bitrex said:
I've been reading up on inductor and transformer design, and was interested to read the following: "Ideal magnetic materials cannot store energy, and practical magnetic materials store very little energy, most of which ends up as loss." So if I'm understanding this correctly, a big 10 Henry filter choke is designed to channel all the magnetic flux through the high permeability steel into the air gap? It seems kind of ironic that all of that copper and steel is required to construct the inductor when the "real work" is all being done by the air gap! I'm having a hard time understanding why air can "store" magnetic energy, but the steel can only "channel" it.
Yes what they are saying is basically true, though it makes certain assumptions. In particular by "near ideal" magnetic material they just mean it has very high magnetic permeability, however they still assume that it has a limited maximum flux density (due to saturation). So it's ideal (or near ideal) in one sense but still very non-ideal in another, and this point is
very important in understanding their statement.
The problem is that stored energy per unit volume is equal to the product of MMF (H in Ampere-Turns/meter) times Flux-Density (B in Teslas). The more "ideal" a soft magnetic material then the
lower is the required MMF needed to reach magnetic saturation. So it can't store much energy because in any non-saturated condition it only has a very small MMF. In an "iron circuit" with an air gap one the other hand, you can achieve the same magnetic flux density but with much larger MMF and hence the energy stored is much larger.
If you ever do any inductor design this point quickly becomes apparent. You try to design an inductor, you use a nice high permeability core with no air gap, and you find you need relatively few turns to achieve the desired inductance. Ok, at first sight you think that this is a
good thing, with very few turns you'll have space to use nice thick wire and have prodigious high current levels and hence store lots of energy. Unfortunately you soon find out that you just can't attain those desired current levels because the core saturates! In the end you realize that you have to go back and include an air gap and use more turns to get the energy storage (1/2 L I^2) that you desire.
Consider a semi numerical example. L is proportional to the magnetic permeability times N^2 (for a given core). So if you reduce the permeability to say 1/4 of it's initial value then you need to double the number of turns. But since the permeability is 1/4x and the turns are only 2x then it's clear that you can now have twice the current before you reach the saturation flux density. At saturation the product of B times H is now actually 4 times as large. Similarly if you include an air gap which reduces the overall permeability of the iron circuit to 1/100 of it's original value then you need 10 times the number of turns (for a given inductance) but you can have 10 times the current and therefore 100 times the stored energy before saturation is reached!
