Cracked Cores
Hi again,
I should have mentioned that a while back i did some experiments with toroid cores. As you know, toroid cores are made up as a 'ring' of magnetic material rather than have two halves, and require special winding equipment to wind in production.
Because the toroid is a complete ring with no two halves to separate, there's no way to put in a gap other than cutting a slot in the toroid. I would have done that, but i figured that the required gap would be too small for me to cut with my diamond saw, so the smallest gap i could make would be too large for the size of the toroid i was intending to work with (small like 3/4 inch or less outside diameter).
This led me to think up a new way to get a gap in the toroid. What i ended up doing was a series of experiments with 'cracked' cores. Yes that's right
Cracking the core by inserting it into a nice size bench vise and slowly turning the handle until the core actually cracks into small pieces. I found that by putting the core into a cloth first meant that after it cracked all the pieces were held in one place by the cloth, but tape around the outside would work too, and that usually i got four distinct pieces.
Once the core is cracked, the pieces are then glued back together piece by piece using super glue. The super glue provides a thin layer so the gap stays small enough. But it does then enter the magnetic path four times, so the total gap is four times the thickness of one layer of super glue. It worked though for the experiments, and i found that even this thin layer altered the permeability quite a bit and so rendered the cores more useful for some applications where before they would never work.
The improvement here though would be to find a better kind of adhesive, one that would not break down with temperature. The super glue probably severely limits the useful temperature range so another kind of glue would be better. Maybe high temperature epoxy.
The thing about the gap in any construction though is that it should be of uniform thickness along the direction of the magnetic path. If it is thinner in one place than another because of poor machining, it could cause the core to saturate. I've never experimented with this though but always meant to. It's unclear whether the core would be less useful because part of it saturates and the other part doesnt, or if the part that saturates somehow affects the part that doesnt, but i would think that some measurements would clear this issue up too. Also, the machining process is usually done pretty accurately but that's mainly to get the faces to match up near perfectly. Those machines are very expensive too