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Double Half Bridge - Worth a shot?

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Yandersen

Sat Jan 05 2013, 07:53AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Dear Albi, should I repeat again what is the best geometry for the coil? Projectile can be longer, coil can be longer - but it all will be less efficient than direct correspondance in length.
20mm length for 10mm caliber? How about 15mm in length including the sharpened tip? And a coil of the same length, ok?
Space between coils can be filled with iron through which hole is drilled for optogate (or fiber - that would be great). This way efficiency only increases - external iron is very good.
kA again? Hundred of amps in short-coil multistage can do much more than you think. For the geometry I have suggested above you may get 1-4J of kinetic increase per stage operating with just a few hundred amps. I will not recommend to parallel IGBTs. How much current one can handle?

DerAlbi

Sat Jan 05 2013, 02:13PM

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

One can handle "nothing" (120A) - but they are great for paralleling. But i know where your recommendation comes from - trust me. Paralleling will let me operate wide withing the SOA - and since i got my hands on 200 lovely cute IGBTs.......

Yan, i know - efficiency-wise - your recommendation is great. However please consider this:

Available Engery: (Cap drops from 350 to 250V): 1.63kJ.
With (good!) 20% eff: where is 324J in the projectile.
Width your prposed geometry: m = 6g.
So velocity = 330m/s (sound of speed

)
Time within a coil: 45us.

This are great numbers, but quite ridiculous. There is only a fixed number of stages possible (30?). So the goal is not _only_ efficiency, but getting as much energy in the projectile as possible in a short time (barrel is only 1m long) so maximize the acceleration.

The faster the projectile gets, the smaller is the energy in the coil due to reduced inductance.

Thats why i want to increase the current and the inductance and try to minimize the suckback with this kind of moving field. Therefore i need a projectile thats longer than a single coil so that >=2 coils can emulate one.

Maybe the whole idea is a waste of time. But sticking to your recommendation results in a verry traditional setup where i dont see the advantage of using multiple halfbridges, what would be the point of this thread.
For me the halfbridge is attractive as long as the efficiency is better than traditional C-SCR-L+D - design. Optimal efficiency by sacrificing the "bang" isnt something i would be happy with.

If the efficency drops to 10% with a longer (20g) projectile i will still have 162J which is still 130m/s - what would be a hell of a bang. (still redicules numbers)

The question is, how to get close to this numbers with limited stages and still "good" (not best!) efficiency.... Thats my thinking:
-> maximizing coil forces
-> high current with "high" inductance
-> how to compensate for the slow current rise time
-> higher coil resolution along the barrel - allows better timing control so one can _somehow_ compensate for slow rise times.

If this is completly stupid, i'd love to hear that - its ok too. But for me, having ultra high eff. but lower kinetic energy would not be much better.

Edit: I have palyed around with optical triggering via fiber optic. I tried 2 setups:
1) IR-Led - Fiber - Gap - Fiber - Transistor <<= doesnt work (fiber to fiber doesnt work with the gap)
2) IR-LED - gap - fiber - Transistor <<= works well.
This makes it possible to seperate the low-current-electronics from the coils. However the space between 2 coils is at least ~4mm for the LED

Yandersen

Sat Jan 05 2013, 09:07PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

2nd setup is a my dream. Just find small LEDs (2.5mm are available). It is also possible to distance the LED out of the little hole where it lights to, so LED-gap fiber is not necessary.

When I talked about 20% I didn't mean that it depends on geometry only. The higher the field the lower the efficiency. Two coils operating on halved current will push more J into a projectile than a full-powered single stage. Over the saturation, doubling the current doubles kinetic energy increase but quadring power dissipation.

If you get 2J per stage, than for a meter barrel with 50 stages you will get 100J; if efficiency will be 20% than shot will consume just 500J. I would be happy with it. :)

DerAlbi

Sat Jan 05 2013, 09:46PM

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

So you would go for the tradidtional halfbridge setup?
Since i have the energy available i would like to put more energy into a stage..

How to do that?

Increasing the current is bad for efficiency. P_loss ~ I^2
Increasing the inductance is bad for timing, but P_loss ~ R

Is there any way to overcome this timing-issue?
Well i start again^^ Instead of having one "large" coil (length equals the projectiles length) with large inductancce (and xyz Amps) i split the coil into two - this halfs the rise time in each coil. The number of turns the projectile "feels" is still the same. The current through every coil is still xyz Amps so i halfed the inductance and got better timing with no influence to the projectile (same current, same sum of turns) - although the current in the caps is doubled. In this way i can double the inductance for every coil again, to get the doubled force on the projectile or just benefit from the timing.

This is verry much the same as if I have 2 coils electrical in parallel and mechanical in series. (Has anyone done this to reduce coil-Losses?? Does it work?) But the advantage would be, that i can make the trigger independent for each subsection of the original coil - so in the end of the day i dont draw necessarily twice the current from the caps.

Can someone evaluate this idea please

Acutally it really comes down to the question:
Do 2 consecutive coils (togehter they are as long as the projectile) connected in parallel, have the same force on the projectile, as one single coil would have? (considering that the current in each sub-coil is the same as it is for the large one and the sum of turns is constant)

Edit:
I did some research to get an idea of how the inductance of two close inductors change when paralleled. The Inductors will be coupled with an couple factor k=sqrt(1-L_stray/L_main).

L_parallel = ((1-k^2)*L1*L2) / (L1+L2 - 2*k*sqrt(L1*L2)).

if L1=L2=L

L_parallel = (1+k)*L/2

I am currently looking for how k develops with coilshape. However since we have only air-coupling (at the time the coil turns on), k will be small (<0.4 hopefully)

Yandersen

Sun Jan 06 2013, 10:19AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

DerAlbi, you make me suspect that you don't know magnetics basics... Pull force is proportional to Ampers*turns (F=c*A*n). Inductance is proportional to the square of the turns number (L=c*n*n). For all coils of the same geometry and the same amount of magnetic energy flushed into, pull force will be the same, independent of the wire diameter coil wound with. Doubling wire diameter decreases resistance and inductance 16 times. The higher the voltage, the faster coil energizes (I=U*t/L) - so you need as much Volts as possible to overcome the timing-issue.
Splitting one coil onto two short ones increases efficiency: even if energy for each coil is halved also, force will be higher. Long coils sucks, short ones are cool. But it doesn't mean that 1mm flat coils will be supergood - not much profit is to make coil shorter in length than it's inner diameter is.

FEMM helps to understand how it all works without actual coil winding. Very close to the real results, really and visualization tells a lot.

DerAlbi

Sun Jan 06 2013, 03:02PM

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

Well, i was not sure about magnetics, but what you say is pretty much that i base my thinking on. You need to consider constraints that are given by the "real word"...
Problems of paralleled IGBTs in a high current halfbridge-setup dictate:
- One needs a lot of voltage safety - switching 350V with 600V IGBTs is hard enough! Thats due to parasitic inductances in the diode path and in the cap-connection cables.
- The limited number of devices and their SOA dictates the maximum current. The SOA needs to be met because pulse overcurrent capability isnt that good AND unsymmetric parasitics corrupt current sharing at turn off so some safety is good too. Also, as you mentioned before, cranking up the current rises power loss quadratically.

=> Both, voltage and current are constant values instead of design-variables here. (ok.. both values can be lowered.. but ... no;-) )

pull force will be the same, independent of the wire diameter coil wound with. Doubling wire diameter decreases resistance and inductance 16 times

..only if you quadruple the current...

not possible.

So pulling force is proportional to N*I.. the only thing we can do is increasing N. As you mentionesd N inifluences the Inductance quadratically. So this is verry bad for timing..
So. My original coil has N windings and the length of the projectile l_coil. With L_c = N^2 Âµ*A/l_coil.
If i split this coil n equal halfs, for each sub coil turns half and length halfs too. Now L = (N/2)^2*Âµ*A/(l_coil/2) = L_c/2.
If i energize both coils with the same current that i would have put into the original coil, the sum of the energy remains constant. Also the geometry doesnt change since i really just cut my original coil into half and used the taps. In the end, Current densitiy is the same, but Inductance halfed.
Imagine now, that my original coils timing was acceptable -> i now have the possibility to add some turns (factor sqrt(2)) to reach the old inductance again.

Splitting the coils increased my pull force by a factor of sqrt(2).

However adding the turns on a half sized coil will increase its outer diameter (factor (sqrt(2)).
Maybe i really should work with femm, however i never did it before. but interesting would be, what happens to the force with the increased diameter. If the effect on force is compensated by bad geometry (e.g. outer turns dont pull the projectile as hard as inner layers do??) the design would useless.

I will try to look at femm, however if for some of you its just a matter of some mousclicks i would be happy if someone would give some support.

Edit: hmmh. I followed a tutorial and build a test setup, put some unrealistic coilcrurrent into it and a integrated "Force via Weighted Stress Tensor" over the projectile and over the coil. And both gives me a force along the z-axis. Somehow the forces are not equal - the force in the ycoil is a few hundred Newton higher than in the projectile - expresses this the contractive forces within the coil or is this somehow a problem in my setup

Yandersen

Sun Jan 06 2013, 05:42PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

The geometry of the coil is it's actual dimensions - if you cut coil in two halves geometry changes. Don't try to estimate parameters based on the original shape - only FEMM or practise will tell what will occur in real.
All parameters can be changed easily by rewinding the coil with a wire of different diameter. So start from the best geometry (repeat again?) than calculate the diameter of wire which will influence on current for required amount of energy. Energizing time will depend on voltage, and as it is a constant, than timing will be a fact for you, limiting the speed at which later stages are still able to operate. In general, each subsequent stage operate on higher current - there is no way to avoid that other than lower the energy so as the current. So ideally, each subsequent stage should be wound by thicker wire and operate on higher current. If you want to use the same wire than voltage should be proportional to the projectile speed to fasten the energizing time in accordance. But this is the senseless way, right?

Give me the exact coil dimensions (in millimeters), wire diameter and turns number and I will tell you the inductance. The rest you can calculate by yourself. How about that?

DerAlbi

Sun Jan 06 2013, 07:33PM

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

Well i would love be be able to handle femm by myself - would be much more flexible

however this turns out to become very hard. Setting up the geometry is easily done.. mastering the LUA-Interface however with its "something is nil"-kind of errors is a harder thing

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