RE: Charging inductors for resonant charging

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Steve,

On 4 Feb 2004, at 8:18, Tesla list wrote:

 > 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?

Is the current "forced" to zero every cycle or simply allowed to
relax and *hopefully* return to zero in a given time?

 > 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.)

Ungapped cores are indeed poor stores of energy. In fact this is
desirable since you don't want a transformer core to store any energy
at all. Energy storage inductors are always gapped. It is not the
decrease in L which does the job (since L can be decreased in
ungapped inductors as well) but the fact that the gap adds
considerable reluctance into the magnetic circuit. Most of the energy
is actually stored in the gap which can take an infinite amount of
flux. Increasing the length of the gap becomes necessary as the flux
level increases, increasing the flux density to the point where the
core is approaching saturation. The core's job is simply to confine
most of the flux to the gap, reducing the inductor's coupling into
other objects. For this reason, a core gap is most effective when it
keeps gap flux in the vicinity of the windings (e.g. in the centre
pole only of an E-E core).

Malcolm

 > 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.
 >
 >
 >