Re: 3 Coil System Was: A photographic tutorial of PancakeCoilwinding...with movies...(fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 24 Jun 2007 13:23:45 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: 3 Coil System      Was: A photographic tutorial of
    PancakeCoilwinding...with movies...(fwd)

Hi Dave,

Tesla list wrote:

>---------- Forwarded message ----------
>Date: Sun, 24 Jun 2007 12:16:20 -0500
>From: David Thomson <dwt@xxxxxxxxxxxx>
>To: 'Tesla list' <tesla@xxxxxxxxxx>
>Subject: RE: 3 Coil System      Was: A photographic tutorial of
>    PancakeCoilwinding...with movies...(fwd)
>
>Hi Bart,
>
>  
>
>>Flat Coil = 13.625" outer diameter, 24 awg, 2" inner 
>>diameter, 252 turns, 
>>    
>>
>
>Did you count the number of turns, or did you calculate them?  If your coil
>is tight wound, there should be about 283 turns at 98% winding efficiency.
>But this is irrelevant.
>  
>
I've counted, measured, and calc'd (44.4 turns per inch). You'll never 
get 283 turns of 24 awg magnet wire in an area (radius of 5.8125") 
unless you have bare wire? Typical magnet wire bare (bwd) is .0201" and 
1 build insulation thickness (wit) is .0012". So the total wire diameter 
(twd) is:

twd = bwd+wit+wit = .0201+.0012+.0012 = .0225" (not including 
variation). I've even measured this wire both bare and with insulation. 
It's right on the money. But not, there are some heavy build 
insulation's out there which are thicker.

6.8125/.0225 = 258 turns. Consider now a little allowance for variation. 
It's only irrelevant if you count the turns, but for anyone calculating 
turns, it is important to do it right.

>I just tested a 13" outer diameter, .25" inner diameter coil at one inch
>from the center and there is a 10 degree to 15 degree angle of flux there.
>
Sounds good.

>I just tested an 11" outer diameter, 4" inner diameter coil at one inch from
>the center and it too has the same angle of flux.
>
>So at 1" from the center of the coil, you do have a weak inductive coupling.
>  
>
Absolutely. There is no doubt that a flat coil and a helical coil have a 
most minimized area of inductive coupling. This is well known and 
understood. For all geometry, the proximity of one coil to another coil 
and their geometry to one another determine their magnetic influence. 
There is no dispute that a flat coil coupled to a solenoid (90 degree) 
will have a weak inductive coupling due to the geometry of the magnetic 
field lines, but we cannot say that it doesn't have inductive coupling 
due to the 90 degree geometry.

For my coil, if I were to raise it 7" above the flat coil, I still have 
inductive coupling, but it gets really small. Mutual inductance is down 
to 38uH with a coupling factor of 0.011. But it still exist. Another 
fact is the maximum coupling for these two coils. That occurs if the 
flat coil were placed in the center of the solenoid. This results in a 
mutual inductance of 581uH with a coupling ratio of 0.172. That is the 
maximum obtainable coupling between these two coils.

>>I then removed the solenoid coil and placed it 1 meter away 
>>from the flat coil. I kept it hooked up electrically the 
>>same. I measured 0 volts across the solenoid with the same 
>>current flow. 
>>    
>>
>
>At what distance does the inductive coupling disappear?
>
Inductive coupling for this coil becomes unmeasureable if the solenoid 
where raised above the flat coil 14" to 15" (measured at base to flat 
coil). Quite a distance, isn't it! Yet, it's still a weak field. This is 
why flat primary coils on helical solenoids for two coil systems is so 
wonderful. It allows plenty of coupling to do the task but also allows a 
soft adjustment where moving the primary position vertically (or 
secondary) makes small increments in coupling helping to fine tune the 
coils energy transfer.

>I would expect the
>inductive coupling would disappear just a few inches above the spiral plane.
>My solenoid coil was mounted about 2.5" above the flat spiral plane,
>specifically to minimize the inductive coupling.  Although, I didn't
>actually check to see that inductive coupling was completely eliminated.  
>  
>
If only 2.5" above, I'm can tell you with complete confidence they were 
inductively coupled. There is no doubt about it. But know this, if 
coupling is low enough, the time of energy transfer is lengthened. The 
time required to transfer the energy from the flat coil to the solenoid 
can seem like an eternity. Spark formation will cease to exist as the 
energy over time factor is too long.

Just like a flat coil, a 2 coil system can be coupled so high that 
voltage gradients along the coil become high enough to cause racing 
sparks. This is very easy to do with a flat coil (and hard to get rid 
of). Flat coils can often produce sparks from the center of the coil to 
the outer edge of the coil along the surface of the winding. It's just 
the nature of the small area, the voltage gradients along the inner to 
outer length, and the non-linear inductance (high in the outer area and 
low in the inner area). On 2 coil systems, we can increase the proximity 
of the secondary until there are no sparks at all. But, we are making 
sparks. So, we find through experimentation, a typical 2 coil system 
runs well without racing sparks at a coupling of about 0.13 or in that 
vicinity. Each coil is a little different how it reacts to voltage 
gradients as they are affected by the coupling, cap voltage, toploads, 
coil length, coil inductance, and resonant frequency.

>I just now tested the 25.25" outer diameter, .25" inner diameter flat spiral
>coil.  Due to its size, the angle of flux at .7" from the center is
>practically zero.  Add to this the fact the solenoid was raised 2.5" off the
>plane of the flat spiral and there is practically no inductive coupling
>between them.  Unfortunately, I no longer have the tall solenoid coil used
>in the experiment to measure the exact coupling.  But I will keep this in
>mind when I build my next coil.
>  
>
You should. If you ever need help on how to measure inductive coupling, 
please ask. It's a very easy measurement. BTW, when I was taking 
pictures of the measurements I made, I left the current flowing for a 
few minutes. The flat coil got pretty warm as I was passing 120 Vac 
across the windings with a constant current ballast to limit current. 
The current was dropping as the copper was heating up and increasing the 
resistance of the copper (a few hundred milliamps).

The coil above did have inductive coupling at only 2.5" above the flat 
spiral. The mutual inductance and coupling values in Javatc are probably 
the most accurate number Javatc will give. You can use Javatc to find 
out those numbers. You can also use Paul's ACMI program or Mark R's 
MANDK program. As a matter of fact, MANDK will tell you the coupling at 
various heights. This might be something you want to look at.

>>The definition of "inductive coupling" = "the transfer of 
>>energy from one circuit component to another through their 
>>shared magnetic field (mutual inductance)". Is your 
>>definition of inductive coupling something other than this?
>>    
>>
>
>We share the same definition of inductive coupling.  We just aren't putting
>our coils in the same configurations when taking measurements.
>  
>

I think we have made some progress to understand views of inductive 
coupling. Good discussion.

Take care,
Bart