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Coaxial LC design for pulse compression using a moving short

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Shrad

Tue Mar 10 2015, 07:57AM

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

@Signification

this is a free tool made available by linear technology, with an extensive library of their switching and analog devices and a good support of general SPICE language which beats anything for the price

I'd like to simulate a decrease of L proportional to the displacement of the projectile, but I still lack some knowledge in coilguns as DerAlbi said

@DerAlbi

I'm not struggling with the most basic things like you say, I know my basics like any guy who has done an EE/System graduation and has done electronics since his young ages, so I guess the problem comes mostly from cultural/linguistic issues in passing ideas

I agree and thank you to point me to those sites as I admit I have not searched those routes yet, so I guess the read will be interesting

I hope that as the projectile shorts turn after turn of the inductor the current slope would get more linear and the amps per turns would keep a better acceleration than with a fixed inductor, but I still cannot consolidate my reasoning

Signification

Tue Mar 10 2015, 01:51PM

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

Thanks (so that's what LT stands for). I will look into this. I forgot to mention that the frequency ALSO changes with R. I have not gone far into this yet but remember taking the advantage of the fact that in a coil, it's R is approximately equal to its H. I first had to determine, somewhat as you, how the reactance of the inductor, XL, dynamically changes with projectile position.

Shrad

Tue Mar 10 2015, 02:49PM

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

XL=1/2pifL if I recall well enough

I assume that R is the main factor for keeping the amps per turn constant with a moving short

I'll read about practical projectile velocities and think a bit

Signification

Tue Mar 10 2015, 03:04PM

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

Shrad wrote ...

XL=1/2pifL if I recall well enough

I think it is actually XL=2pifL = wL since an inductor fights higher frequencies, and Xc=1/2pifC = 1/wC, since Xc decreases with frequency.

Shrad

Tue Mar 10 2015, 07:51PM

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

you're right, I inverted the two formulas

Signification

Tue Mar 10 2015, 08:30PM

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

I think there is something that will help provide answers we both need--If you know, or another member, know please post it--I will keep searching. It is what--I guess would be called--"INSTANTANEOUS REACTANCE" of a coil. L. I remember wondering about this in physics years ago--but now, during coilgun research this knowledge would be vital. When dealing with inductive reactance (XL=wL) and capaitance reactance (Xc=1/wC) all conditions I have seen involve persisting sine waves such as i(t)=sin(wt) continuously over time. However, what is the reactance when we are dealing with just the first quarter of a sine pulse?? As is the case in coilguns. The college physics textbook formulas only give this reactance for a steady continuous sine wave. Since I am looking for the wL of an inductor as just a fraction (0 to pi/2) of the current waveform goes through it, I would guess it has something to do with the instantaneous rate of change of current (di/dt) along the pulse. So, I guess the answer wouldn't be just a constant number of ohms. Perhaps, since XL=wl and I=V/R, then the answer is proportional to: R=V/Iwt??

DerAlbi

Tue Mar 10 2015, 08:53PM

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

What you search is called fourier transformation. A quater sine pulse is not possible without involving higher frequencies.

In general its not very descriptive at all to think about a Coil or Capacitor as it is a imaginary resistance. The only thing that really discribes its behavior is the components differential equation.
But you should know that since you elaborated about that just some posts ago.
Thx by the way, i am indeed a inexperiences english user, i hope i can keep up with andy at least

Signification

Tue Mar 10 2015, 09:33PM

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

Pretty much -nothing- other than a pure sine wave is mathematically describable without a Fourier description. Which is the basis of the guess I made of R=V/Iwt. I think for a certain squarewave it was R=16V/piwt or something. My next step was to review "impulse response" in my old Fourier Analysis book. I am not concerned with imaginary resistance, just another way to look at the behavior due to this coil current pulse.

Shrad

Tue Mar 10 2015, 09:40PM

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

in LTSpice you can right click on a graph an show the FFT

for the LC circuit above without a switch to limit the discharge time to the first positive half wave the FFT showed a nice peak at fres

when critically damped like this, I assume there would be another peak related to the EMI generated (as would tell the negative megavolt peak in the simulation)

DerAlbi

Wed Mar 11 2015, 12:10AM

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

X=V/(Iwt) only applies to sine wave.
on an Inductor the only thing that counts is dI=(U/L)*dt.
Or in you notation style dI = U/Ldt. hahaha

as said before, differential equations is the only thing that is important to Coilguns. there is no way to get around calculus. Coils arent some resistance and inductances doesnt change in presence of a projectile... its motor theory. Please get rid of your thought with Reactance stuff. it will help no understanding.

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