Why a ferrite core?

[jadaro2600]

So they sell these special ferrite cores which apparently reduce eddy currents with a special compressed bead structure.

What's the point?

I've seen people using these ferrite cores for fly-back on a collapsing magnetic field - why would you need to get rid of an eddy current in the core?

[Nali2001]

It's all about the reduction of core losses - mostly heat.
Take 3 cores. One solid steel, one from normal transformer laminations and a ferrite.

You will notice that the solid steel core will soon get so hot that the wire insulations will melt. This heat is eddy current losses. Plus such a core has a relatively low frequency range in which it can operate.

The core made from transformer laminations has more electrical core resistance from plate to plate and hance the eddy currents will not form that easily since there is a high resistance in the core itself. Its like you trying to get a 1000amp current to run though a hair sized wire. It will not from because the thin wire has to much electrical resistance. Same with transformer laminations the thinner the laminatons and the better the insulation the lower the eddy current / heat losses, well they are not perfect. You probabbly felt a warm transformer before so they are still lossy and also oppereate at a relaticly low frequency, but that is more due to core hysterisis.

Then you have the fast materials like Ferrite. The advantage here is that the core itself is non conductive. And hence the eddy currents can not flow. Or very very little. Disadvantage is that these sintered materials have low maximum field strength like 0.4T or so. And steel like 1.3T. But the low field strength is not that much of a problem since the ferrite core can work in high frequencys like 30khz instead of 60hz It is a fast material, but at low field strengths. So in other words, ferrite really shines in high frequency stuff. Not so much in the low frequency's like 50hz, due to the low max field strength.

Then there are the fancy materials like noncrystalline materials, like the well known Metglas. These are very thin tape wound cores and so have very little eddy current losses, and have a high frequency operation range at relatively high field strengths, but they are more expensive than other core types.

Regards,
Steven

[jadaro2600]

I see, so the lower the eddy current losses, the less heat is produced.

[Nali2001]

True, and eddy currents also interfere with the 'flow' of the 'wanted' magnetism in the core. Plus a cool core can handle higher field strengths. The more you heat a core or a magnet the more it looses its magnetism.

Eddy currents:
http://www.youtube.com/watch?v=iABmUEH5s0k&feature=channel_page

[jadaro2600]

What if I used one of those cheap magnets ..the kind you can drill through, for a core?  Or what about compacted rust?