Bob Scott

I'd like to understand electromagnetics a bit better. What is the definition of electromagnetic flux? I'm attempting to make a better equation for flux density.

Is flux static, like a measure of the magnetic field intensity, as the lines of force that you can see using iron filings over a bar magnet?

Or is it defined as dynamic only, applying to AC fields only? The equation for B (AC flux density) in Gauss in my Dad's old engineering books applies to AC voltages. These books are really terse.

The ambiguity is in the word "flux" itself. When something is in a state of flux it is changing.

Papabravo

Flux is like fluid flow. It is a characteristic of vector fields.

A fluid flowing through a pipe is a vector field, that is there is a velocity vector at each point in the field.
An electric field is a vector field, that is there is an electric field vector at each point in the field.
A magnetic field is a vector field, that is there is a magnetic field vector at each point in the field.
Atmospheric temperature is a scalar field, that is there is a scalar quantity at each point in the field.

Vector fields exert forces on real particles moving through them, scalar fields do not.

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Sceadwian

Flux does not always mean change. There are of course static electric fields, lightning being an obvious example. There are static magnetic fields, any polarized magnetic material is an example. One of the meanings for flux under the physics subdefinition of flux according to dictionary.com is:

A quantity expressing the strength of a field of force in a given area.

Static electric flux is easy to define, the number of electrons in a given area, moveable flux is different as the bulk of electrons in matter are all 'in use' so the net electric flux is zero. Magnetic flux is a bit more complex, I know little to nothing about exactly what gives rise to a static magnetic field, something I should probably look up one day. Electromagnetic theory is based primarily on the interaction of electric and magnetic fields which implies them changing, and non-moving electric or magnetic flux is pretty boring stuff.

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Bob Scott

Thank you very much Papabravo and Sceadwian! That clears that up for me.

Papabravo, thanks for refreshing vector/scalars for me. Thanks Sceadwian for seeing it at a different angle. The more points of view that you can get of a subject, the easier it is to understand intuitively.

microtexan

Why are you trying to make a better equation for flux density?

Bob Scott

I thought it would be easier to design a saturating switching power supply transformer with a different equation for B.
The existing B=V*10^8/(4.44*A*N*f) uses AC voltage and frequency. I think current is more suitable.

So I've come up with some new ones:

B=0.1415*Al*N*I/A

B is in Gauss
Al is the core's inductance in milliHenries at 1000 turns.
N is the number of turns that pass through the core.
A is the core's cross sectional area in cm^2.

I've also come up with:

Al= *0.89*A/Le

= the core material permeability.
A is the core's cross sectional area in cm^2.
Le is the core's magnetic path length in cm.

This way you can easily see that the Al value is dependent only on core material and core geometry.

You see I had some trouble verifying the published specifications of an Amidon toroidal core FT240-77 made of type 77 material that has a characteristic permeability of 2000. The specs in Amidon's leaflet said it had an Al value of 3130. Their catalog said it was about 2750. My own test with an oscillator to determine inductance found the real thing to be closer to 1690.

I can elaborate on the derivation of the formulas but only if you're interested.

Papabravo

I don't mean to be impertinent but a saturated inductor in an SMPS sounds like a really bad idea. Are you sure that is what you want to do?

__________________
We never have time to do it right; but we always have time to do it over.

Bob Scott

Oh, you would never sound impertinent.

The saturation is part of the oscillation cycle. As soon as the core saturates, the driving transistor turns off. It's been done before in photographic Xenon flash units and... remember the old Mark 10 capacitive discharge ignition? See:

Attached Images

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