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Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Hey everyone, I would like to build a coil gun and had some questions I can't quite find answers to. I just want to understand the concept better before I start.
My understanding is high Henry/slower coil is better in lower stages of the gun (first few stages), and low Henry/faster coil in higher stages of the gun right?
Lets just say someone was making a multi-stage (say 100 stages) coil gun but "HAD TO" make all coils 100% identical, same size/turns, same gauge wire...
What would an experienced person choose? High H/slow coils, or low H/fast coil?
Wouldn't a bunch of low Henry/fast amp rise coils give more velocity in the end? Or would a bunch of high Henry/slow amp rise coils give more velocity in the end? Which of the 2 options would be more efficient? Is it even possible to know?
Power supply would be 3, 12v batteries in series so 36v, @ about 100amps.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
The reason that a larger inductance is generally better for the first few stages is that the projectile is moving slower. Slower projectile means a longer pulse. However, this is when using capacitors. An LC circuit will have a certain time constant, or how long it takes to go through a single period. In your case, you don't have a capacitor so the only factor your inductance will change is the rise time of the pulse. You want the rise time to be as fast as possible. So in short, I would make all your coils have a pretty small inductance, say around 10uH. That's 10E-6 Henry.
Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Thank you for your reply. Just what I needed to know. If I make my coils small inductance like you said say 20uH, at 100a 36v will the first coil even get my 3.5gram projectile moving though? Also I am considering using a capacitor but mainly just to collect the flyback voltage to make my setup more efficient (not sure if that's what that will actually do). PS what do you think about my circuit? BTW I know nothing about designing a circuit, I'm pretty much a beginner so I have a lot of questions. After the IGBT turns off, how long of a pulse is the flyback voltage compared to the IGBT pulse? Will this cap that I added help my coilgun? What about the size of the cap?
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Remember that the strength of the magnetic field is proportional to the current through the coil. So yes, 100A will be enough to move the projectile.
Your circuit will not collect the reverse voltage on the inductor. Really, it is best for a beginner to put the diode in anti-parallel with the coil and short any of the reverse current. Trying to recycle it is just too complicated for your first design.
As for the capacitor, it will help provide a kick of current, but once it drains the battery will be supplying all the current. Another consideration you need to make is how to turn off the IGBT in time for the projectile not to experience any suck-back.
Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Ok so I can still keep the cap to help give it more kick at the begging of a pulse. My IGBT has a antiparallel diode in it, is that enough? And well my IGBT will be powered by a MOFSET 4amp driver, driver will be powered by a very fast LED opto-switch circuit from a RPM counter, so I am assuming here that my IGBT will have super fast ON/OFF.
Important question about the flyback voltage. How fast is the flyback voltage? Lets say that my IGBT has a ON pulse of 1ms, is the flyback voltage pulse just as long so 1ms? Or is the flyback voltage way faster since it way higher voltage? Also if my circuit is 40v, what voltage is the flyback 400v? 1000v?
I need to know how fast the flyback voltage is so that I can then move my opto switch bit away from the coil to compensate for the flyback and try to avoid suckback.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
You're not going to get a force from the flyback voltage. Don't move your optical sensor to compensate.
Voltage is the derivative of current through your inductor. The reason such a large voltage is created is that you cut the current off suddenly. This creates a step whose derivative is theoretically infinity. Of course, in real life when you cut the current off it isn't a real step. So the flyback voltage is proportional to how fast you stop the current flowing.
Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Oh ok, I was under the impression that flyback voltage would create some pull force too. So what do I do to stop the osculation of fly-back v? Is my antiparallel diode inside the IGBT enough?
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Coil-gun Project, questions I couldnt fine answers to
