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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
As I understand, disc launchers work fantastic with very very fast rise times.
So, a sine wave (of sorts) isn't the pinnacle of fast rise times, so I was contemplating, why not use the capacitive energy to discharge into a square wave?
The rise times would be so fast....
Problem I see; Switches, which need to withstand such "oomph" in a short time. SCR's wouldn't work, as you would need to shape the wave into a square. So, why not just use IGBT's? If this works correctly, you could have a VERY large capacitor bank (value wise) and still get decent disc launching action out of it.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
I think a square wave would be really difficult to get with an RLC discharge. The sinewave-shaped discharge you get is a consequence of the RLC physics, not the switching device. The inductance will always slow the rise time, and the limited capacitance & finite resitance would always lead to the current decaying immediately after reaching it's peak instead of staying at a plateau.
If you had a HUGE capacitor bank, you could abruptly shut off the current with an IGBT before the current decays significantly to approximate a square wave, but then you would be using only a tiny fraction of the available energy in an unnecessarily large capacitor bank.
That being said, an IGBT can still give you efficiency gains for a coilgun at turn-off if you can cut the current at the right time to reduce suckback.
Registered Member #3411
Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33
Theres no suckback in a disk launcher, you actually want the current to stay high for as long as possible, in order to propel the armature via the inducted current in it for as long as possible. Obviously at some point the armature would/should be far enough :D
Registered Member #2004
Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39
Because of the relationship between field strength and distance, wouldn't it be better to aim for something like a saw-tooth wave? It really depends on the length of the pulse I'd think...
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
There would be no difference between SCR and IGBT. The SCR shorts the caps into the coil, how can it be any faster? Like was said earlier, the RLC circuit is what creates the waveform, not the switching method.
Registered Member #3950
Joined: Wed Jun 15 2011, 12:45AM
Location:
Posts: 51
Like Saz43 said, square waves are nearly impossible to get with a capacitor bank. However, the higher the voltage and lower the capacitance of the capacitor bank, the faster the rise time and discharge time would be, provided the cap has the same ESR.
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Sine Wave vs. Square Wave
