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Old 15th March 2011, 01:36 PM   #11341
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Originally Posted by jlsem View Post
How is this possible? Eddy current formations themselves are independent of frequency and although eddy current losses are proportional to the square of the frequency due to inductive heating of the core, I don't see how the "bucking" current could cancel primary induction current to the degree we see losses of permeability at high frequencies. I believe the decrease in permeability is due more to domain switching issues.

John
Eddy currents are proportional to the rate of change of flux. So higher frequency will produce higher loop voltages, hence higher loop currents, and resultant higher bucking flux.

Domain switching may be fine for magnetic material, but not copper, aluminum, brass.. The proximity of conductive materials to the time varying field of an inductive storage device tends to exclude the magnetic energy. The result is a lowering of the system inductance. Inductance is defined via an equation which relates the total amount of energy stored as a result of a current. Lenz effects reduce the total energy stored, therefore lowers inductance.. That will appear to be a change in the permeability of the flux path.

Cheers, John
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Old 15th March 2011, 01:42 PM   #11342
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Unfortunately, transformers are known to have problems similar to tape heads, and this is WHY they are LAMINATED in the first place, and Ferrite cores are used at virtually everything above the audio bandwidth. Too much eddy current loss with mumetal or steel.

The method of how laminations are produced is interesting, but not necessary to know implicitly, in order to select a transformer or a tape head, but the difficulty of making THIN laminations implies WHY some transformer manufacturers would go for thicker laminations, just to save time and trouble. Of course, at line level, the difference between thicker and thinner lams may be almost irrelevant, but at MC levels, I consider it important.

Now I, at this time, have elected to forgo using a transformer in my all-out design. The transformers that I have been exposed to, though recommendation, catalog, or direct measurement, all concern me with various problems. However, this does not say that a transformer is not useful for another design, especially one that NEEDS it.
You see, without a transformer, I have a BALANCED INPUT and a self noise of 10-20 ohms. Most here would start with an UNBALANCED INPUT and a self noise of 60 ohms or more, usually much more. Here, an input transformer can give you a balanced input, and lower the equivalent noise to 5 ohms or so. Given eddy current losses, DC winding losses, etc, I would not expect much better than this, EXCEPT for special transformers made for ribbon microphones, for example, that have their own 'tradeoffs'.

Some of the other features that transformers offer such a bandwidth limiting and galvanic isolation is rather pointless in my design, but it could be helpful in others.
The original reason for me to concern myself with low noise design, starting 43 years ago, when Ortofon showed me the door, after I mentioned that I wanted to build a solid state replacement for the transformer (they thought I was some sort of a nut), and how Ortofon adapted my patented design (without paying me of course) 10 years later, to REPLACE their input transformer, shows that transformers are difficult and expensive to do right, and should be avoided if possible, with low noise solid state electronics.
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Old 15th March 2011, 02:03 PM   #11343
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Originally Posted by simon7000 View Post
Good practice is to remove the sharp edges with a bath of vibrating abrasive material. It also should be dulled when the sides are "pickled" in an acid bath to reduce the surface conductivity to reduce the eddy currents.

In a proper stamping the punch actually only goes 1/2 way or less through the material before it pushes out. If you look at any punched metal piece on the side (magnification may be required) you should see part of the edge is smooth followed by roughness. The smooth is where the punch pushed. The rough is the breakaway!

When you have sharp edges remaining it can mean the die is worn and has too much clearance. If it is sheared too much of a gap between the knife and table will leave a burr.

Basically an edge means worn tooling or just poor quality.

Also the varnish used is not as an insulator but to reduce movement and mechanical noise.
My goodness..what a production..

1. Buy the material in sheets a foot square.
2. Apply the bonding film to both sides of the sheets (or buy the sheets coated).
3. Stack the sheets to the desired thickness using a fixture which applies a force normal to the sheets. Either use a weight and gravity, or a flat plate with lots of bolts and bellvilles. Preload to the proper force.
4. Cure the stack in a furnace with the proper time/temp profile.
5. Send the finished stacks to a wire EDM facility with the core profile.
6. Debur the edges where desired.

Wire EDM is easily capable of 50 micron accuracy. 25 is doable, but it may cost a bit more.

With half mm lams, this will get you 98-99% packing factor. 1 mil lams, maybe 60 to 80% depending on film thickness.

See...problem solved...simple.. (I should be on the "Daily Show"..)

Cheers, John
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Old 15th March 2011, 02:11 PM   #11344
DF96 is online now DF96  England
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Core noise temperature is almost certainly the same as the core physical temperature.

Calculating noise for a particular frequency is no problem, provided you have resistance or loss measurements for that frequency. A frequency dependent resistance is still a resistance, so at a particular frequency it produces thermal noise which can be calculated from its resistance and temperature.

JC is not making things up. The paper he referred to shows this. It is unfortunate that poor nomenclature and, at times, weak grasp of physics has made this debate much longer and harder than it need have been. Frayed tempers and SHOUTING have not helped either.
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Old 15th March 2011, 02:21 PM   #11345
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Quote:
Originally Posted by DF96 View Post
Core noise temperature is almost certainly the same as the core physical temperature.

Calculating noise for a particular frequency is no problem, provided you have resistance or loss measurements for that frequency. A frequency dependent resistance is still a resistance, so at a particular frequency it produces thermal noise which can be calculated from its resistance and temperature.

JC is not making things up. The paper he referred to shows this. It is unfortunate that poor nomenclature and, at times, weak grasp of physics has made this debate much longer and harder than it need have been. Frayed tempers and SHOUTING have not helped either.

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Last edited by mrfeedback; 15th March 2011 at 02:25 PM.
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Old 15th March 2011, 02:25 PM   #11346
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Originally Posted by DF96 View Post
Core noise temperature is almost certainly the same as the core physical temperature.

Calculating noise for a particular frequency is no problem, provided you have resistance or loss measurements for that frequency. A frequency dependent resistance is still a resistance, so at a particular frequency it produces thermal noise which can be calculated from its resistance and temperature.
Agreed for the most part.

It's the noise proportional to frequency part that is the interesting part... One can use a temperature term which is proportional to frequency (which I don't like), or transfer the losses to the coil resistance and have a frequency dependent resistance term..which I like better..

I believe using a freq dependent resistance would be mathematically easier, and I believe the actual core physical temperature can be used to proportionally increase the resistive term.

However, as I said (in a roundabout way), I am far too challenged to work the math required for an analytical approach to calculating the actual noise of a part exhibiting frequency dependent eddy losses.

Quote:
Originally Posted by DF96 View Post
JC is not making things up. The paper he referred to shows this. It is unfortunate that poor nomenclature and, at times, weak grasp of physics has made this debate much longer and harder than it need have been. Frayed tempers and SHOUTING have not helped either.
I've not said he is making things up. I've claimed that he made a statement/correlation between lam thickness and resultant noise with no physics justification....so was ignored for the most part. I understood the mechanism, so was NOT going to allow JC's assertion to die away. He was correct.

Frayed tempers and shouting?? You've been nothing but pleasant, I've not shouted nor been angry...did I miss something?

Cheers, John
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Old 15th March 2011, 02:29 PM   #11347
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Originally Posted by gpapag View Post
This means that the metal strip has to be treated for electrical isolation application after stamping and abrasive bathing. I was under the impression that laminates were purchased with the insulation already applied.

Is this acid pickling the electrical isolation application I wrote above?
If yes, do you know what acid is used for silicon iron laminates?


Yes, I have looked with a X10 loop.


I thought so too.
When it comes to very small thicknesses, how easy it is to see these defects?
Do the small signal transformer manufacturers test for such electrical shorts between laminations? (Large transformers can be checked by monitoring the excitation current not exceeding a certain upper limit)


OK. But it comes handy as such (isolator) sometimes, No?

Regards
George
Makes sense to pickle before stamping, easier to handle. But even with my sheet metal toys I have never made a transformer.

Acid pickling is the insulation techniques as mention in my college days, never tried it myself or seen it done, so I have to say "I don't know." (These words may cause some on this thread to pass out!)

One varnishing technique I am familiar with involves putting a load of transformers in a vacuum chamber, removing most of the air and then letting in the varnish. That gets it into every nook and crany but does not guarantee it will hit every possible short. I do not know if anyone sprays the material before stacking.

Jacco,

Maximum punch life I am told is when the shear zone is 40-50% of the thickness,


John,

A turret press is used for quantities of 1000 or less. Wire EDM probably 50 or less. A special die in a commodity press is used for greater numbers. That is based on costs. Of course you have a bigger budget!

If you look at an E-I core you punch two "E"s and two "I"s at the same time and get almost no waste! That was worked out long before CNC (Computer Numerical Control) machines existed.

Last in general the normal clearance you require is that the die hole should be larger than the punch by 10-15% of the thickness of the material being punched. In a turret press the punch and die rotate in a giant massive C frame. The accuracy is better than .002 inches for alignment. (Bearings, machining tolerances even thermal expansion play a role in this.)So that is why .032" thick sheet metal is the commodity item .014" is doable. But any thinner just gets smeared! Of course if you wanted to punch thinner stuffer a smaller lighter and more accurate press could be made.

ES

Last edited by simon7000; 15th March 2011 at 02:32 PM.
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Old 15th March 2011, 02:49 PM   #11348
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Originally Posted by simon7000 View Post
John,

A turret press is used for quantities of 1000 or less. Wire EDM probably 50 or less. A special die in a commodity press is used for greater numbers. That is based on costs. Of course you have a bigger budget!
Ya think????

A typical lamination is 1mm thick and a meter by a meter. Salient features have to be in the 25 micron accuracy range. Stacking a two meter long final "device" prior to cure and keeping a 15 micron lam to lam alignment is VERY difficult.

One way is to use a two step stamping, with the required features locked at pass 2. Another is to cure lams together after one stamp, then wire EDM the final features.

I specified wire EDM because it's easy enough to send out to a vendor, it relieves us the issue of actually trying to machine to those tolerances, and the machining cost is easily determined (by quote).

If I were in the business, I'd have in stock various overall thickness cured laminate sheets with half or 1 mil thick starting foil, and I'd send these cured sheets out based on custom work orders.

It'd be cost effective for small quantity/fast turnaround..

Cheers, John

Last edited by jneutron; 15th March 2011 at 03:01 PM.
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Old 15th March 2011, 03:06 PM   #11349
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Originally Posted by jneutron View Post
Eddy currents are proportional to the rate of change of flux. So higher frequency will produce higher loop voltages, hence higher loop currents, and resultant higher bucking flux.

Cheers, John
In theory, the opposing current of the eddy currents mainly prevents the flux from penetrating the core material completely resulting in uneven flux distribution. This effect is almost completely ameliorated by using thin laminations in combination, to a lesser effect, with using high resistivity laminations. I'm sure you are aware of all of this, but my point is that in high quality audio cores hysteresis loss swamps eddy current losses and does more in contributing to a tenfold decrease in permeability when increasing frequency from 400Hz to 20kHz.

John
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Old 15th March 2011, 03:16 PM   #11350
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Originally Posted by jlsem View Post
In theory, the opposing current of the eddy currents mainly prevents the flux from penetrating the core material completely resulting in uneven flux distribution. This effect is almost completely ameliorated by using thin laminations in combination, to a lesser effect, with using high resistivity laminations. I'm sure you are aware of all of this,
Yes. And I'm confident that everybody contributing to this discussion is also. The thread is just too long to review now...
Quote:
Originally Posted by jlsem View Post
but my point is that in high quality audio cores hysteresis loss swamps eddy current losses and does more in contributing to a tenfold decrease in permeability when increasing frequency from 400Hz to 20kHz.

John
I would expect hysteresis losses more as an amplitude based loss mechanism, not so much a frequency based one. For the application JC initially spoke, I'd still believe eddies will dominate. Selecting a thin lam certainly will reduce eddy losses, so I'd expect hysteresis to dominate at lf. I'd have to sweep-measure Ls-Rs to see just how big the eddy loss is.

Cheers, John
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