Tesla's Pancake Coil Patent....a closer look

[ion]

Attached is the Tesla patent for coils that are claimed to neutralize self inductance.

I'm sure this subject has been beaten to death on all FE forums but a critical look at the patent and it's claims would be a good thing, in an attempt to find the "magic" properties if they exist of such a coil that are often assigned to it.

I don't claim to know a lot about these type coils, a I have only very briefly experimented with them a long time ago. Maybe we can discover something by going back and taking a closer look.

Starting with the main two claims at the end of the patent, we will later try to dissect the text of the patent.

Neutralizing self inductance. What does that mean?

verb (used with object), neutralized, neutralizing.
1.
to make neutral; cause to undergo neutralization.
2.
to make (something) ineffective; counteract; nullify:

As I understand it, neutralizing self-inductance would leave just normal inductance i.e. the inductance of each of the turns added together without the squaring factor of self inductance.

Since neutralizing self inductance is the gist of the claims, this should be easy to test.

We need to take two equal lengths of wire, wind one length as a spiral single wire in the usual manner according to Fig 1.

Wind the other in the spiral layer bifilar wound manner as described in Fig 2 of the patent.

Compare the two coils for inductance. One, the non-bifilar should follow the number of turns squared law (n^2) for self inductance of a spiral coil.

http://www.deepfriedneon.com/tesla_f_calcspiral.html

The pancake bifilar version of the same length of wire should follow the linear law, just the straight number of turns relating to inductance. So it should have a lot less inductance than the Fig 1 version with self inductance.

You can use the same formula, just leave off the squaring term.

Has anyone attempted this? Seems like it would be a good first step in determining the properties of the coil.

One could also compare the prior two winding versions with a straight solenoid winding.

I suppose the next test would be to ring the coils with white noise and examine the spectral response in order to determine self resonance modes. I lack the equipment to do this so if anyone is interested, please do.

[Smudge]

As I understand it, neutralizing self-inductance would leave just normal inductance i.e. the inductance of each of the turns added together without the squaring factor of self inductance.

That is not what I would understand.  Self inductance is simply what we normally call inductance and is the inductance of a single coil.  The word self differentiates it from mutual inductance which involves more than one coil.  So my reading is simply that the special coil arrangement is designed to have some self capacitance that neutralizes its inductance at a certain frequency, i.e. it is then in resonance with its self capacitance.  But I could be wrong

Smudge

[Vasik041]

I also understand in way that such coil have high self capacitance

and neutralization means  neutralization of reactive component of impedance (1/wC = wL)


PS see also https://en.wikipedia.org/wiki/Loading_coil

[Verpies]

How about looking at it as a bunch of turns connected in series.  On turn also constitutes a coil.
There is mutual-inductance between the coils(turns) which alters the self-inductance of the whole.

If all of the turns(coils) are wound in the same direction then their mutual-inductance tends to compound and yield a higher self-inductance.
If half of the turns(coils) are wound in opposite directions then their mutual-inductance tends to cancel and yield a lower self-inductance.  In an ideal case, approaching zero.

In a bifilar coil of the 1^st type - all of the turns are wound in the same direction (a.k.a. bifilar Litz coil).
In a bifilar coil of the 2^nd type - all of the turns are wound in the same direction, but the voltage (potential between adjacent turns) is maximized and so is their effective mutual capacitance.
In a bifilar coil of the 3^rd type - half of the turns are wound in the opposite direction to the other half.

Notwithstanding the opposite signs of the inductive reactance and capacitive reactance and their cancellation at resonance.
Mutual capacitance is a factor in coil design, too, as depicted by Partzman.

[Verpies]

and neutralization means  neutralization of reactive component of impedance (1/wC = wL)

...or X_L + X_C = 0

[ion]

That is not what I would understand.  Self inductance is simply what we normally call inductance and is the inductance of a single coil.  The word self differentiates it from mutual inductance which involves more than one coil.  So my reading is simply that the special coil arrangement is designed to have some self capacitance that neutralizes its inductance at a certain frequency, i.e. it is then in resonance with its self capacitance.  But I could be wrong

Smudge

I'm not intimately familiar with all the Tesla writings. I would guess if used the word "mutual" inductance in this or other patents, we could then differentiate "mutual"  from "self". The only thing he mentions is the "mutual relations" Sec 1 par 45.
I see him talking about self inductance and inherent capacitance as two different but related
quantities.

from verpies:

If all of the turns(coils) are wound in the same direction then their mutual-inductance tends to compound and yield a higher self-inductance.
If half of the turns(coils) are wound in opposite directions then their mutual-inductance tends to cancel and yield a lower self-inductance.  In an ideal case, approaching zero.

In Fig 2 we see that the coils are wound to aid, not oppose induction, else he would have connected the innermost wires together to cancel induction.
What he claims to be dong is to create a larger difference in the volts per turn between the two bifilar pair, to force the capacitance to store more energy.

In the 1953 edition of the "Radiotron Designers Handbook" there is no mention of "self inductance" anywhere in the book, not even the section on "Inductors in AC Circuits". They only talk of "mutual inductance". Surely the radio designers knew about capacitance between turns in an inductor. I took self inductance to be an early term used by Tesla and others which was later replaced  by "mutual inductance", perhaps my mistake.

If I have made a semantic error, no need to go further, we can scrap the thread, but it would be good to nail down the history and meaning of the terms for future reference.

Regards

[TinselKoala]

Attached is the Tesla patent for coils that are claimed to neutralize self inductance.

I'm sure this subject has been beaten to death on all FE forums but a critical look at the patent and it's claims would be a good thing, in an attempt to find the "magic" properties if they exist of such a coil that are often assigned to it.

I don't claim to know a lot about these type coils, a I have only very briefly experimented with them a long time ago. Maybe we can discover something by going back and taking a closer look.

Starting with the main two claims at the end of the patent, we will later try to dissect the text of the patent.

Neutralizing self inductance. What does that mean?

As I understand it, neutralizing self-inductance would leave just normal inductance i.e. the inductance of each of the turns added together without the squaring factor of self inductance.

Since neutralizing self inductance is the gist of the claims, this should be easy to test.

We need to take two equal lengths of wire, wind one length as a spiral single wire in the usual manner according to Fig 1.

Wind the other in the spiral layer bifilar wound manner as described in Fig 2 of the patent.

Compare the two coils for inductance. One, the non-bifilar should follow the number of turns squared law (n^2) for self inductance of a spiral coil.

http://www.deepfriedneon.com/tesla_f_calcspiral.html

The pancake bifilar version of the same length of wire should follow the linear law, just the straight number of turns relating to inductance. So it should have a lot less inductance than the Fig 1 version with self inductance.

You can use the same formula, just leave off the squaring term.

Has anyone attempted this? Seems like it would be a good first step in determining the properties of the coil.

Yes, this has been done several times. For example I have three such matching coils with very close to the same amount of wire, one wound monofilar and two wound and connected in the Tesla Bifilar manner according to the patent.
Using my ProsKit MT-5210 LCR meter, the monofilar coil measures 712 microH (118 turns of #27) and the two bifilar coils measure 685 uH (118, 59+59)  and 661 uH (116, 58+58). The monofilar coil is probably tighter than the two bifilar coils and may actually have slightly longer wire as a result.

I also have two solenoidal coils of #34 magnet wire, one monofilar and one TBF, and they measure 316 uH and 322 uH respectively. They don't have the same amount of wire as the pancake coils though.

So I'm saying that there is no significant difference in inductances between the TBF winding and the monofilar winding in my pancake or solenoidal coils. YMMV.

One could also compare the prior two winding versions with a straight solenoid winding.

I suppose the next test would be to ring the coils with white noise and examine the spectral response in order to determine self resonance modes. I lack the equipment to do this so if anyone is interested, please do.

I wish I had a Spectrum Analyzer.  But it is easy enough to find the self resonances of these coils using a scope and a fast risetime square wave.

[TinselKoala]

As far as the inductive and capacitive reactances cancelling... this does not mean that those parameters disappear at resonance! Consider two 150 kg (fat) kids on a see-saw. Just as when the see-saw is empty, the loaded see-saw balances. But this doesn't mean that the masses of the kids vanish! They cancel their effect on the balance but they do not vanish. Clearly the coil still has inductance and capacitance even though its impedance (except for its ohmic resistance) "vanishes"; this is shown by the fact that it still has alternating magnetic and electric fields which can induce alternating voltages in a nearby coil.

[forest]

This patent is Tesla riddle. The key is the title of the patent.
Do you remember Evgray motor ? The famous experiment with a coil able to throw an object high above from a spark from a battery ?

Why is the title so weird ?  What is Tesla trying to tell us ?

He is trying to tell us that in proper conditions proper  coil magnetic field depends on frequency of the input current...

That's why he said about his coil having energy 2500 times larger for the particular frequency and type of current...

For me it seems that the proper usage of this coil for electromagnet is the singel wire transmission line.. like in Evgray motors...

[TinselKoala]

Then why didn't Tesla use it for his later patents, such as #645576?

The high-tension coil A in the form of a flat spiral was composed of fifty turns of heavily-insulated cable No. 8 wound in one single layer, the turns beginning close to the primary loop and ending near its center.

http://www.youtube.com/watch?v=pZJzRBsf9f0

[Verpies]

In Fig 2 we see that the coils are wound to aid, not oppose induction, ...

Yes, I also wrote this in the description of the 2^nd type of this coil.
Half of the turns "oppose the induction" in the 3^rd type.