Think you may be overlooking a key distinction. The reversal I’m describing on departure isn’t caused by resistive decay of the induced current,
That is exactly it. You understood me ...but you don't believe me. That's OK - we will get there.
...it’s caused by the geometry of induction itself. Even in a superconducting coil, the induced current is not free to “just persist” in one polarity.
In a shorted superconducting coil that is exactly what happens! The induced current persists indefinitely. There is a superconducting coil on Earth in which the current has been persisting for over 20 years !
As the rotor’s pole sweeps past the coil face, the flux linkage changes sign.
Ah, but the magnetic flux that penetrates a shorted ideal coil cannot change (by penetration I mean a loop integral of the flux). That is the beauty of it - it is the key to understanding the entire electromagnetic induction phenomenon.
The current induced in the shorted loop is such that the internal magnetic flux generated by that current cancels any change that the external flux is trying to make, thus keeping the total flux penetrating the coil
constant and preventing any flux lines from cutting the coil (i.e. changing your flux linkage).
This simulation shows this principle beautifully.
Oh, ...and and one more thing: The magnitude of current induced in an ideal shorted coil by an external flux source DOES NOT DEPEND on the speed with which that external flux changes. Nanosecond or a year - the induced current ens up the same magnitude.
By Faraday’s law, the induced EMF must change sign as well, which forces the current to reverse.
Faraday's law (induced EMF ℰ by a changing external flux) breaks down with an ideal shorted coil because when R=0 and i=ℰ/R then this would means that i=∞ for any ℰ>0, which is ridiculous.
In my opinion analyzing coils through the prism of induced EMF (ℰ) and Ohm's Law: i=ℰ/R, is exactly the kind of wrong path that prevents people from fully grasping electromagnetic induction.
So the attraction I see on departure is not a symptom of copper losses
Oh, yes it is ! ...and the transfer of coil's stored energy to the load, too, of course,
—it’s the inevitable consequence of the flux derivative changing sign across the coil’s two walls.
The external flux's time derivative dΦ/dt has absolutely no influence on the magnitude of the current induced in a shorted ideal coil. Like I already wrote: The flux can change in a nanosecond or in a year - it makes no difference.
You are just so used to common coils leaking their stored energy/current all the time (like Mr. Faraday), that you think that the speed of the external flux changes makes a difference in the induced current (because the external flux represents energy replenishment to the leaky coil) ...but I got news for you - to an ideal shorted coil that speed (rate of Φ change) makes not an iota of difference.
Superconductivity would only eliminate resistive losses and extend the magnitude/duration of the induced current, but it would not prevent the polarity flip.
Not only - it also prevents the flux from cutting the coil and changing flux linkage which completely defeats the Faraday's Law. Not a small feat.
Also, what "polarity flip" ? The direction of induced current in a shorted ideal coil does not change direction by itself.
In other words:
Approach phase → flux increasing → induced current opposes increase → repulsion.
I agree with the proviso that when you write "flux increasing" you mean external flux, because the total flux (internal + external) that penetrates an ideal shorted coil stays constant.
Also, you skipped one arrow - it should have been:
Approach phase → external flux increasing → induced current generates an internal flux → internal flux opposes the external flux (or increase of total penetrating flux) → repulsion and the total penetrating flux stays
constant.
Zero crossing → flux change = 0 → current crosses zero at coil center.
Departure phase → flux decreasing → induced current opposes decrease → attraction.
No, in an ideal shorted coil you do not start from 0 flux. The current induced during the approach phase still circulates in the coil and it generates internal flux that opposes any changes in total flux penetration attempted by the external flux. This means repulsion. If you were starting form zero internal flux, then you'd have attraction on departure ...but you don't because an ideal shorted coil has not lost its current (and the internal flux it generates) that has been induced during the approach phase.
That sequence is dictated by Faraday’s law and coil symmetry, not by resistance. Superconductivity strengthens the effect but doesn’t alter the fundamental reversal.
...
Superconductivity would only amplify the magnitude and persistence of those induced currents, but it wouldn’t eliminate the polarity flip dictated by rotational symmetry.
Nothing of the sort. Faraday's law is not applicable to ideal coils because when R=0 then Ohm's Law i=ℰ/R dictates that i=∞ which is ridiculous.
The Quantitative Lenz Effect (QLE) nips the Faraday's Law in the bud before the bastard has a chance to act, because QLE does not allow for the change of the flux linkage through an ideal shorted coil.
His little brother (the qualitative Lenz Effect) which you all know and love, defines only the sign of the induced EMF in the parent law. The qualitative Lenz Effect is the minus sign in the Faraday's parent Law: ℰ=
-dΦ/dt. Both of them say noting about the quantity of the induced current. The big brother (QLE) - does ...and it slaughters the brother and the father.