Hmm.. I guess that makes sense. If the core is solid material, there is only so much flux that can be contained in it.. Which is why it saturates, but it has a high permeability before that point. Air can hold a much greater flux density, if not unlimited, before saturating.. I'm not sure if air even has a limit. Only thing with air tho is that it has a lower permeability.. which is what causes the lower inductance. Good to know!
I don't understand how you can make a ring core with a cross section area of 1 sq cm and a length of 5 cm. I don't see how that is geometrically possible.
Is the equation I've come to, L = ΦN / I correct?? If not, please state where I have gone wrong. Thanks!
The formula that you have come up with implies to me that the Inductance is proportional to the number of turns.
In fact the inductance is proportional to the square of the number of turns (This was mentioned in one of the formulae before you rearranged things).
michinyon:
I don't understand how you can make a ring core with a cross section area of 1 sq cm and a length of 5 cm. I don't see how that is geometrically possible.
The length of 5cm is the axial length of the coil, not of the entire core.
JohnLincoln:
The formula that you have come up with implies to me that the Inductance is proportional to the number of turns.In fact the inductance is proportional to the square of the number of turns (This was mentioned in one of the formulae before you rearranged things).
You are correct when you say that inductance is not directly proportional to the number of turns.
The equation L = ΦN / I on face value seems to say that turns and inductance are directly proportional... This is not the case. You must remember that the number of turns is a component of Φ. So the other 'N' in the equation is actually hidden in Φ.
β = Φ / A
and
β = [ μNI ] / ℓ
and
L = [ N2μA ] / ℓ
by rearranging these three equations you come up with L = ΦN / I... So yea.. you can't take it on face value, you have to remember that some of the variables are made up of other variables!
localbroadcast:
I am really interested to know how the air gap does this. Can you help me understand this? I'm really curious.. because the cores I just talked about in my previous post come in 2 types.. without gap and with gap.. so obviously what you are saying has some basis of truth! Maybe russelz can chyme in with some helpful words of wizdom?
MarkT has already explained this but perhaps I can offer another way of looking at it. Think in terms of magnetic reluctance being analogous to resistance in an electrical circuit. The reluctance is a measure of how many ampere turns (think volts) are required to produce a given flux (think current) in the circuit. Now the core material is in series with the air gap. The core material is non linear while air is linear. If the reluctance of the air gap is much greater than that of the core then the reluctance of the two in series is largely determined by the air gap and is almost linear.
As John stated above the inductance is proportional to the square of the number of turns. Al quoted in those data sheets is the inductance for one turn.
I've always found TDK to be very good for ferrites.
Russell.
Oh, and its B, capital-b, not β (beta)!
I am really interested to know how the air gap does this
Define "air gap".
My understanding from work is it is a term to describe the gap between two halves of a core , like a rectangle split in two that can be assembled with the two halve either touching or separated by an air gap
that consists of a shim or shims made of appropriate material (such as Nomex paper).
Is that what you mean ?
russellz:
Think in terms of magnetic reluctance being analogous to resistance in an electrical circuit.
That is a very good analogy and is quite a good aid in helping someone understand magnetic circuits. I forgot about the parallels the two systems have... When I was in school years ago I always found it helpful to think of it in those terms.. and you can also compare the two to plumbing as well.. voltage and oersteds can be thought of as the water pressure, and amperage and flux as the amount of water flowing.
I guess I was having a hard time understanding why I would want to increase the amount of reluctance (decreasing permeability) in my core.. The circuit I am working on will aim to have a current that is relatively constant, so the value of B, H, and μ will be pretty stable. Because of this I can design the inductor to operate in a region that is not in saturation and has the best μ value.
If I had a circuit where the current was not constant and the μ value were wandering around a greater range of values, then I suppose this would be where the benefit of better linearity becomes useful?
MarkT:
Oh, and its B, capital-b, not β (beta)!
Thanks for this.. For some reason I had it in my head that whenever I wrote B back in my university days I wrote it in a fancy script.. which my memory said was a beta symbol. Guess not!
raschemmel:
Define "air gap".
Yes.. the air gap I'm talking about is the gap in a core... With the TDK ferrite cores I posted, they come in 2 pieces shaped like the letter 'E'. When put together, they make a rectangular core with 2 windows. When there is an air gap, the center leg of the E will be shorter than the other 2 legs. The coil is wrapped around this center leg. I think the center leg is called the yoke, but don't quote me on that.
There are other meanings for 'air gap' in electronics... such as the air gap between the rotor and the stator in an induction motor.. But I'm discussing the one in the core of an inductor or transformer.