filling air gap in transformer cores

[maudio]

Hi,

When assembling a transformer core there will always remain some airgap even when you don't want it (toroids excepted).

And it's not insignificant either: I found out that I can raise the primairy inductance of a coil on a double c-core by over 30 % just by squeezing the cores together with even more force
The gap will not only lower inductance but also increase leakage inductance. Bad thing...

So I was wondering if I can put some "magnetic field conducting" stuff between the two core halves to fill the gap

I've been thinking about ferro fluid such as used in tweeters but I am afraid it will probably leak out sooner or later.

Better would be some glue/kit with such properties? Does it exist? Or do I have to mix up something myself by adding some iron particles to it?

[EC8010]

Originally posted by maudio
And it's not insignificant either: I found out that I can raise the primary inductance of a coil on a double c-core by over 30 % just by squeezing the cores together with even more force

Yes, but ask yourself what will happen when some DC flows in the transformer. The gap is a magnetic resistance that helps make the primary inductance more nearly constant with (AC) level and DC.

[poobah]

It depends on what your motivation for doing so is. Yes, you increase the inductance but this also bring about core saturation at lower current levels. It will NOT increase the basic power handling capability of the transformer... that tends to come on a pounds per watt per LF response basis.

In OPTs and even power transformers, that gap is your friend.

Can you tell us what you are trying to achieve?

[poobah]

Damn EC... you beat me!

[maudio]

Can you tell us what you are trying to achieve?

Sure, I am winding an esl-transformer and my aim with this idea was to keep the number of turns needed as low as possible. The basic maths being:
Less turns = less capacitance + less leakage = more bandwidth

So therefore I wanted to maximize the inductance/turn. I also think reducing the gap will decrease leakage. I measured an increase in resonance frequency when reducing the gap.

I know less gap will lower the core saturation limit but that is not my primairy concern as I plan on using rather large cores. DC currents are also non-existent in the application.

[poobah]

If you are certain that there will NO DC... then by all means. But listen to the cat... the gap tends to make your design prediction of L better. In other words don't go to extremes.

The bulk of your reduction in leakage inductance will be achieved through interleaving anyway.

See if he beats me again...

[maudio]

Originally posted by poobah
If you are certain that there will NO DC... then by all means. But listen to the cat... the gap tends to make your design prediction of L better. In other words don't go to extremes.

Ok, so I understand that too little gap might raise a few new problems. But am I wrong in thinking that a toroid core is wound from one long strip of iron and will not have any gap at all? Are they using material with higher losses for toroids to compensate?
Well I guess it's all a matter of taking things into account in the design stage.

The bulk of your reduction in leakage inductance will be achieved through interleaving anyway.

With only 1 layer of primairy wire and 4 layers of secundairy there isn't much to interleave... I'll probably just add another layer of prim on top of the secundairy to sandwich it in between.

See if he beats me again... [/B]

[poobah]

Yes... a tape wound core can be considered gapless.

There are lossy toriods usually made of dusts and ceramics. They are popular in power inductor designs.

You may consider winding your primary trilfilar or quadfilar to improve interleaving... capacitance will be your deciding, or limiting, factor then.

What type of core are planning on using?

[jcx]

lapping and clamping high perm ferrite core faces is necessay to get the best core coupling/least gap

to a 1st order aproximation the leakage inductance is not changed by the core - the leakage inductance is a coil geometry issue, it is the coil inductance from flux not linking the 2 windings - the core is usefully placed where the flux does link the windings

[maudio]

to a 1st order aproximation the leakage inductance is not changed by the core - the leakage inductance is a coil geometry issue, it is the coil inductance from flux not linking the 2 windings - the core is usefully placed where the flux does link the windings

makes a lot of sense, Jcx. I came up with this because I saw the frequency of the resonance peak (the one due to Lleak and Cpar) of my unloaded transformer shifting slightly upwards when reducing the gap and reduced leakage seemed a logical explanation to me at the time. But it might as well have something to do with the changing parameters of the primairy.

Poobah, right now I use some c-cores that I salvaged from old mains transformers. So I don't have much data on them, they are proably quite standard. Made with 0.3mm (0.01") laminations and core area is around 13 cm^2 (just over 2 square inch). I plan to stack two of them doubling core area. Maybe further down on the road I'll look for some special high-mu cores but for now they will do I hope.

I don't think I fully understand your suggestion about trilfilar or quadfilar primairy winding. Do you mean parallelling several wires for the primairy? I was planning on using 5 or 6 parallel wires for the primairy on my next try-out, but my objective was just to fill the entire winding space (62 mm / 20 turns) while maintaining low winding height. I don't see how that would reduce leakage. But I guess you mean something different... Maybe you can explain this a little further?

thanks to you all,

Martin