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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
If you're going to use that brick, you won't have to pay as much attention to the coil. Just make it so that the pulse is very long. Since you can turn off the IGBT, you don't have to properly tune the coil, in fact you will get better efficiency if you use a huge amount of capacitance that doesn't drain much when you fire. Look up half bridge coil guns for more info or just ask specific questions in this thread.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Turkey9 wrote ...
If you're going to use that brick, you won't have to pay as much attention to the coil. Just make it so that the pulse is very long. Since you can turn off the IGBT, you don't have to properly tune the coil, in fact you will get better efficiency if you use a huge amount of capacitance that doesn't drain much when you fire. Look up half bridge coil guns for more info or just ask specific questions in this thread.
Well, it's only one IGBT, so... I can't do half bridges with it. I also got some brick diodes for the anti-parallel thing now. Powerex ones.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Nice. It doesn't have to be half bridge, but the theory is the same. Having a switch that can be turned off before the current is done flowing opens up a lot of doors for experimentation and huge increases in efficiency. Having a square voltage waveform on your coil will impart more force on your projectile than a sinusoidal one usually found in coil guns. Just play around with it, you'll be able to do a lot.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
So, when I look to switch off the IGBT, I don't want the pulse to look sinusoidal, but like a square wave? Do I just switch it off before the pulse has a chance to go negative, or do I need to switch it on and then off really quickly, to prevent much voltage change at all? (i.e. a large capacitance, or large coil?)
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
If you have a large enough capacitance, you will end up switching off before there is much voltage change.
This is what I would do: Get enough capacitance to insure that the projectile will reach the center of the coil long before the voltage goes negative. Time the signal to the IGBT so that it turns on and then off right as the projectile reaches the center of the coil. The slower the voltage changes when the IGBT is on (the larger the cap bank and/or coil) the greater the work the coil will do on the projectile. This would mean the greatest speed without wasting energy in the oscillations of a normal coilgun.
Sorry if this doesn't make sense, just ask for clarification!
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Turkey9 wrote ...
If you have a large enough capacitance, you will end up switching off before there is much voltage change.
This is what I would do: Get enough capacitance to insure that the projectile will reach the center of the coil long before the voltage goes negative. Time the signal to the IGBT so that it turns on and then off right as the projectile reaches the center of the coil. The slower the voltage changes when the IGBT is on (the larger the cap bank and/or coil) the greater the work the coil will do on the projectile. This would mean the greatest speed without wasting energy in the oscillations of a normal coilgun.
Sorry if this doesn't make sense, just ask for clarification!
Er... So, basically instead of having other parts of the voltage "pull" the projectile back into the barrel, you just switch the IGBT off before it has a chance to go negative basically? And, I have 6600 uF of capacitance; That's as big as I can get at the moment.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
well with the typical coilgun "ringing down" through the coil, the projectile isn't exactly in the middle of the coil when the current peaks, because the current still has to drop down to zero before it leaves or else it will experience suckback. with the igbt switching off the signal when it peaks, current from the caps no longer flows into it, so the current in the coil drops more quickly down to zero (not instantly though)
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Yeah Inducktion, you have it right.
6600 uF is a good amount to start playing with. I'd just start experimenting and see what works best for you. Just make sure that you have a freewheeling diode in the igbt or add one, because the reverse voltage spike will still occur.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Turkey9 wrote ...
Yeah Inducktion, you have it right.
660 uF is a good amount to start playing with. I'd just start experimenting and see what works best for you. Just make sure that you have a freewheeling diode in the igbt or add one, because the reverse voltage spike will still occur.
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Working on a coilgun, have questions...
