What happens to energy in a saturated inductor?

[Circlotron]

Say we have an iron cored inductor with neglible dc resistance and an air gap in the core.
Apply dc volts and the current runs up and the air gap accumulates energy.
When the inductor saturates the inductance drops. This means that the stored energy must be less than before it saturated. What has happened to that energy? Or does the stored energy just level off even though the current keeps climbing, even faster now that the inductance is dropping?

[apelly]

The latter.

Inductance is that initial accumulation of energy.

[calexanian]

The what would then be steady flux of a saturated core could be considered stored energy. If you suspend the stop the current flow however, that flux will disappear either through internal dynamics or via an external path, and typically in the opposite polarity. Essentially how a boosting circuit works. the coil changes the stored flux into a potential that rises until it finds some path or leakage.

[james_s]

It's a bit like a bottle, if you keep putting water in the bottle eventually it will be full. After that any additional water you try to force in will either pour out or burst the bottle.

[Circlotron]

It's a bit like a bottle, if you keep putting water in the bottle eventually it will be full. After that any additional water you try to force in will either pour out or burst the bottle.

But as the bottle becomes full it reduces in volume. That’s what makes it tricky.

[mrkev]

You wrote it yourself. When core is saturated, you can't add any more flux and therefore inductance drops. Energy doesn't drop, it's still the same, any aditional current just doesn't add to the amount of flux.

It's a bit like a bottle, if you keep putting water in the bottle eventually it will be full. After that any additional water you try to force in will either pour out or burst the bottle.

I don't think that bottle is a good example for inductor. On the other hand, there is not a good analogy for saturation in any ordinary example I can think of (like turbine. turbine is a great analogy to inductor, since when you push f.e. water throught, you are storing energy in the spinning mass. When you stop water suply, spinning mass will try to push water through with the same speed /as a pump/. You could maybe say, that at the certain speed, it starts somehow slipping and cannot turn any faster.)

[mrkev]

It's a bit like a bottle, if you keep putting water in the bottle eventually it will be full. After that any additional water you try to force in will either pour out or burst the bottle.

But as the bottle becomes full it reduces in volume. That’s what makes it tricky.

Not really, in that analogy, inductance would be the capability of intake of water, which would remain constant until it's almost full.

[james_s]

I struggled to think of a really good example and came up with the best I could think of, it's not always straightfoward. Either way the point is that you can't keep pumping more energy into a saturated inductor, at that point it just acts pretty much like a wire until you stop pushing.

Perhaps a compressed spring is closer? You can push it in until it is fully compressed against the stop and then it just acts like solid metal until you let go of it and release the stored energy.

[tautech]

You don't normally run inductors into saturation for a few reasons.
Stress on the inductor.
Heat generated in the inductor and associated switching elements.

If the inductor values are know calculations can be made to state switch ON time and the energy stored.

When doing this sort of stuff a current probe is invaluable to measure current rise at the point of saturation WRT switch ON time.

[mrkev]

I struggled to think of a really good example and came up with the best I could think of, it's not always straightfoward. Either way the point is that you can't keep pumping more energy into a saturated inductor, at that point it just acts pretty much like a wire until you stop pushing.

Perhaps a compressed spring is closer? You can push it in until it is fully compressed against the stop and then it just acts like solid metal until you let go of it and release the stored energy.

It's the same kind of example, which would fit much better to capacitor - because you have potential energy. Kynetic energy is better analogy, but with just that exception of saturation...

Perhaps if you'd imagine yourself on stationary bike that has pedals directly connected to heavy fly-wheel without any break. If you press down on pedals, it will be hard at start, but when the fly-wheel starts spinning, it will become much easier. And same thing will happen every time you try to speed up. But when you stop pedaling (or slow down), energy stored in fly-wheel will force your feet to keep moving.

Saturation would be limit on the fly-wheel speed, after which it will just slip. So no matter how fast you try to pedal, you woudln't be able to force it to spinn faster.

Analogy to inductance in that case is the capability of resisting any change in pedaling speed.

[calexanian]

Some switching controllers will pass current until saturation. they have current sense feedback and look for the saturation point to get max current per pulse. I designed with one once. I did not like it, but it did work and had a super fast voltage correction response time.

[T3sl4co1l]

Nothing, it's still there of course!

The energy storage rate (with respect to time) in fact accelerates rapidly.

This is because you've specified that the source is an unstoppable voltage source, and the load has "negligible" DCR.

For a circuit where the current must stay within some nominal range, this isn't a good example.

As for how this works, in terms of inductance and energy and parameters, what happens is the incremental inductance is decreasing.  You've still stored energy and flux in the unsaturated range, and that doesn't simply go away.  It's the slope that's changing, not the whole history.

This is different from mechanical changes, like if you magnetize the core then pull it apart (or, say, yank the armature out of an energized solenoid).  You're putting mechanical work into the system here, so the electrical energy need not be conserved.

Likewise if you saturate the core with a crosswise magnetic path; the total energy flow (from all windings) will be conserved, but energy can be pushed between the windings through transformer or inductor action.

Tim