E/I transformer laminations; why?

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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
 
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
 
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.

Later,

John Inlow
 
Re: A pic, yeah, I'll have to do that....

carpenter said:
I'll probably have to go slow and easy with a rubber hammer. Ha ha.


John,

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...
 

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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!
 
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. ;)
 
circlotron,
i agree with what you said 100%, i handbuilt all the trannies used in my amp projects, i started out with solid state amps in the late 60's or early 70's while still a freshman in college. that time, the shops making transformers do not yet understand the solid state transformer requirements as tube amps were still very much in season then...so i had no choice but to make my own. having knowledge of transformer theory helped me make up my own trannie...though i had to dismantle a good transformer meant for tube amps to get the desired turns ratio, figuring out wire size was easy....i have built a torroid transformer which i used for a low powered leach amp...the biggest i made so far is one with an EI core with 2 1/4 center leg and stacked to about 5.5 in depth, all made by hand....
 
Re: stack depth

carpenter said:
How deep can you stack the laminations?


I see Joan2 rolls bigger than me. The maximum I've done is EI 150 (1 1/2" center leg) with a stack of 3 1/4") I would generaly limit the stack to about 2 times the center leg size.

Last year I made 2 transformer of this size for my Aleph 4 rated at around 750 VA each. Secondaries were 12 ga.

Regarding the bobbin former what I usualy do when doing non standard stacking is splitting a standard plastic former with a jacksaw put it over the wood mandrel and fix it with some tape and I'm ready to start winding.

Big bobins should have an internal hole about 0.005" oversize regarding the lamination core to accept the poor gometry of the bobin.

If doing a non standard bobin or in instances that I don't have a standard plastic bobin former I make a former using Fish paper usualy 0.010" thick as you can see a small bobin on one of the previous pics.

The following pics show tranies for the Aleph..
 

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Re: thanks Joe......

carpenter said:
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.


OK now it's clear.

Still they seem a little big, but you can play with the stack hight to size them correctly.

I hope your amps are monoblocks!!! :rolleyes:

PS,
John, you are a bold guy!!!
 
well now.....

I ordered 750 I/E sets in a smaller size (7.5" x 6.5") #26 gauge. The order stacks out to roughly 14" which gives me a stack height of 3.5" apiece for the four chokes. With shipping (140 lbs cost me $65.00!!) the cost comes to $272.51. The 11 gauge wire cost me $160.00. Everything combined, I'm investing $432.51, which is a bit more than the $300.00 I had originally planned on spending. Still, four of these chokes would probably cost $1200.00 to $1400.00 from Hammond if I were to gauge cost by weight. So I'm cutting the cost by about 2/3s. I'll keep you folks posted on my results.

John Inlow
 
Re: Well, I'm certainly impressed!

carpenter said:
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!


Coming back to some of your questions, tis is the way I do it, so feel fre to experiment.

Layer interleaving with craft paper or 0.002 mylar is always a good idea but I only do this for output transformers or voltages over 200/250V. Interlayer voltage difference with 45V total should be very low. To put this in perspective you see bobins in high power electric motors rated for 220VAC winded ramdomly without any winding insulation (except the varnish of the original magnet wire), but they do varnish the whole stator when bobins are finished.

On magnet wire varnish you will find different qualyties and materials though, and also doubled varnished types. But for our type of work I find the regular single varnish type good enough, at least on what I can get arround here.

If you care to varnish your finnished chokes buy a galon of air drying electrical varnish and dip the whole assembly on it. A good idea is to heat the chocke first, hanging from a piece of wire over an electrical heater to something arround 50°C or more, this will dry most moisture and will help the varnish to penetrate the winding.

Air Gap:
The way to create an air gap with EI laminations is simple, just put all the "E" from one side of the bobin and all the "I" to the other side in order to close the magnetic path.

To create the gap you just need to put a nonmagnetic separator between the E and the I. Doing this you will be creating a double gap since you are sepatating the central core (full magnetic path) plus both 1/2 outer magnetic path.

So, if your able to calculate a certain gap you should divide by 2 to get the correct physical amount of the gap for EI chockes.

The problem you will have is to hold the magnetic structure. The esay way is to use a frame as you can see on commercial products, but on big and nonstandard pieces such as you are doing this will be something you will have to work on your own. Not difficult though.

I'll show a pic of a gapped choke, for a LP filter, I did reacently. this was for testing purposes but I still have it like this.
 

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