STEORN DEMO LIVE & STREAM in Dublin, December 15th, 10 AM

[mondrasek]

The torque for this motor setup comes from attraction between a magnet and a ferromagnetic core and I think you surely get less attraction force to a core with a permeability of 20 than to a core with a permeability of 1500.
So I think this is the relationship.

To choose a good core for our job here you have to select cores with the lowest saturation values, among the ones with a permeability over several hundred or higher.

Thanks Gyula.  Exactly the type of information that I was looking for.  And exactly the concern I had! 

Do you, or anyone else, know how to relate the permeability of a ferrite material with it's ability to be attracted to permanent magnets of a given strength?  I mean, you said a permeability of over several hundred or higher...  but how low is too low, and how high is unnecessary?  Any idea how to optimize the selection? 

[wings]

Thanks Gyula.  Exactly the type of information that I was looking for.  And exactly the concern I had! 

Do you, or anyone else, know how to relate the permeability of a ferrite material with it's ability to be attracted to permanent magnets of a given strength?  I mean, you said a permeability of over several hundred or higher...  but how low is too low, and how high is unnecessary?  Any idea how to optimize the selection? 

Naudin first choice:

http://www.feryster.pl/polski/nanoperm.php?lang=en
http://www.magnetec.de/pdf/vergleich%20nano-ferrit_1.pdf

http://jnaudin.free.fr/steorn/indexen.htm

[gyulasun]

Thanks Gyula.  Exactly the type of information that I was looking for.  And exactly the concern I had! 

Do you, or anyone else, know how to relate the permeability of a ferrite material with it's ability to be attracted to permanent magnets of a given strength?  I mean, you said a permeability of over several hundred or higher...  but how low is too low, and how high is unnecessary?  Any idea how to optimize the selection?

Is it not linear? If you have a permeability of 1 or 2, there is no or very very little attraction. If you have 20, then it increases twenty times and so on.
The higher limit is only material dependent, the important data to be connected to here is saturation characteristics I think.
The lower this latter value is the better. Nowadays perhaps you find among the off the shelf toroidal cores only the ferrites as the lowest saturation core materials.  I would prefer using those with rectangular and narrow B-H curves I think. We ought to dig out old knowledge in magnetic core history...

rgds, Gyula

[PaulLowrance]

I've been designing a mosfet circuit that will utilize two small trigger coils to turn the coils on & off. The 1st trigger coil will turn the current on. The 2nd trigger coil will turn the current off. So the 2nd trigger coil will be situated a bit after the first. Has anyone here already designed this. It would save some good time.

Also, what's the most inductance anyone's used for the toroid so far? Initially I'm going to try the two pre-wound coils, but they'll be good for low rpm because of their high inductance. Later I'll wind the Metglas cores and see what they get. 

[mondrasek]

Is it not linear? If you have a permeability of 1 or 2, there is no or very very little attraction. If you have 20, then it increases twenty times and so on.

Exactly.  So, what is optimal for any given permanent magnet? 

Here is a specific example.  Let's say you have a 10mm diameter x 10mm long cylindrical magnet that will "hold" with a force of 100N if on a smooth surface of a steel block that is thick enough and has a surface area large enough to satisfy all needs for "attractive" target material.  Now if we replace that steel block with something with less permeability, say 50% that of steel, will the permanent magnet only hold at 50% the force, ie. 50N?  If not, at what decreasing level of permeability will the permanent magnet no longer be able to maintain 100N of attractive force?

Likewise, will the attractive force increase to greater than 100N if the permeability of the target material also increases over that of steel?