Re: Mike Marcum Ferrite Cores

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Matt,

On 18 Oct 2005, at 11:15, Tesla list wrote:

> Original poster: mecortner@xxxxxxxxxx
>
>  >Just about anything including signal grade ferrites (Philips 3Hx,
>  >even 4Ex series) at those frequencies. The grain size is easily
> small
>  >enough to obviate eddy currents. Hysteresis losses can be reduced by
>  >gapping the material if they are a problem. If you have some
> ferrites
>  >on hand and know the grades, you can try and obtain data from the
>  >manufacturers or simply "Googling" them. Low hysteresis losses are
>  >signalled by a small enclosed area in the B-H curve. At low
>  >frequencies AND high signal amplitudes you are going to need high
>  >inductances to minimize the risk of core saturation which, if the
>  >core is gapped as well, implies large core size. A definite design
> is
>  >going to depend heavily on design requirements and there is not
>  >enough data given to go on (yet).
>
>  >Malcolm
>
>
> Thanks for responding Malcolm, actually all My ferrites
> are just flyback type or SMPS cores from randomly junked
> equipment. Enough material to make a real small generator
> core, sort of a toy, but I don't think I could extrapolate
> that to a larger version. I was hoping to find a inexpensive
> part number from a distributor for the scale mockup, as
> always the bugs are cheaper/easier to fix when things are
> a bit smaller. Having said that, about all the material I
> need now is a quart jar full of the powder.
>
> I do understand the B-H curve but on this type of gen there
> are no magnetic reversals to deal with. The material only
> has to let go of the residual magnetism before the next pulse.

And that is where the B-H curve comes in. The curve shows the flux
density (B) vs amount of magnetomotive force (H) applied. Note that B
tailors off asymptotically as H is increased as the core domains
align leaving fewer to be aligned. It also depicts the amount of
remnant magnetism on the B axis when H is reduced to 0 (as your
application does). Maximum utilization of a given core is achieved by
operating it with reversals (as in a half or full bridge drive
configuration).

> However the core saturation could be an issue? I guess I should
> describe the thing, it's one of Tesla's own patents, a varying
> DC sine output generator. It uses a unique coil geometry that
> is 90 degrees to the magnet face with the back side shielded
> by the core, this removes the magnetic drag even when it's
> under load. which is the opposite of normal generators.
> A friend has a book with the patent number in it, I'll get
> it tomorrow and post so you can see what I'm talking about.

Great.

> The magnets I'm using are grade 38 (38 mega-gauss) neo's,
> 3/4" dia X 1/8" thick, the core cross section will be
> 3/4" X 3/4". The problem here is the coil generates it's own
> magnetic field when loaded, and that will add to the magnet's
> field, this all flows through the 3/4" X 3/4" ferrite core.
> How much gauss will go through a core cross section this size
> before saturating?

You're going to hate this answer but it depends on the core material.
In general, the more square the B-H curve is, the faster the core
will saturate (i.e. with less applied magnetic field).

Malcolm