Quantsuff sent me a link to a short video of a toroid being wound using the cheating method. The toroid is broken in half, the turns are wound on and the two halves glued back together. Quite some time ago I participated in a discussion about doing this or a similar method and I’m not certain what the results would be.
Some of the discussion was about storing energy in the air gap. For instance, a conventional E and I type transformer can be built two ways. One seen in power transformers is to alternate the Es and Is so that there is no air gap, and effectively the transformer has a solid core. The other way is to stack all of the Es together and all of the Is together and then put the core on the Es, add a layer of paper over the bare ends of the Es, and put the Is over the paper. All of the Is are spaced away from the Es by the thickness of paper, creating the air gap. This method is used in audio transformers, and I forget what the advantage is .
Back to the broken toroid. My take on this is very confused. First off, we have all of the inductors made by putting two pieces of ferrite together, either with or without glue. These include the small transformers often found in power supplies which have the equivalents of Es and Is, and are assembled after the windings are put on. Also there are the pot cores, similarly assembled after the bobbin is put on. These have flat faces on the two halves, and are assembled with no spacer other than the glue itself. These are used for all sorts of purposes, including power supplies. Breaking the toroid core in half and gluing it back together is similar to the above kinds of inductors, as long as the core breaks cleanly and there are no lost pieces when they are glued together.
But then there is the whole reason why the toroid is used, with no breaks or halves to be assembled. Obviously breaking and gluing the halves back together is different than a solid core. How much different? Does gluing it back together give it an air gap? Or does gluing it back together give it the same qualities as a solid core? How much difference is there between the two? Remember that the ferrite core material is made up of tiny magnetic particles suspended in a non-magnetic glass-like material. If it is broken and glued back together, is the break much different than the gaps between particles that are already there?
One thing I should add is that the toroids and other cores come in various materials that have various permeabilities. One thought is that if I have high permeability and low permeability cores, does breaking them in half affect one more than the other? My guess is that the high permeability core will suffer the most.
I have never seen a toroid split in half and reassembled in any of the hundreds of inductors that I have worked on in various kinds of equipment, mostly power supplies. If there was negligible difference between an unbroken and broken toroid, then one would think that it would be used more often. One thing that may influence using this method is that toroids may be difficult to break in half cleanly, with a larger than acceptable loss in bad breaks. But I have never ever seen this broken toroid method used in any project, other than the link above. This leads me to conclude that there is not a negligible difference, that it’s not an acceptable method of winding a toroid.
But the broken toroid method is not much different than assembling two halves with glue. This two piece method seems to be acceptable to many if not most transformer designs. Could a toroid be made in two halves to allow easy winding? Perhaps the whole reason for making a toroid is so that the magnetic field has the shortest path, a circle, and if it’s disrupted by a break, the shortest path is no longer so short, and the toroid is more like a two part core. There is no longer the advantage, so the core might a well be a two part core.
I’ve read that the pot core, which is a two part core, is very good at confining and conducting the magnetic field. it holds a nylon bobbin that makes it easy to wind the coil. You just insert the two halves and either glue them together or put a screw and nut through the center hole. Some pot cores have a clip that serves to hold it together and it also has solder tabs to hold it to the circuit board. I don’t see pot cores often, probably because they are more expensive than other coil forms. I often see the two part ferrite transformers with E type halves. And one thing that needs to be looked at is that the typical power supply has a mix of toroids and two part transformers. If the transformers had the same quality inductor as the toroids, one would think that either one or the other would be used, unless there was a large enough price difference to justify using one or the other.
I’ve raised several questions and speculated a lot, and by now I’ve made it clear that I really am clueless about much of this field (no pun intended). I really need to study some documents to find out more about the field.