How do you prevent eddy currents in a transformer?

[Mr. Astrophysicist]

How do you prevent eddy currents in a transformer? (I know the answer, just testing you guys).

[John Cuthber]

One way is to not install the transformer in an electrical circuit of any sort but just use it as a paperweight.

 

Seriously, do you think that is much of a test?

[studiot]

I asked my [insert] favourite politician [/insert] this one,

 

The reply was

 

Rename them Walter currents.

 

Edited February 21, 2015 by studiot

[Enthalpy]

Or live in France, where they call them Foucault currents because he was a Frenchman. English-speaking countries call them "eddy" for the same reason.

[John Cuthber]

Or live in France, where they call them Foucault currents because he was a Frenchman. English-speaking countries call them "eddy" for the same reason.

Or, perhaps because the French don't know what an eddy is.

 

eddy

ˈɛdi/

noun

1. 1.

   a circular movement of water causing a small whirlpool.

   "the current was forming foam-lipped eddies along the bank"

   synonyms: swirl, whirlpool, vortex, maelstrom; More

verb

1. 1.

   (of water, air, or smoke) move in a circular way.

   "the mists from the river eddied round the banks"

   synonyms: swirl, whirl, spiral, wind, churn, swish, circulate, revolve, spin, twist;More

[Mr. Astrophysicist]

Answer: Laminate the core material; it could not be much more simpler.

[dimreepr]

You’ve been watching the big bang theory; the trick is to understand why.

[John Cuthber]

Answer: Laminate the core material; it could not be much more simpler.

Do you think that's the only way?

[Enthalpy]

Most forum members here knew the "laminate" answer, and so did the original poster, so maybe t's more useful if I bring some elements that are less universally known - and my apologies in advance to all who know them already.

 

Transformer steel contains silicon. Despite reducing the saturation induction, silicon is very useful to increase the resistivity of steel hence reduce the eddy currents. It's also one convenient means to deoxidize the steel, which reduces the coercitive field hence the hysteresis losses.

 

Lamination thickness acts on the eddy current losses squared, but so does the frequency also, and the stray capacitance of the laminations' insulating oxide would let capacitive current pass anyway, so above some 400Hz to 10kHz, the game with thinner laminations is over.

 

The usual next step is iron powder, which is easily finer than a lamination. The powder is oxidizer superficially to insulate and then sintered. Though, the iron filling factor is worse, dropping the saturation induction, and since oxide layers are in all direction, the induction must cross them and the permability drops, so iron powder is used where laminations can't go well, in the kHz range.

 

Above iron powder, and up to coreless inductances in the100MHz+, is the domain of ferrite. The material itself (Fe, O, and variable amounts of Mn and Zn, for the most common ones) resembles more a ceramic than a metal: very brittle, and a high resisitivity that reduces eddy currents without having to interrupt them with an insulator.

 

Though, I used once the market's best ferrite core for antennas (made by Philips then), and this one had longitudinal slits, with dents in its cross-section, to reduce the eddy currents further - it was damned brittle. I needed Litz (=braided) wire for the windings, a good insulator between the ferrite and the winding, a decent insulator (no Pvc) on the terminal wires, and then I got Q=400 at 457kHz, wow. With that ferrite antenna, a bandwidth <1Hz and many more optimizations, I reached a sensitivity of -170dBm and made the equivalent of an avalanche transceiver

http://en.wikipedia.org/wiki/Avalanche_transceiver

but with 110m range instead of <20m, that is, a power improvement of 30,000+.

 

Eddy currents also appear in the windings, typically at 10kHz-1MHz (above, the current doesn't penetrate the wire anyway, so the effect has a different name). The standard answer is to use braided (Litz) wire.

 

Even at lower frequencies, some transformers with a low voltage side must use one single turn of copper, as a foil rather than a wire then. Any (leakage) induction through this foil creates huge eddy currents because of its width, so the answer attempt (it's imperfect) is to split the other winding in two and put one half nearer to the core than the foil winding and the other half outside the foil. By symmetry, the leakage induction is minimized.

 

At one inductor for 20kHz, with a cut U core of winded steel, I had an intended gap of several mm. I suspected the lateral spread of the flux above and below the gap to create losses in the laminations since they were not parallel to the induction there. I added a copper foil right around the core at the gap, making 1.5 turn but insulated, and made the winding again. My intention was to guide the flux with the foil to stay parallel at the gap; whether it worked that way or differently, it did reduce the losses.

 

I must have forgotten some more methods.

[MigL]

Most AC transformers are laminated plate because of the low frequency.

If you've ever messed around with the switching power supply inside your computer ( don't do it, seriously high voltages and enough retention on the inlet caps to seriously injure you ) you would have noticed comparatively small ferrite core transformers because of the high switching frequency of these supplies.

 

I didn't know Foucault's first name was Eddy.

[CasualKilla]

You can laminate the core, I beleive in the direction of flux flow works best. Or if your rich you can make the core out of a highly resistive materiel that still has decent permeability. Peddy = I^2 * R. Induced EMF will be constant so by reducing the current by increasing resistance of the current flow path you decrease losses.

 

The only way to truly prevent eddy current is to turn of the transformer as I don't believe there will ever be a way to make the resistance of the current path infinity.

[studiot]

 

as I don't believe there will ever be a way to make the resistance of the current path infinity.

 

 

Vacuum cored Tx?

[John Cuthber]

 

Vacuum cored Tx?

Unfortunately, it doesn't quite work. There will always be some leakage flux and that spreads out- eventually, it will hit something conductive.

Very low is possible, but zero isn't.

[studiot]

 

Unfortunately, it doesn't quite work

 

Ever the too practical Scot.

 

[CasualKilla]

Unfortunately, it doesn't quite work. There will always be some leakage flux and that spreads out- eventually, it will hit something conductive.

Very low is possible, but zero isn't.

LOLOL. But studiot is right though, a vacuum cored TX in an isolated universe consisting entirely out of vacuum will have 0 eddy current losses. But the problem is, how does one create the transformer coils without conductors.

 

The game is on.