Re: Mike Marcum Ferrite Cores

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hi Matt,


What I was referring to, is if you take a magnet
and measure it's lift ability in pounds and then
place it on a chunk of ferrite with a certain
cross section and length, then re-measure it's
lift ability through the ferrite piece, it will
be something less. But how much less is what
concerns Me. That will give Me an idea of how
thick to make the core.


Right, i just didnt understand why you were comparing them to 
magnets, until i read below...

The magnets are on the rotor, their field will
penetrate thorough the core, but the coils will
produce their own field under load that is 90
degrees to the magnets field. If the core
saturates then some of the coils field will leak
out of the stator and hit the rotor magnets,
creating drag. For instance you'd have the north
pole of the magnet hitting the north field of a
stator coil as it moves toward the coil.


When a core saturates, it simply does not produce any more magnetic 
field.  Its reached its limit, so your inductor basically turns into 
a resistor (goes from energy storage device to energy dissipative 
device).  This is why saturation is to be avoided.

>I have also been working on a CCPS, inspired by Marco's project Thor
>power supply. I have a webpage for it that really needs updating,
>but might be of interest to some of you:

Nice work there!
By the way, how do you actually measure a
parameter like saturation in a core?


One method would be to perhaps wind a few test turns on the core and 
drive it with an AC source.  Some math would be required to get the 
values in the ball-park, and then you could simply watch the current 
through the inductor and see if it behaves as it should, or if it 
suddenly rises out of control.  For example, say you are applying a 
square wave voltage across the inductor, the current should ramp up 
linearly over time (usually very fast, so an oscilloscope is 
needed).  If the core saturates, this linear rise in current will 
take a sharp turn upward.  Note what current level this happens at, 
as well as the number of turns on the core and the measured 
inductance of the coil.  Plug all that back into the formula i gave 
before and this tells you exactly what flux density you achieved when 
saturation set in.

There may be an easier way to test for saturation, but this is how 
ive done it in the past.  I didnt include many details, if you are 
truly interested in an exact method for testing saturation point, 
maybe I or someone else can help you out there.

Steve

---- Matt Cortner ----