Variable Inductor Help

Hello Mike,

Basically the principle will work. When you saturate the toroidal core, the permeability of the toroid will reduce, for all directions of flux. Because the toroid's winding will take some space, the overall permeability of the pot core will be less.

I see some things that you have to bear in mind. Because of the DC winding on the toroid, the flux of that pot core will also go through the copper. Although virtually no EMF is induced in the DC winding, there will be eddy current loss in the DC winding, because of the flux through the pot core (that must also go through the copper. Probably you inductor will have somewhat more losses (depending on frequency and thickness of wire and volume of wire of DC winding).

There will be some small coupling between the winding on the pot core and the winding on the toroid. That is because of some flux goes through the air in the middle of the toroid. Whether this is a problem, depends on the application. With proper winding technique, you can avoid this.

By having the inductance control flux spatially 90 degrees with respect to the AC flux, I believe your AC inductor will be more linear (provided AC flux

Hello Mike,

I forgot to mention that I see no link to the image.

Best regards,

Wim PA3DJS

Hi Wim, The picture is posted in alt.binaries.schematics.electronic. Thanks, Mike

No, magnetization is a VECTOR and it doesn't matter that the circumferential field is saturated (maximum magnetization in the theta direction) because the pot core induces a magnetization in the axial direction (tilts the magnetization from full-amplitude in the theta direction, i.e. gives a component at right angles).

So, without changing the magnitude of magnetization, the direction can change. This means the axial component of magnetization responds to the applied field, and that means the toroid is fully magnetically active, just like it wasn't saturated by its winding.

Energy conservation suggests that this would cause some coupling of the windings at saturation (regardless of the geometric shape of the windings, which do not couple magnetically). It might generate some interesting harmonics of the drive frequencies.

Hi Whit, I think you said it won"t work. Work with me here to help me understand what you said above.

I thought the magnetic domains would be locked in position by the dc and the lower flux within the potcore would not be able to move them around, and this would result in a lower AsubL. Oh, I think I just gained some insight. As you said it's a VECTOR so the domains do move. It would seem to me that being a vector that the domains have a limited rotation and not a full 180* swing each half cycle. Wouldn't this limited swing reduce the AsubL?

I don't know where to go with that.

Thanks for the input and anything more you can add, Mike

Hello Whit,

Assume the DC driving field is horizontally directed, the AC field is vertical and the magnetic material is anisotropic but non-linear.

Magnetization is a vector and has a maximum (non-linear system). If you assume the DC (horizontal) field infinite, a finite AC field will not be able to change the angle of the resultant field, hence the direction of the magnetization vector. So there will be 0 contribution of the magnetic material to the AC field.

In general, once the core has been saturated (by the DC field), , the vertical polarization component is inversely proportional to the DC field (assuming AC field

In the large-signal regime, yes, it gets reduced. In the small-signal model, however, the derivative of the total magnetization with respect to small axial field is zero, so the axial field isn't coupled to the total magnetization. The second derivative does not vanish, so the even harmonics (second harmonic) of the exciting signal do show significant effect.

The toroid winding current creates a second harmonic of the potcore drive, SO one can imagine using wide-dynamic-range sensing of that second harmonic to measure the toroid current. A sensor based on this effect could measure microamps to kiloamps in this way (which is way cooler than using op amps and resistors).

A physics lab needed lots of high voltage; they built a four-arm transformer core, sawed a gap in one arm and wound a secondary on it, then overdrove the other three arms with three-phase power. The saturation at peaks caused current in the secondary at

The power company hated it. The power factor was dreadful, and (at that time) the customer wasn't liable to pay extra on account of the line losses that resulted. That little billing issue has been resolved nowadays, so you know this is an OLD story.

In analogy, a taut string resists sideways deflection; still, a piano string at nearly full stress allowable for steel DOES deflect when the little felt hammer hits it.

In particular, the quantity of interest is the axial component of the magnetization, and NOT the angle (which is, in the small-angle sine/tangent approximation, the ratio of the axial and toroidal components). As a small axial addition to the magnetization vector does not (to first order) increase the magnetization amplitude, so saturation doesn't prevent axial magnetization.

Taking the toroidal field to a large number reduces the angle (as you say) without reducing the axial component (so it doesn't change the result).

Hi Whit,

As the contribution of the absolute magnetization (= length of vector) does not change in the saturated regime, reduction of angle, gives proportional reduction of vertical (axial magnetization). Am I wrong?

Best regards,

Wim PA3DJS