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Coaxial LC design for pulse compression using a moving short

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Andy

Thu Mar 05 2015, 02:59AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Ill argue sideways ;) the sparkcap can be replaced with any switch, im more constraing on the inductor value change be open and closing the secondary coils, the other parts were a cheap way to recylce some of the lost energy, depending if you close it quickly more time, but low eff, if you slowly increase the resisance high eff to a point, but quicker discharge.

The idea is pretty much the you change the L of a coil when the energy is stored in the field.

Shrad

Thu Mar 05 2015, 01:09PM

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

DerAlbi wrote ...

Skineffect or better the skindepth gives you an equivalent thicknes of the conducting layer. No matter how thick your copper is, if you have 1.5MHz the resistance of a round wire is then the same as a holow cylinder with a wall thickness of the skindepth. (for a round conductor of course.
On a PCB-ground plane for example if the frequency is high enough there is no point in calculating trance resistance based on the full 35Âµm copper.. its the skindepth that counts. Same with your 3mm wire. Its more like a 3mm cylinder with a quite thin wall.
For the wire in the coil you could counteract that by making many parallel (isloated) wires. But you dont get the copper fill that way so your resistance is at least 130% of that what you expect.- practically its maybe 200%.
But it has huge impact on your capacitor ESR. There you cant have many parallel plates in your coaxial design which makes it way more resistive.
in other word:
"outer = 15cm perimeter so 7.5cm wide and 2mm width which gives 0.0000222 Ohms"
that 2mm assumption is nothing more than a wish! use the skin depth.

I thought about designing the thing so that the discharge would be over before one period of oscillation of the equivalent LC so didn't account for AC properties, that's the reason I mentioned to create a design to shape the pulse as so the pulse duration matches the moving short travel duration

DerAlbi wrote ...

Your capacitor is calculated wrong, because the coil in beween the plates will have a potential too! The effective capacitance is then not caluclated by the distance of the 2 plates, its caluclated by the distance of one plate to the nearest inductor outline.
If you still dont understand: think about it this way: you have a conducting stuff inside the dielectric. That must do something weird to your capacitor and cant be ignored therefore.

I was thinking that my spiral coil not being between the two plates of my capacitor, it would not reduce the capacitance between the two tubes

DerAlbi wrote ...

Now lets think about the concept a little bit more: youve got 2 Joules stored and your half wafe is 0.3us. for a 20cm length coil. That makes 10cm acceleration distance (neglet the moving short, thats a bonus). That proposes a constant acceleration (if the world would be ideal

) of 1.05e6 m/sÂ² and an end velocity of 0.315m/s. LOL

if you have a 100% conversion efficiency you would launch a projectile that is as heavy as 40kg.

This is out of my reach as is, thus me wanting to discuss in order to better understand the implications and flaws of the idea

DerAlbi wrote ...

Of course thats only with the most perfect ridicules assumtions which are far off the practical world. But as you can see: the ballpark figues are completely ridicules.

The problem is here, tha you start completely wrong.
You should define a Projectile and an energy specification first. Then work out which coil shape would be the best for the projectile and the required energy transder and THEN think about how to apply your concept. However i think the last step wont be so successfull. The stuff you can realize in your setup is by it own far away from that whats known to be efficient.

The problem is, from my point of view, that as everyone expects a projectile to be a cylinder of ferromagnetic material which is essentially a steel rod, or maybe a bead for some originality, everyone will design it the classical way and follow the same path again

I don't tell anyone that it's the way to do it efficiently or anything like that, just that as a coaxial capacitor is practical for high power pulses and multiple stages induce losses and timing constraints, it may (I insist, may) be interesting to follow simpler approaches

this is just to nourish my humble mind and the pleasure to discuss technical topics

DerAlbi

Thu Mar 05 2015, 04:19PM

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

I thought about designing the thing so that the discharge would be over before one period of oscillation of the equivalent LC so didn't account for AC properties, that's the reason I mentioned to create a design to shape the pulse as so the pulse duration matches the moving short travel duration

So you argue that the effective time is even less which increases frequency even more. So AC-Properties are even more important.....

I was thinking that my spiral coil not being between the two plates of my capacitor, it would not reduce the capacitance between the two tubes

Right, i double checked, my Fault

The problem is, from my point of view, that as everyone expects a projectile to be a cylinder of ferromagnetic material which is essentially a steel rod, or maybe a bead for some originality, everyone will design it the classical way and follow the same path again

The problem is, that your approach right now leads you to completely ridicules technical specifications. At least iterate over it. Make changes, make ballpark calculations.. mace changes.. calculate again. Just be practical.
Determining the projectile specification will NOT lead you to the same path all the time. You still get the oppertunity to use your apporach of delivering the energy into the projectile, however you at least should know what orders of magnitude you try to achieve. You design some kind of frequency thats compeltey ridicules. Actually the frequency MUST be caluclated in order to fit to the projectile, there is no other way around.

If an initial specification of what you want to achieve can make you idea impractical or not worth persuing, thats at least an answer to everything in this discussion. If its immeadiately obvious that you cant achieve ~20J into a ~40g projectile... then.... ? Its impractical.
If it still works.. then fine! You design a new method of energy delivery, nothing else. I dont see how this could lead to the usual paths.

You argue right now, that not knowing the practicality of the idea for the sake of the idea is a good point of view to estimate if its practical. I disagree. Sorry.

hen918

Thu Mar 05 2015, 06:32PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

DerAlbi wrote ...

I thought about designing the thing so that the discharge would be over before one period of oscillation of the equivalent LC so didn't account for AC properties, that's the reason I mentioned to create a design to shape the pulse as so the pulse duration matches the moving short travel duration

So you argue that the effective time is even less which increases frequency even more. So AC-Properties are even more important.....

I was thinking that my spiral coil not being between the two plates of my capacitor, it would not reduce the capacitance between the two tubes

Right, i double checked, my Fault

The problem is, from my point of view, that as everyone expects a projectile to be a cylinder of ferromagnetic material which is essentially a steel rod, or maybe a bead for some originality, everyone will design it the classical way and follow the same path again

I think what DerAlbi is saying is, you shouldn't take lots of new concepts and put them all together. Think of one concept, then design outwards, finding and solving the problems from the middle, If you find a problem impossible / impractical to overcome, find a new, or modify the original concept.

Shrad

Thu Mar 05 2015, 09:43PM

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

DerAlbi wrote ...

The problem is, that your approach right now leads you to completely ridicules technical specifications. At least iterate over it. Make changes, make ballpark calculations.. mace changes.. calculate again. Just be practical.

the thing is I have a work which eats my time from 7:00AM to 6:30:PM and then I have to get things done in the house and spend the half hour I can with my son before he goes to bed... so I have not had time so far to refine and iterate

all I can do is spend ten minutes now and then to read here and there and google a bit

DerAlbi wrote ...

You design some kind of frequency thats compeltey ridicules. Actually the frequency MUST be caluclated in order to fit to the projectile, there is no other way around.

if you take into consideration the time the capacitor takes to discharge and that the projectile has reached the end of the solenoid during that time, do you really care about AC and resonant frequency? I would think that it is the equivalent of a decreasing DC, no? again, maybe that is totally unpractical, I have not made calculations to check if the projectile would have reached that distance when the pulse has ended and not after several periods...

if there is AC, wouldn't the field even go back and forth in direction? the projectile would then just vibrate, no?

Signification

Fri Mar 06 2015, 01:24AM

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

Shrad, you wrote:
That is valid if the circuit used to make the discharge time long enough for the projectile to move to the end of L keeps a somewhat constant energy flowing during the discharge
---------------------------------------- ---------------------------------
Do you mean a constant energy maintained during virtually the entire discharge OR constant DECREASE (to zero) in energy during the shorting interval?

DerAlbi

Fri Mar 06 2015, 04:11AM

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

a decreasing DC

Thats called a Oxymoron.

DC is by definition constant. Thats it. DC. If it decreases it inherently has a bandwidth and therefore frequency content ("AC").

Aa fourier analysis of the current waveform would show you the frequency characteristic. That would be mainly a peak at the resonant frequency which is a little widened and flatened out due to the exponential decay.

Shrad

Fri Mar 06 2015, 08:13AM

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

I mean that the time constant for the C discharging would coincide with the time taken by the projectile to travel the barrel

but maybe I'm totally off

if you discharge a cap in an inductor, it will ring, OK this is AC, but if you disconnect the inductor at the moment C is discharged (positive part of one period of the AC frequency of the LC), do you really have to take care of all this?

I have to simulate this with a vswitch but I can't figure the hysteresis value for now

DerAlbi

Fri Mar 06 2015, 11:01AM

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

Every change in voltage or current implies a bandwidth. It doesnt mater how small or short the change is. Think about it, as you would buy an oscilloscope: to see the waveform on the oscilloscope you would no doubt agree that you need a decent speed scope with a certain Bandwidth. Because you know that you cant follow the waveform with your multimeter with the DC-Voltage/Amps range.
Why would you need a fast Oscilloscope, if it was DC?

And no matter what, a certain frequency will behave according to the laws of electrodynamics. Therefore Skineffect is alsways present and it will increase the ESR for the higher frequencies.

For inductors thats not a problem, because you can use parallel wires. The equivalent would be a multy laver capacitor with multiple plates. But then you are back to the usual design again.

But the actual problem is here, that you chose wrong components. Just reduce the freuqency.. and everything is all right. you cant accelerate a practical projectile that fast anyway. (To make is light will reduce size and therefore the effect Âµr changes dramatically at small scales,) Thats why you shouldnt design someting vice verca. Start with specifications, then look for solutions how you can meet them!

Shrad

Fri Mar 06 2015, 01:26PM

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

I'll simulate a vswitch driven by a pulse of the same duration as the first positive part of the oscillation and see where it goes

I'm totally OK with you DerAlbi, but as I stated I have no experience with coilguns and don't even want to build one, that was for the sole reason of discussing the idea

if we take out the surplus things in the concept, I would keep in mind the fact that a moving short would displace the magnetic center and there would be no suckback... what is your opinion on this?

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