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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Ok, I was looking at an old GDT I once made out of the wrong core material, and it got me thinking.
Say I have a ferrite ring to form a magnetic circuit, and I want to store as much flux as possible in that circuit in the shortest amount of time (ie drive it to the point just before saturation). But what would be the quickest and most efficient way to do this, with a large mmf (ampere turns) value or with a low mmf value?
I've been googling and wiki'ing, but I don't fully understand reluctance. I guess what I'm trying to say is 'Is the maximum rate at which you can drive a core to saturation defined by its reluctance in the same way the maximum rate at which you can drive a current through a wire or coil is defined by it's inductance?'
So that would mean with a high reluctance, it would be most efficient to use a high mmf value (many turns, low current) to 'slowly' drive the core towards saturation, at the maximum rate defined by the reluctance of that core material. The current could therefore be self limited by the resistance of the large number of windings, resulting in the fastest AND most efficient way to saturate the core (correct number of turns, and therefore current for the reluctance of the material you are trying to saturate). Is this paragraph correct?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Bear in mind that this post is powered by a large amount of beer so YMMV.
The question you're asking doesn't make sense. The amount of magnetic energy that the core stores when driven to saturation is always the same. So it doesn't really matter how you drive it, no method is more "efficient" than any other.
Saturation is always caused by the same amount of ampere-turns, no matter the amount of windings you use. But since I=(1/L)*integral(V dt), and L is proportional to turns squared: the more windings you have, the longer a given voltage will take to get the current to the saturation level.
So the quickest way to saturate the core is to slap a very high voltage across a winding with only a few turns. And according to the first point above it's no more or less efficient than any other method.
Saturation is usually regarded as a bad thing that we try to avoid, except in certain circuits (saturable reactors for power control and pulse compression)
Reluctance is the magnetic analog of resistance. It's a measure of how many ampere turns you need to force a given flux density.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
That makes sense now, thanks! So the only limitation in driving the core to saturation would be the ability to provide a sufficient current for a specified period of time, determined by the inductance of the whole mess.
I was thinking that the core provided some kind of resistance to accepting flux, so it could only accept it at a rate slower than you could physically apply the current, hence my thinking that applying the current slowly would be more efficient. I suppose I need to think less and read more
That's funny, this post is also fuelled by a fair amount of beer and I think I understand it now.
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reluctance/ampere turns
