Re: Current Limiting and Impedence

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Gerald  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Steve,

I think I agree with everything your saying so please take my comments in 
that light.

> Original poster: Steve Conner <steve@xxxxxxxxxxxx>
>
> Ballast inductors don't work without air gaps. That's just the way the 
> world is. If you think you have a gap-less ballast working, it's probably 
> behaving as a saturable reactor.

It seems like with enough turns (or enough area), you can prevent the 
gapless ballast from saturating, but without the gap, the inductance will 
be larger and less current will flow.


> A ballast inductor is like a spring in that it stores energy. The resonant 
> charging process in Tesla coils (whether running off AC or DC) is rather 
> like a spring being compressed when the gap fires, and as it bounces back 
> it shoots the tank capacitor to an even higher voltage than the supply. 
> It's sort of like an electrical pogo stick.

Yes, this is pretty fundamental.


> Iron cores are not good at storing energy because they magnetise too 
> easily. It only takes a very little H to make a lot of B. To take the 
> spring analogy, a gapless iron cored inductor would be like a spring made 
> of some weak floppy substance such as noodles. As soon as you put a 
> voltage across it, it barely puts up a fight (hardly any current flows) 
> and then before it has stored any worthwhile energy it bottoms out 
> (saturates because B got too big) and looks like a short circuit.

I believe the first part is a direct result of too high inductance.  If the 
gapless ballast doesn't have to saturate though.


> This kind of behaviour is fine in a transformer where you want the 
> magnetizing inductance to draw as little current, and store as little 
> energy as possible. But in a ballast, where you are trying to store 
> energy, it's about as effective as a pogo stick made of noodles.
>
> Putting the air gap in introduces a substance that doesn't magnetise so 
> easily. It's like upgrading from a noodle spring to a steel one, and you 
> go bouncing happily down the road.

I agree (I think).  If the inductance doubles (lets say) by removing the 
gap,  the current allowed halves and the energy stored (0.5*LI^2) is one 
half.  Seems like the way to make a comparison is to compare a gapless 
ballast with inductance L with a gapped ballast of different physical size 
to get the same inductance L.  Now, what would be the difference in 
performance???  Assume that neither designs saturate.

I wonder if the real issue is physical size between the two designs.  If I 
take a gapped ballast and remove the gap, the inductance goes up.  I will 
need to reduce the number of turns to get back to the original inductance. 
This will increase the volts per turn that will push the ballast closer to 
saturation unless I increase the cross sectional area.

Gerry R