Hi.
Toroidal core transformers are supposed to confine their magnetic field within them without no external emissions.

Why is the flux leakage larger with a EI core ? Or is it not ?

If the central arm of the 'E' is cut out, becomes a toroidal core; right ?

 

There is magnetic radiation outside of the core with an EI style.
With a Toroid, the core is contained completely within the coil.
To make a EI a toroid, the centre strip would have to be eliminated and the coil wound on the outside cores.
Max.

 

wrong. air gap. leakage.

no air gap on toroid. no leakage.

 

A toroid coil has much less stray magnetic field, but they will still interact if they are excessively close to each other.E-I core transformers mostly have no air gap, but still there is a flux field both in and surrounding the iron/steel core. It does leak out the corners, and it does tend to grow outside where there is no coil to capture the flux. So if you wound coil over all the E-I core there would be less leakage flux, but an impossibly xpensive transformer to produce.

 

wrong. air gap. leakage.

no air gap on toroid. no leakage.

Not all toroidal are gap less, some are done with a gap for a purpose. https://www.powerelectronics.com/content/why-have-air-gap

 

If the central arm of the 'E' is cut out, becomes a toroidal core; right ?

Many large VA toroidal transformers working on mains frequencies are made from strips of iron wound in a toroid shape.

 

The preferred power transformers that my workplace used in equipment manufacturing were like a coiled core of a single iron ribbon with the windings on them.

Core was something like :

 

Core was something like :

Toroidal !
Form of.
Max.

 

Found the exact image; called 'r-core' transformer. Suppose r for ribbon.

 

Found the exact image; called 'r-core' transformer. Suppose r for ribbon.

Toroidal are made the same way with a continuous steel ribbon, coiled into a circle.
Max.

 

Air gaps are usually included to avoid magnetic saturation. At least that is the explanation that I was given long ago. I suppose it would, but it also tends to reduce the effective inductance, I think.

 

Air gaps are usually included to avoid magnetic saturation. At least that is the explanation that I was given long ago. I suppose it would, but it also tends to reduce the effective inductance, I think.

From my earlier link, the first paragraph -
"What exactly is the function of the air gap? When asked this question, most engineers respond, “It prevents core saturation.” Although this may be true in certain cases, it's not true in general. In fact, in a transformer, the air gap will not prevent saturation caused by excessive ac voltage polarization, as we will see. Further, the air gap has several other critical functions" https://www.powerelectronics.com/content/why-have-air-gap

 

Very fortunately I have not had to design transformers, toroid or otherwise. I have built a few and added a winding to a couple, but have not had to start from zero and create a design. And I am quite grateful for that.

 

The core of a transformer consists of a magnetic substance named cold rolled grain directed silicon steel (CRGO steel), with a strong permeability. The core is constructed of thin CRGO sheets to reduce eddy losses.

Ferrite cores are used in high-frequency applications, because of lightweight and low loss instead of the iron cores.

 

Very fortunately I have not had to design transformers, toroid or otherwise. I have built a few and added a winding to a couple, but have not had to start from zero and create a design. And I am quite grateful for that.

In the "SchematicsFor Free" website there is one paper about designing audio transformers. And there are other sources as well. The important thing is to have enough iron to transfer all of the energy without magnetic saturation, and it is also important to have enough turns to provide adequate magnetization. A caution is that many of the formulas give minimum values with the presumption being that minimum cost is the primary goal. But the cheapest transformer is NEVER the best for the application.

 

Now THAT's a transformer!