Why does transformer flux need to be reset in every half cycle?

[T3sl4co1l]

Relays turn off extremely slowly, especially with clamp diodes: milliseconds to tens of milliseconds.

They do indeed go faster if you provide a higher voltage for freewheeling: often, a zener diode (from ground), or TVS diode (across the terminals).

Tim

[ZeroResistance]

Relays and solenoids typically have a relatively large coil resistance. My guess is this would aid in dissipating the stored energy with the aid of a free-wheeling diode.

The current flowing in the inductor would eventually get dissipated by the high DC resistance of the winding.. but what about the property of the core itself .. if it can retain magnetism when current has long gone to zero... then that flux would stick around for some time...

[ZeroResistance]

What about transformers with gapped cores or transformers using materials like powdered iron cores that have distributed air gaps, and these are known to store energy in air gaps (although I don't know how) ...

So Would a transformer with air gap walk to saturation faster than a core without air gaps for the same Volt / sec applied?

[T3sl4co1l]

Since flux depends on core area, not gap, it doesn't matter!

Indeed, if you set up a saturation tester (typically by driving a pulse into the winding -- just as the simulation above), and you use an inductor on a gapped core, then you can adjust the gap manually, in real time, while testing.  As you adjust the gap length, the slope of the current varies (because of the inductance -- more gap = less inductance = more magnetizing current), but the time where saturation begins will remain nearly constant.  Because, for constant applied voltage, the same flux is reached at that moment, (almost) independent of gap (or inductance or current draw).

Conversely, because the resistance of a real circuit is nonzero, a gapped core is less likely to saturate in a full-wave circuit.  Why?  For the same reason the current eventually reaches zero on the diode-clamped inductor, or in a short-circuited (non superconducting) inductor.  The increased magnetizing current causes more voltage drop across circuit resistance, which drops a voltage, and thus a flux, that's proportional to current.  It's a negative feedback process.

When you solve the equation for that feedback process -- that is, where applied voltage is dependent on flux -- you get an exponential decay type solution: the inductor-resistor time constant.

Test this in the simulator by connecting a voltage source to an inductor, and setting the inductor for a nonzero initial current, and a nonzero series resistance.  You will see the average drifts back to zero, over time!

The rate at which current decays is the L/R time constant.  Written as a function, I(t) = I(0)  exp(-t * R/L).

Tim

[chris_leyson]

What about transformers with gapped cores

If there was a large DC component in the inductor current, say for example in the case of a flyback transformer, than you would gap the core. A gapped core lets you use a higher MMF or higher current. Saturation flux density, Bsat, remains the same as this is a function of the number of turn and core area.