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Yandersen

Sat Dec 08 2012, 08:04PM

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

Aha! Here is the fixed schematic:
Driven Coilgun

So, initial state:
C1 is charged to the [battery voltage-0.6V];
C2 has no charge;
IGBT' gate is at the ground level, as well as SCR2 gate and cathode.

When the projectile approaches the coil, you trigger your IGBT by charging it's gate to around dozen of volts, right? Then current starts to rise in the coil. When projectile is inside the coil, you are switching off the IGBT by lowering it's gate to the ground. During that process voltage across IGBT rises, making SCR2 able to handle the current flow and redirect it to the C2. When current depletes, all unused energy is stored in the C2, and SCR2 closes, capturing the energy inside that cap.
When projectile is outside the coil, you trigger SCR1 to return energy back into C1, so next shot coil will start charging from higher voltage, thus reusing some of the energy from last shot. When current depletes, voltage across C1 is higher than the battery voltage, and C2 is 0.6V below the ground level. SCR2 will not allow the current to flow back into C2, because it's gate at this moment is at the ground too. So we returned to the initial state with only one difference - next shot coil will be charged faster and use little less of battery.

Ash Small

Sat Dec 08 2012, 08:08PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

I'm pretty sure some brushless ESCs for RC cars incorporate regen braking. If your theory was right, that wouldn't be possible without destroying the batteries.

Well, there are several factors to consider here, firstly, in the case of regen. braking for RC cars, maybe the batteries are configured so that they only re-charge at the designed rate?...Although, as far as I'm aware, all the Lithium based batteries require the charge rate to be reduced to a few % of the initial charging rate as charging progresses (from what I read when I was designing a simple charger that runs off a 12V car battery for a Nintendo DSi, although they are lithium ion batteries in the DSi.)

I read quite a few stories about peoples cars going up in flames while they were re-charging LiPo's 'in the field', and the insurance companies not paying up because the chargers were faulty.

As others have pointed out, all the 'authorative sources' say that the 'proper charging procedures' must be adhered to for safety and reasonable battery life.

I assume a 'momentary burst' won't overheat them sufficiently to cause them to burst into flames, but it will still degrade them, I'd assume, and repeated bursts will presumably overheat them sufficiently to eventually cause a fire.

Maybe someone should run some tests to see what effect this has on them?

(I'm still of the opinion that, if they are near fully charged, as they would presumably be at the beginning of a day's 'shooting', they are likely to suffer some degradation.)

EDIT: I've just read the three previous posts by Hanzie and Yanderson. I think Yanderson is probably on the right track, placing a diode in series with the battery, etc. As far as Hanzie's point is concerned, I'm still trying to get my head round exactly how recupreated energy is used to slow the car down. Most of the KERS systems in formula 1 use some form of lithium battery as far as I'm aware (Williams and, I think, one other team use flywheels to store the recuperated energy) Red Bull consistently has problems with their batteries overheating, and several times in the past couple of seasons have had to disable the KERS system until the batteries cool down again. Maybe all you need to do is monitor battery temperature?

Yandersen

Sat Dec 08 2012, 09:11PM

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

Sorry for the multiposting, I've just realized that IGBT will actually start to close some time after it's gate will drop down to 0 volts. If so, then this schematic will ensure that SCR2 opens after IGBT is closed:

ben5017

Mon Dec 10 2012, 01:42PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Got some work done on the X3CC this weekend. Both accelerator stages are up and running. As of now, I have no way of compiling performance data. How do you guys estimate velocity? Am I better off buying a cheap chronograph?

Anecdotal results: I was pleasantly surprised with the power from one stage + injector. The results from adding the second stage showed a noticeable increase in power, although I was expecting a bigger jump in performance after how well the first stage worked. This leads me to believe that it is nowhere near tuned at the moment.

Since I finally have a multi stages coil gun, I can now start experimenting and work out ways to optimize the design.

Hanzie

Mon Dec 10 2012, 03:52PM

Registered Member #6534 Joined: Sat Sept 08 2012, 12:22PM
Location: The Netherlands
Posts: 32

Ash, those RC cars slow down by giving very little reversed throttle like they would when they drive backwards. As the car still drives forward, this works as a brake. I don't know how else to explain it, but as far as I'm aware there's no energy going back to the batteries. This wouldn't really be a whole lot anyways, so it's not really worth the trouble of putting such a system inside. Real-size cars may use the technique though.

As for the coilgun; Looking awesome so far Ben. ^^

Yandersen

Mon Dec 10 2012, 10:26PM

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

I check speed of mine by the oscilloscope and a piece of pipe with two optical gates distanced by 10cm from each other.

Ben, try a shorter projectile - it will give your coils a little more time to dessipate energy before suckbacking. Saz at least puts a bunch of serialized diodes to deplete current faster. BTW, I've tried a comparably little 1N5408 diode as dempher diode for the coil which had 100A running through - it sustained the surge pulse without even mentionable heating. Consider using small diodes - for a single surge you don't need iron monsters.

ben5017

Tue Dec 11 2012, 09:54PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

yaz: I am trying to gain some aerodynamic stability with the longer thinner projectiles, and hopefully improve the effective range of the carbine. The scar 1000's projectiles were shorter and fatter, and were unacceptably unstable, some would shoot stable and straight, others would buzz end over end. for the x3cc I made sure I found a barrel that fits absolutely perfect. I need the long heavy slugs to hit tip first in order to have sufficient penetrative power, hopefully these two changes in addition to the increased velocity will do the trick.

-As for my diodes. I just put those on there to test everything is working as expected. Now I will begin upgrading the various systems. Finding the optimal diodes, and flux augmentation are the first things on my list.

-Should I be looking for diodes primarily by there amperage and speed ratings?

-You had mentioned adding 5W zeners in series with my diodes? Can you explain how those would help and where the energy will get dissipated? What voltage zeners should I be looking for? 25V?(same as my battery)

-So IGBT half bridge, Nonpolar caps in parallel , and zeners in series with my diodes are 3 different ways to limit suck back?

Saz: did you ever measure the velocity of your stages with out external iron? Where did you get your powered iron?

Yandersen

Wed Dec 12 2012, 04:24AM

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

Ben, what is the voltage rating of your IGBT? Let's say it is 600V. Then put 3 zeners of 180V rating IN SERIES to get a little below 600V (540V you will get). The leftovers of your current will dissipate on those zeners almost instantly. You may parallelize few such strings of zeners to be sure it is not overwhelmed. The idea is to higher the voltage drop. When IGBT is switched off, current is circulating through the coil and zeners expiriencing R*I*I dissipation power running through the coil and V*I power dessipation over zeners (V is a voltage drop over the single zeners' string).
Just do this and forget about any suckback. But make sure nothing touches the coil during the voltage pulse shot... :)

ben5017

Wed Dec 12 2012, 03:09PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Yan: Good guess, they are 600v, just ordered (24) 5w 180v zeners. Should I still be putting more dempher diodes in series also? Or with the addition zeners is this now not necessary? Will my IGBT experience any significant different conditions, like a spike in amperage or voltage that I should be concerned about?
Can you explain what will happen if something touches the coil, I assumed the enamel wire had enough dielectric strength that 600v would not be a problem.
Those 5W zeners can dissipate all the current in the coil? Over one hundred amps? Should I expect them to experience significant heating?

Yandersen

Wed Dec 12 2012, 09:28PM

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

No, just one dempher diode. It will dissipate almost nothing. Your IGBT will be scared by voltage spike, but I doubt it will be over 600V, so their pants will stay clean. Good question about the wire. Hardly it will blow inbetween the layers, but keep ends wrapped with additional insulation and away from each other. 540V is pretty dangerous, but you want to dessipate energy the fastest way possible, right?
Zeners will dissipate almost all energy stored in a coil (it is E=L*I*I/2, so you can estimate it if you know your coil inductance and current). The rest will be dissipated on coil resistance. Current will linearly decrease, and voltage will stay constant while the last electrone will die. When current stops, voltage will drop instantly from 540V to 0V. It will not be long. In fact, there are no any faster way to dissipate energy rather than let the coil shot a HV spike.

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