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carpenter 14th June 2003 09:11 PM

E/I transformer laminations; why?
I'm interested in constructing my own E/I transformers. I'm wondering why the industry uses E/I laminations rather than solid iron. Does anyone know the answer? It would be a snap for me to fabricate a large, solid "I" shaped iron bobbin and bolt iron to the outside stems of the "I" after I wind it up with magnet wire. Is there any reason why this wouldn't work?

Thanks for your time,

John Inlow

carpenter 14th June 2003 10:31 PM

Well, I found the answer. Sorry for bothering you folks. The article is located here:

transformer laminations and eddy currents

Here is the text:

Eddy current loss
Eddy current losses occur whenever the core material is electrically conductive. Most ferromagnetic materials contain iron: a metal that has fairly low resistivity (roughly 10-7 Ω m). The problem is intuitively obvious if you consider that the magnetic field is contained within a 'circuit' or loop formed by the periphery of the core in the same way as it is contained within a turn on the windings. Around that periphery a current will be induced in the same way as it is in an ordinary turn which is shorted at its ends.

What is needed, then, is some method of increasing the resistance of the core to current flow without inhibiting the flow of magnetic flux. In mains transformers this is achieved by alloying the iron with about 3% of silicon. This lifts the resistivity to 4.5×10-7 Ω m. Depending upon the amount of silicon this material is called 'transformer iron', 'electrical iron' or 'armature iron'. The alternative name 'silicon steel' is a misnomer because steel is iron alloyed with carbon; and carbon does no good in a transformer core. The silicon does, though, increase the mechanical hardness of iron in the same way as carbon - try sawing up a transformer core and you'll discover this quickly.

lamination concept In any resistive circuit the power is proportional to the square of the applied voltage. The induced voltage is itself proportional to f×B and so the eddy losses are proportional to f2B2. The flux is also related to the size of the loop. Figure PLM shows how the idea of lamination is used to reduce the power losses caused by eddy currents in mains transformers. The same principle applies to motors and generators too. Using a solid iron core (as in cross-section B) results in a large circulating current. So, instead, the core is made up of a stack of thin (~0.5 millimetre) sheets (cross section C). Here I have shown only four laminations but there will normally be many more. The lines of magnetic flux can still run around the core within the plane of the laminations. The situation for the eddy currents is different. The surface of each sheet carries an insulating oxide layer formed during heat treatment. This prevents current from circulating from one lamination across to its neighbours.

Clearly, the current in each lamination will be less than the very large current we had with the solid core; but there are more of these small currents. So have we really won? The answer is yes, for two reasons.

Power loss (the reduction of which is our aim) is proportional to the square of induced voltage. Induced voltage is proportional to the rate of change of flux, and each of our laminations carries one quarter of the flux. So, if the voltage in each of our four laminations is one quarter of what it was in the solid core then the power dissipated in each lamination is one sixteenth the previous value. Hurrah!

But wait; it gets better. Look at the long thin path that the eddy current takes to travel round the lamination. Suppose we made the laminations twice as thin (we halved d1). The path length of the current isn't much changed; it's still about 2×d2. However, the width of the path has halved and therefore its resistance will double and so the current will be halved. The bottom line is that eddy current loss is inversely proportional to the square of the number of laminations. Iron losses should be between about 1 and eight watts per kilogram at 50Hz and 1.5T for good transformer steel.

This idea of dividing up the iron into thin sections is carried a stage further in the iron dust cores. Here the iron is ground into a powder, mixed with some insulating binder or matrix material and then fired to produce whatever shape of core is required. These cores can function at several megahertz but their permeability is lower than solid iron.

Hope this is interesting to someone,

John Inlow

JOE DIRT® 14th June 2003 10:33 PM

Thanks John!!!....good read;)


carpenter 14th June 2003 11:28 PM

thanks Joe......
I've been trying to discover a way to fabricate my own large chokes using the E/I laminations approach. I've ordered 80 lbs. of 11 gauge magnet wire and will be procurring 4 very large E/I sets next weekend. Digi-key want about $250.00 each for a 100mH/10amp 35 lb choke. I hope to make four of them for $300.00. They'll create the current source for my balanced Zenlites. I'll use the inductors instead of the lightbulbs. This should boost the efficiency from 8 percent to around 40+. With the 11 gauge wire I may find that I have less inductance and more amperage capability. Time will tell.


John Inlow

JOE DIRT® 15th June 2003 12:44 AM

11`ll have your hands full winding them...thats 1/8`th thick wire...would like to see a pic when your done


carpenter 15th June 2003 03:03 AM

A pic, yeah, I'll have to do that....
I'll probably have to go slow and easy with a rubber hammer. Ha ha.


apassgear 15th June 2003 03:25 AM

Re: A pic, yeah, I'll have to do that....
2 Attachment(s)

Originally posted by carpenter
I'll probably have to go slow and easy with a rubber hammer. Ha ha.


I have rolled 12 ga inductors and transformer in the past and seldom used a hammer (but will depend also on how much of the window you are planning to use) but 11 ga will probably be a challenge.

You need to protect your fingers with masking tape or gloves. If you need to use a hammer you can use a regular steel one taking care that the face is smooth and no dents or burrs. Modern varnish is quite tough and wont chip.

For inductors you will need to calculate the gap between the E & I.

For better results you will need a sort of a hand lathe utilizing a wood mandrel to fix the bobbin.

This are some pics of what I use but you can do it of wood very easilly...

apassgear 15th June 2003 03:28 AM

2 Attachment(s)
And another one...

carpenter 15th June 2003 04:06 AM

Well, I'm certainly impressed!
In fact, I might be able to fabricate a devise such as yours. Would you care to discuss further the winding technique? i.e. is it necessary to isolate with paper (or what have you) the winding layers on a choke that will see 45 volts? Do you have to oversize the bobbin to allow the E/I laminations to fit more easily. And, how do you create an air gap? If I were more well read I could ask more questions; afraid I'm still in the learing curve. I'm all eyes!

Circlotron 15th June 2003 12:01 PM

About the only part you can make at home.
There is something *really* satisfying about winding your own trannies and chokes. Real grassroots-level diy. Also, because most people are scared of winding their own stuff for some reason, you will find yourself feeling exceedingly smug afterwards. *That* will make your system sound much better for sure. ;)

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