Inductor/power supply choke operation

[Y2KEDDIE]

I'm familiar with the formula for Inductive Reactance, and realize Inductance, is directly proportional to frequency change . I also know as the DC current increases the Flux increases until saturation . At saturation (critical) Inductance is .at a minimum because the Flux no longer changes. hence a choke has maximum Inductance when there is minimum DC current through it. Chokes typically have a small air gap in the laminations to counter saturation Am I correct?

I've heard of swinging choke, but not sure on principle how they work.
Is there an electronic way of varying the gap/ preventing saturation. I'm thinking if so, one could build a power supply with LC filtering to work through an extended range low to high voltage.

??

 

[Harald Kapp]

Am I correct?

Almost. In an ideal inductor, inductance is neither proportional to frequency, nor to frequency change. Rather it is a constant value given i units of "H" (Henry) or fractions thereof.
The impedance of an inductor changes with frequency, being (ideally) 0Ω at DC and rising linearly with frequency: X(L)=2*PI*f*L
This is because the change in flus is proportional to the frequency (change of current vs. time), not the change in frequency.

You're right about the air gap. Technically there are different ways for realizing an air gap. Ferrites for example have a "distributed" air gap where the total length of the gap is summed from the number of very small air gaps between the ferrite's corns.

I've heard of swinging choke, but not sure on principle how they work.

I haven't heard of this arrangement before, had to look it up. I don't think there is an electronic wy of varying the gap since this is a mechanical feature. One could imagine an actuator that changes the gap's length, but this would probably be slo - too slow for meaningful use.
You can, however, change the flux within the core and thereby influence the saturation properties of the inductor, by adding a second coil through which you send a control current. This control current will add flux to the core which will increase or decrease the total flux, thereby influencing the onset of saturation.

 

[Y2KEDDIE]

I was mis-using the terms inductance and Inductive reactance.(Big difference, thanks I stand corrected).

I thought I had read in a text, that chokes inductance is greater when no current is being drawn, and decreases as it reaches saturation. I best revisit; probably was Inductive Reactance.

 

[Harald Kapp]

I thought I had read in a text, that chokes inductance is greater when no current is being drawn, and decreases as it reaches saturation.

That's correct. The inductance of a saturated core sinks to a very low value.

Plus: a real inductor is never ideal. Parameters like the ohmic resistance, winding capacitance and others have to be taken into account.

 

[Colin Mitchell]

If you are talking about an inductor (choke) in a power supply, it needs to have an air gap.
Suppose you put an inducotr on a car battery and illuminate a globe.
Current will flow through the inductor and produce magnetic flux.
You can prove this by opening up the inductor and picking up a kilo of nails.
If the inductor has a closed magnetic circuit and no air gap. The core will be saturated when a small current flows.

That's the DC workings of an inductor.

Now we put the inductor on the output of a power supply that has RIPPLE.
Connect a globe and everything works fine.You cannot see any problems.
But connect an amplifier to the output and you will hear hum.
This is because you want the inductor to work in AC mode.

 

[Y2KEDDIE]

So, technically, as the current fluctuates through an air-gapped power supply filter choke the Inductance changes, and therefore its reactance. The inductance changes because the physical conditions are changed. Varying the flux is the same as increasing or decreasing the core material, dimensions, etc. The inductance of a solenoid will change if you change the core material , change the spacing between turns etc., externally.

I would guess when a choke is designed , its inductance (Henrys) are specified to be at a specific current; the nominal operating current. Any current less is more inductance, any more current approaches saturation. I'm thinking this is similar to specifying a bleeder resistor, which aides regulation. One has to draw some current for it to be effective.

 

[Colin Mitchell]

Say you have a 12v power supply with no ripple.
And say you have a big inductor.
When the 12v enters the inductor it produces magnetic flux and it emerges as 12v (or very close to 12v).
Say you have a 12v power supply with 100mV ripple.
When the 12v enters and starts to rise, it produces extra magnetic flux and this flux cuts the turns of the winding and produces a REVERSE VOLTAGE in each of the turns and by the time the 12v emerges, the 100mV has been cancelled.
But suppose the inductor is smaller and the 12v saturates the core.
When the 12v rises, the core does not carry the increased magnetic flux to the other turns and output rises 100mV.
If we put an air gap in the core, the magnetic flux created by the 12v will flow around the magnetic circuit and when it comes to the air gap, it will be lost in the gap. This will result in the flux density being much lower than before.
Now, when the voltage rises, the core will be able to pass this rise to all the other turns and the 100mV will get cancelled.

 

[Harald Kapp]

its inductance (Henrys) are specified to be at a specific current; the nominal operating current

... and below.
The change in iinductance becomes noticeable when the flux reaches a value near saturation.
Depending on the type of core material there is an (almost) linear range of flux below saturation. You'll have to look at the magnetization curve (e.g. here). Renowned manufacturers of inductors specify a max. current for the inductor. This is the currrent that can be forced through the inductor with a specified max. loss of inductance. Unfortunately there is no common standard for this parameter, so one manufacturer will specify max. current for a 10% decrease in unductiance, another will specify max. current for 30% decrease in inductance. Nothing beats studying the datahseets very closely.