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Coaxial LC design for pulse compression using a moving short

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DerAlbi

Thu Mar 12 2015, 10:11AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

...have fun. its a shame you dont want this solution..

]justignoretheunknown.zip[/file]

Shrad

Thu Mar 12 2015, 10:41AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

we don't even know the actual differential equation you are talking about, and you did not provide information which would drive some search for someone who doesn't know

I find it logical that I want to approximate first, dig math later

if you know the business that much it wouldn't pose a problem to explain why the complex math is needed for what appears to be a simple problem to novice eyes

to my eyes, it appears as a voltage which decreases over time to a value of zero, and I don't really care of the rules which determine the discharge curve... the fact that the current has a maximum at a certain value and reaches zero seems to be satisfactory enough, whatever the imaginary and real parts or differential equation

if the complex math is needed to simulate the decrease of L to zero over the span of time the current and voltage reach zero, please explain, or I see no goal of saying over and over that the solution relies on complex equations that we are not aware of

if you're not open to explaining this from start to a point where we are able to find things, then please let us progress at our own rate and don't complain

Uspring

Thu Mar 12 2015, 11:29AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

DerAlbi wrote:

..have fun. its a shame you dont want this solution..

justignoretheunknown.zip

AFAIK this is incorrect. For time varying inductances you have

V = L * dI/dt + I * dL/dt

DerAlbi

Thu Mar 12 2015, 12:47PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Uspring: AFAIK you are correct

I didnt want to make it more complicated and easier to understand. Small formula is better than a big one and considering the Simulation there is not so much difference since he got 50R in series. Also the inductance doesnt change so much, as long as the motor is moving..
Your formula still lacks the term for BackEMF which should have a bigger influence. (V = -F*v/I)

Shrad: the equations were mentioned multiple times within this threads alone (last by Signification), in wikipedia about indurctances and i really dont see how one can actually miss them if one doesnt ignore them on purpose.
There is no way to explain the necessity of math or equations besides the fact that this is the way how the system is understood and described. I keep telling this over and over and stay ignored just to be blamed that i dont explain them... wtf. The real problem here is that you dont want to hear about how complicated stuff is. You want to live in an easy world with rainbows everywhere and where you find a breaking new concept which all the bright people out there didnt find for no reason.
If you "don't really care of the rules which determine the discharge curve" then take a piece of paper and daw your current curve as you want it to be, i am sure physics will bend to your will. they you concept will work for sure and.. heeey next goal is to extract vacuum energy, right?

Signification

Thu Mar 12 2015, 12:59PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I can't seem to open DerAlbi's zip file for some reason! From Uspring's statement it looks like DerAlbi may be treating L as a constant thus still missing our point of view???

I have not had time to think about it yet, so the following is a partial start:

@Shrad,
Let Lf be final inductance (=zero), Li, initial inductance (max inductance). Similarly, let Nf final 'number of effective turns (zero), and Ni, initial number of turns all turns). Now looking at the following version of the equation for the inductance of a single layer inductor (solenoid):

L=Uo*pi*(r^2)*(N/L)*N,

where r=radius of inductor, N/L=turns per unit length, N = number of (effective) turns, we see here that the only variable for L as the projectile moves down the barrel is N. Thus we have L as a function of N or L=f(N) We can simply write this as a liner equation with initial and final coordinate pairs: (Ni,Li) and (Nf,Lf) with slope=(Lf-Li)/(NF-Ni).

Next, for the N variable changing as a function of time, a 'velocity' may be represented as v(t)=dN/dt; with initial condition N(0)=Ni. And for L: v(t)=dL/dt; initial condition: L(0)=Li.

This is my first thought written live. Before continuing, and dealing with time (t), current (I), etc, I have some ideas, but would rather further review the entire thread (I think there is another one). I admit I have not read it 'at least twice', and will further try to retrieve DerAlbi's zip file.

DerAlbi

Thu Mar 12 2015, 02:30PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Try again... then the raw ASC-File... I did not miss the point, i just simplified it for the first impression. It does not take into acount that the field energy is preserved while inductance changes. It seems useless to be correct anyways. todays motto seems to be "if theres a peak and a zero crossing, its enough".

]renamefileextensionto_asc.zip[/file]

1426170457 2906 FT169480 Not Missingthepoint

But thanks for the doubt.

Signification

Thu Mar 12 2015, 04:17PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

DerAlbi wrote ...
. It seems useless to be correct anyways. todays motto seems to be "if theres a peak and a zero crossing, its enough".

NO WAY!!

99% of the time, people really interested will listen if you present it in a proper manner--ask (don't tell them) what they need, likewise listen to the reply, and then work up (together) from there.

Your information has been very helpful, but 'picked out' of your negative responses--you don't leave options to receive questions.

Shrad

Thu Mar 12 2015, 06:59PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

@DerAlbi

I do not deny that there is AC and that it influences the behavior, and I do not deny that you can calculate the complex math... I know that there is a trigonometric relation between I and U and that one doesn't go without the other

simply, I have not had extensive math during my studies and that was nearly a decade ago, so I have forgotten almost all the sine and cosine things as well as integer and imaginary terms

if you deliver the complete equations at first I simply cannot understand them and my brain just refuses to connect

I'm totally receptive to successive approximation and my brain will accept to iterate through the increasing complexity to a certain level

what Signification explains for reducing the differential relation to a simple function of the turn ratio is just perfect, it explains how to approximate and why to approximate that way, and it lets me understand better

now, I can apprehend the problem a bit deeper and will surely meet another aspect of the problem which will require to get a step further into math

btw I shouldn't have expressed that I wanted to modify the discharge current curve to my needs, but rather that the moving short would have improved the acceleration

and I know for sure that it has been done before in hypervelocity experiments these last decades

for what it's worth, my last response doesn't seem to have been posted, and I was thanking you for the .asc file which made things clearer

Uspring

Thu Mar 12 2015, 07:29PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

DerAlbi wrote:

Your formula still lacks the term for BackEMF which should have a bigger influence.

I have some reservations about that statement. Think of a superconducting loop of wire carrying a current and a piece of steel near it. When the steel is sucked into the loop, its inductance will increase but the flux inside the loop won't change since there is no voltage around the loop. The flux is given by L*I. Since L increases, I will decrease by the same factor. The energy, being 0.5*L*IÂ², will decrease. The difference might be just the energy, that is spent sucking the steel into the loop. Back EMF is, as you write V = -F*v/I, is also a statement of energy conservation.
This isn't a rigorous proof and might be faulty. Just a thought.

DerAlbi

Thu Mar 12 2015, 11:28PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

If thats just a thought.... its a good one. you might be right, that just one of both "attachments" are at work. they could both be the same. havent checked that.

that would mean:
I * dL/dt = F*v/I
dL = integral(F*v/IÂ²) since F ~ IÂ² that would give dL = integeal(someFactor*v) sice the higher v makes shorter integration time dL is a constant within 2 integration-bounds. Yep. makes sense. cool! thats actually new to me. I knew both attachments to the formula, but i discarded one of both depending on the simulation i run (mechanical vs electrical).

That makes the most powerfull coil shape the shape that changes the inductance most in presense of the projectile. Which would be a single layer coil (best coupling factor) which has a bad L/R... aaawww i am sensing a true optimum there

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