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Re: Current Limiting and Impedence



Original poster: Steve Conner <steve@xxxxxxxxxxxx>


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

Could you elaborate a little more on the lack of an air gap.

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.


If you go back in the archives you'll find this topic discussed in a lot more technical detail by me and many others. But for a more seat of the pants explanation-

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.

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.

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.


Steve Conner