transformer core flux propagation speed

Hi,

I was curious about how fast the winding induced flux will propagate through a transformer core (ie a ferrite core) assuming a single primary winding on a toroid? Would it be possible to make a resonant transformer using core geometry (ie replacing the toroid with a shape that has a sine wave on the toroid). If the switching frequency is high enough, maybe it is possible to utilize the finite flux propagation speed (ie. by using resonant flux switching to give different simulated urns ratios etc)

cheers, Jamie

Reply to
Jamie M
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If the core isn't significantly conductive, then the speed of flux penetration should just be the speed of light in the core, which would be c/sqrt(mu) -- so, c is around 300 meters/microsecond, and you know the relative permeability of the core.

The core doesn't have to be a special shape.

There are, in fact, resonators that are made with high dielectric constant ceramic, as 1/4 wavelength shorted coax segments. Because of the dielectric constant, the velocity is much lower than the speed of light in a vacuum, so the resonators are more compact.

I remember when they first came out, with much ballyhoo -- I don't know if they're still popular.

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Reply to
Tim Wescott

Roughly sqrt(mu_r) times slower than c. Notice that in real materials, past a cutoff, mu_r depends strongly on frequency, so above 10kHz-10MHz (depending on formulation), the response is strongly dispersive.

Notice that using ferrite for signal propagation works just like any other propagation medium, so along with time delay comes phase shift, and with phase shift comes rotation from magnetic to electric fields. To achieve stable propagation you'll have to determine what geometry allows the electric field to propagate.

There are microwave resonators available which do this; the operation is analogous to optical total internal reflection, except the puck is fractional wavelength sized, so it acts like a conventional resonator as well. I believe they are made from ferrite or dielectric (either of which has an index of refraction greater than air).

Tim

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Reply to
Tim Williams

El 14-04-12 0:15, Jamie M escribió:

hello Jamie,

The phenomenon is known, even standing waves in the magnetic medium because of the air/ferrite boundary.

Besides high mu_r, ferrite materials have a eps_r > 1, it can be >1000 (MnZn ferrite). This reduces the propagation speed significantly. unfortunately, most manufacturers don't specify eps_r' and eps_r'' versus frequency for their power ferrites.

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Reply to
Wimpie

Hi,

Thanks, I guess for experimenting it would be good to find a core material with low propagation speed and also a high saturation current density, maybe MnZn ferrite powder mixed with epoxy to mold it into a custom shape and also increase the saturation current could work? I am not sure about the properties of ferrite compared to ferrite+epoxy for a transformer!

cheers, Jamie

Reply to
Jamie M

high

simulated

Adding a bunch of lower mu_r and eps_r material interstitially will only lower the effective value of mu_r and eps_r.

?-)

Reply to
josephkk

Google "YIG-tuned oscillator design"

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

At the risk of saying I am hijacking this thread (might need a new topic), what about core noise and dynamic range ??

What I mean is.... If you had a super quiet wide range hall effect or other magnetic sensing device in the gap of a (ferrite ?) core, what would be the smallest AC and/or DC signal change you could see ?? I know there is some noise floor in there but it's kind of hard to read some of the lit I've googled.

boB

Reply to
boB

Actually, I think the noise in ferrite cores was just Johnson noise and might get louder with hotter cores maybe but is hysteresis maybe an issue too at low low levels ??

boB

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Reply to
boB

And maybe the noise goes up with permeability ?

I guess I am asking if there are any magnetics companies that have noise specs for their cores. I don't believe I have ever seen any from the usuall sources.

boB (again)

Reply to
boB

There's also Barkhausen noise, due to the stick-slip motion of magnetic domains.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

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Fun home experiment: attach an audio amp to transformer, then run DC through another winding. Change the DC up/down and listen for the domain boundaries to 'crash'

Reply to
Robert Macy

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This isn't directly due to thermal noise (flipping magnetic spins, domain fluctuations), which will be much weaker (though possibly noticable around the Curie temperature).

Very soft materials (low remenance) should be quieter than those with high remenance; blatant example, applying a field to a permanent magnet won't cause any substantial flipping until the entire coercive force is applied (~1e5 A/m for NdFeB, IIRC), at which point the whole thing changes quite rapidly.

Tempted to set up an amplifier and try listening to a core. Should be able to get a small ferrite toroid up to Curie with only the soldering iron handy.

Tim

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Reply to
Tim Williams

Won't there be a mean time delay associated with the domains snapping around? They will have, in effect, some sort of inertia.

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Reply to
John Larkin

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Dr Philip C D Hobbs
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Reply to
Phil Hobbs

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Dr Philip C D Hobbs
Principal Consultant
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Reply to
Phil Hobbs

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Think more sequential than average. Actually, a domain wall does not have 'pressure' on it until an adjacent wall flips. Then the field can build against it. Sort of like dominoes.

For me to understand what was going on, I used to envision a field of wheat, blowing in the wind. The wind hits a stock and it tends to bend over allowing the next stock to 'see' the wind and so on. Some stocks are stiffer than others so the wave is not so uniform. Importantly, It makes a great image for picturing wave propagation. Plus, *IF* the wind changes direction before the field is down, you can start to envision the standing wave patterns moving across the field, even see how the stocks in one section are not even going down the right direction, but the opposite direction, so instead of helping, they are hindering. Anway, any allegory that helps intuitive understanding has some value, look at what Tesla came up with after watching ??, which was an imperfect allegory, too. He came up with the induction motor.

Reply to
Robert Macy

Barkhousen was a term I was semi familiar with and now that Phil mentioed it, I wonder how much that could interfere with an AC signal amplifing when the DC current is changing appreciably... Now I will go look at the wiki link. I'd expect harder magnetic material to make more BH noise and soft material to make more johnnson noise but I'm just guessing. Hopefully any real work I do is way below the curie temperature but it makes intuitive sense that there would be some hot action up there.

A Wikiing I will now go !

boB

Reply to
boB

Looking at the B-H curve on that page, it doesn't look like particularly hard material, but it might not be magnetically "to scale" but was just a convenient curve to use for illustatration. Since I'm set up to do the test in the lab already, I will have to try it on ferrite at least while varying the DC current. It might even be worse than any other noise but I haven't noticed it yet.

I wonder how much gain is needed ? Maybe a hall effect device is too noisey to eve hear this Barkhausen effect and that's one reason the wiki artilce mentions using a coil ? Looks like it could even sound like somewhat of a zipper noise.

OK, so I went to the listed youtooob link and they show a guy rotating a magnet near an antenna loop looking thing. This is kinda confusing to me cuz I don't see how that is going to excercise through the B-H loop as shown in the wikipedia article.... And it's a magnet, not a soft magnetic core. Something doesn't seem quite right there.

boB

Reply to
boB

OK, finally found a decent demonstration online

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This one makes sense.

boB

Reply to
boB

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