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This machine had a handle decades ago. It was replaced with a motor. I have many different types of transformer/coil winders.

ronsimpson said:

This machine had a handle decades ago. It was replaced with a motor. I have many different types of transformer/coil winders.
View attachment 124188

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Well I was going to post a picture of the ones we used to use but image search seems to be broken at the moment, but the gears were enclosed and the turns counter was more like a car odometer.

Prototype said:

like a car odometer

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Built this one for miniature transformers. Counter reads in 1/10 of a turn.

Not pictured, I have some really big coil winders. You would think that is all I do.