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How do I measure various parameters of a ferrite toroid

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T3sl4co1l:
Bingo. :-+

Note that, in case of a rod core, there is fringing flux everywhere, so we could use a sensor to map out the field around it, but there is no one singular number to say what's happening.

(Rod core inductors saturate somewhat more softly than closed style cores do.  The middle reaches Bsat first, which causes mu_eff to drop, so the inductance also begins to drop.  As current continues to rise, the saturated volume grows, mu_eff drops further.  Eventually the ends also become saturated, and the inductance drops to its air-cored value.)

The canonical way to measure flux, is with Faraday's law.  It's simply a voltage on a winding, then, and we can integrate to find flux (a passive RC integrator, or active op-amp integrator, can be used).


This is used in compensated current transformers, for instance:

In a regular CT, the sensed current is transformed (stepped down by ratio) and dropped across a "burden resistor" and sensed as a voltage.

In a compensated CT, there are two secondary windings: one driven and one sensed.  An error amplifier reads the sense winding, and servos the driven winding to force the sense voltage to zero.

Zero sensed voltage, means zero core flux, means freedom from core saturation and bandwidth down to DC!

The driven winding has DCR and LL voltage drops, but as long as the error amp's output has enough voltage to overcome those drops, it can successfully force the induced voltage to zero.  No current is drawn from the sense winding, therefore no voltage is dropped across its DCR or LL, and the sensed voltage is exactly the induced voltage (EMF).

Finally, current is sensed in the driven winding (usually with a shunt resistor), and you have a complete current sensor.

Because EMF is servoed to zero, the flux is very nearly zero (within limits of the error amp's accuracy: input offset, and voltage gain), so such a sensor is theoretically DC-capable, though because we're sensing through a transformer, we do need to do the good ol' calculus "Plus A Constant" chant.  In any case, a real one can significantly extend the bandwidth of a CT, from say ~kHz (for a regular CT) to fractional Hz. :)


All this, to say through example: yes, we can measure flux, or at least something very closely related to it (Faraday's law, EMF = -dPhi/dt), and we can apply calculus with some passive or active circuitry (integrator or differentiator) to get that last step, plus or minus a constant, or within some limits of DC stability. :)

And also yes, there are two fluxes, core flux (for a single turn), which is B * Ae, and circuit flux, which is B * Ae * N.  It may be helpful to carry turns as a unit, so you remember flux density is flux per area per turn, and also magnetic field intensity is amp-turns per meter, H = N * I * l_e.  (Which means, since B = mu*H, mu must have units of flux per amp per meter per turn squared.  Which is good, because A_L = mu * Ae / l_e gives us the necessary per-turns-squared in the formula!  Dimensional analysis, it's good stuff. :D )

Tim

coppercone2:
compared to the other selectable elements, what is accurate enough for measurement of these parameters?

can you get anything useful trying to 'voltnut' this shit?

for small signals/filters you would end up being interested in the distortion resulting from manifestations, for chokes your gonna end up having heavy margins because of modern active semiconductors, for electromagnets your gonna end up testing it

is there useful things in power electronics or maybe for protection you can get out of precise measurements of these parameters that I am missing ? (with equipment more advanced then the kind described in this thread).

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