RE: Charging inductors for resonant charging

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

 >If the current is forced to 0 every cycle, how can it
 >build up over many
 >cycles and cause saturation?

I don't know. All I know is that every time I've tried to use an iron core
without airgap as a choke, it has always saturated. The last time I saw this
was in the line choke in my thyristor controlled rectifier, it needed a 1mm
airgap to make it work. Not only was the current discontinuous, but it
reversed every other half-cycle.

Maybe the problem isn't "flux walking" but simply that ungapped iron-cored
inductors don't like to store much magnetic energy. I reckon this is because
magnetic energy is 0.5*L*I^2, and increasing the airgap increases Imax as
fast as it decreases L. (The energy stored in the airgap can be felt as
mechanical force, on a previous version of my line choke it was enough to
overcome the clamp bolts and crush the airgap shut.)

Of course, from this argument, we can say that the best energy storage
inductor has an infinitely large airgap, in other words no iron at all. This
would be true if it weren't for copper losses, which are increased as the
airgap gets larger, because you need more turns to achieve the same
inductance. In practice the lowest overall losses are found at the point
where the copper losses and core losses are equal.

Of course if you use superconducting windings, then there is no contest, the
air core wins hands down. Superconducting air-cored coils have been made to
store several MegaJoules at currents of 1000s of amps.

Steve C.