Air core transformer question -- help

On Mon, 4 Jan 2016 20:36:18 -0800 (PST), snipped-for-privacy@gmail.com Gave us:

Use for RF.

coupling efficiency suffers at low freq.

Removing the core reduces the coupling between the coils.

If you did the experiment with a 50 or 60Hz switching frequency, you pretty much turned your transformer into a couple of loosely coupled wires.

Oops, why did you want to do that? As Tim said there is now little coupling. The voltage is given by the changing magnetic flux through the secondary... no flux, no V.

George H.

The iron core couples almost all of the flux from a winding to all other windings. The leakage inductance is a tiny fraction of the total inductance. When you remove the core, the leakage inductance becomes dominant, and the flux from one winding does not couple well at all to the other winding. There are ways to improve the mutual inductance in air-core transforers, such as bifilar winding (interleaving the turns of the two windings). But, now you know WHY they put all that iron in transformers. it is not a mechanical support, it is to capture and couple the flux between windings. And, your results are certainly proof of that!

Jon

The reason that there's no flux, is 99% that the B field in iron is magnified from the field (H-field, in physics terminology) that you get with no core. The rest might be that your windings are no longer well-coupled because the core no longer links them, only their geometry does. It is possible to make a transmission-line transformer, very well flux linked, by pairing the primary and secondary wiring, and winding that pair. Your transformer wasn't wound that way (when there is a core, it doesn't matter).

Bifilar winding is used in cored transformers when you really want to minimise leakage inductance. There are situations where even a little leakage inductance can be undesirable.

And for precision ratio transformers, each half of a bifilar winding can be identical to the other to one part in 10^9, which can be handy.

Most ratio transformers use bundles of more wires, and can't do better than about one part in 10^7.

See "Coaxial AC Bridges" by Rayner and Kibble.

For your own benefit, instead of removing the core, you might instead reconfigure it so that an adjustible gap is possible.

EI stampings can be grouped with 'all E' and 'all I' groupings.

Gradually increasing the gap in paper-width-thick increments will show you what's happening as you change the effective permeability of the magnetic core from the initially high value of silicon steel, to that of free space. ( uo = 4 x pi x E-7 )

RL

On Thu, 07 Jan 2016 09:58:04 -0500, legg Gave us:

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For an awful long time, it seems that there has been no end to the mischief possible, in coveting other people's (or even 'unoccupied') space.

Opinions or beliefs, which occupy no physical space what so ever, seem also to end up costing somebody........

Just give it a name and some bright spark will figure out a way to make a buck from it.

RL

Thanks to each of for your helpful comments. Initially, the astable multivibrator that I am using to excite the bifilar winding had an oscillation frequency of approx. 5.5 KHz (as estimated from o'scope traces), but now, after some modifications, the oscillation frequency has been boosted to 100 KHz. Now the output is 8 Volts AC, across the primary of the air core transformer, and that is sufficient for my initial proof of concept design.

If you want a higher frequency transformer there are all sorts of ferrite cores that you could maybe use. Is there a lot of power involved?

George H.

Of course. Just ask NASA.

On Sun, 10 Jan 2016 04:53:23 -0600, John S Gave us:

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Serious enquiries only.

Cheers

Phil Hobbs

On Sun, 10 Jan 2016 05:11:40 -0800 (PST), Phil Hobbs Gave us:

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Cheers

Phil Hobbs

Nah, they're an infinite money sink.

Faradays law - Voltage induced = N X rate of change of flux. No flux is reaching the secondary!

On Sun, 10 Jan 2016 09:37:03 -0500, Phil Hobbs Gave us:

I'll come close the deal right after I claim my power ball jackpot of