# DC inductor changing reluctance -- effects?

## Main Question or Discussion Point

Hi, I would like to know what happens to the flow of DC current and voltage in an inductor if we change the reluctance of the inductor core repeateadly, Just a DC power source attached to an inductor in series with a lightbulb for example , now as the DC pass through the wire wound around the inductor core we somehow change the reluctance at a certain rate , would the bulb flash with that frequency or would nothing happen at all ?

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berkeman
Mentor
Hi, I would like to know what happens to the flow of DC current and voltage in an inductor if we change the reluctance of the inductor core repeateadly, Just a DC power source attached to an inductor in series with a lightbulb for example , now as the DC pass through the wire wound around the inductor core we somehow change the reluctance at a certain rate , would the bulb flash with that frequency or would nothing happen at all ?
How do you propose changing the reluctance? There are 2 ways I can think of, and one way would couple an AC signal into the bulb...

How do you propose changing the reluctance? There are 2 ways I can think of, and one way would couple an AC signal into the bulb...
You meant into the inductor?

berkeman
Mentor
You meant into the inductor?
Well yeah, but the inductor and bulb are in series, if I understand the OP's question right.

Anycase, change of the reluctance will lead to change of the energy in the magnetic circuit . That should induce voltage and cause change of the current flowing in DC circuit.

well a lightbulb is basically a glowing wire so I treat the bulb in series as just a continum of the wire I put it there as a means of seeing the swing in current/voltage.
the main question is can we make the current oscillate through the load (bulb) from a steady DC supply using just an inductor whose core reluctance is varied back and forth repeatedly.
berkeman for this idea i leave out the ways one could vary the reluctance of the core as I want to understand if the varied reluctance would have an effect on the DC flowing through the winding wound around the core in the first place?

you probably thought something similar to magnetic amplifier but the difference is that they saturate the core using DC and then AC experiences either a large or small impedance , but since DC is steady what would DC experience?

berkeman
Mentor
berkeman for this idea i leave out the ways one could vary the reluctance of the core as I want to understand if the varied reluctance would have an effect on the DC flowing through the winding wound around the core in the first place?
My point was if you use a winding to vary the saturation level of the inductor (and hence the effective reluctance), you are basically making a transformer anyway, so of course the current to the bulb will vary.

But I think @zoki85 is also correct that any mechanism of varying the reluctance will vary the current through the inductor. My 2nd method that I had in mind is mechanical.

But the bulb is in series wtih the primary winding which is connected to the DC source, will varying the reluctance of the inductor core by the help of a secondary control winding do anything to affect the DC that flows through the main winding and the bulb? I sure know this works if the current through the primary and the bulb would be AC as it is used in magnetic amplifiers, but does this work for DC too? In other words can we affect the flow of DC in a wire wrapped around a core by changing the reluctance of that core?

your mechanical way of doing that is probably changing an air gap right?

berkeman
Mentor
For using another winding to modulate the saturation of the inductor, I picture it just like a transformer. With one winding being the inductor winding connected to the bulb, and the driven winding wrapped around the inductor. So it's not so much that the AC winding is affecting the DC current -- the current through the bulb is the sum of the DC current and the AC current from the AC source.

On the mechanical method, yeah either modulating an air gap, or adding/removing material from the inside of the inductor (like pulling a slug out of the middle of a set of cylindrical inductors or something similar. Modulating an air gap would be a pretty easy experiment to set up, but I would use a load resistor and an oscilloscope to see what the effects were, instead of a light bulb. The effects may be too small to notice without an oscilloscope.

So it's not so much that the AC winding is affecting the DC current -- the current through the bulb is the sum of the DC current and the AC current from the AC source.
That's what i thought , otherwise we could use the mag amp for DC too but it only works for AC because AC current increases or decreases dramatically if passed through a winding which is wrapped around a core whose saturation point is increased or decreased.

If I would use the primary winding jus to pass DC through then change the core reluctance and the secondary winding would have the bulb in series wtih it, then the secondary would get induced current with the frequency of that used to change the core reluctance and the amplitude corresponding to the change in reluctance in each period right?

I still wonder , from the perspective of the DC flowing through the winding , how it is affected when the core to which the winding is wrapped around is changing reluctance repeatedly,I suspect that before saturation the core acts similary to a capacitor charging , it should be taking energy out of the DC current?
and if i add something on the secondary winding as a load the energy taken to magnetize the core should increase proportionally , so when i first thought about the bulb in series wtih the primary i thought it should flicker as the core goes from nothing to saturation and back

berkeman?