3kj induction CG questions / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

3kj induction CG questions

previous 1 2

Move Thread

LAN_403

ramses

Thu Feb 07 2008, 09:46PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

I just had a thought and proved it small scale:
The diodes don't have to be rated for the full voltage as long as they are between the load and the switch. the voltage drop across the switch during turn on and the fact that the coil is conduction constantly, so the voltage drop should be high there too. The surge current could probably be increased as long as the power remained the same (400a at 8.3ms could be 800a for 4.15ms) I have only tested my theory small scale (650v not the full 1800v)

the switch used to give really nice sparks when closed, but at 1800uf instead of the previous 4800uf (although at the same energy) there is less sparks. since my electrodes may be made of "interesting" materials (steel wool, aluminum foil, sewing pin, etc.) cool sparks are sort of important to me. will this change once i shoot at the full 3kj?

I got a 1 ft length of AWG 4 solid bear copper wire at a hardware store for under $2. it may be worth more as copper than as wire. with 4800uf at 375v, the results were horrible! it didn't even make it an inch down the barrel. granted the tube was too big and the coil was standard insulated and wound with a form... but still, that's horrible.

caps are "bussed" together 4x3 for 1800v at 1800uf. "buss bars" are doubled up sections of aluminum sheet. kinda pathetic, but i really didn't feel like buying a 8" by 20' roll of flashing for $25.

further testing(playing) has been stopped until I figure out why the barrel(pen tube) constricted and is now catching the projectile.

any suggestions welcome, ramses

Danielle

Fri Feb 08 2008, 03:38AM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

the coil shrinks when you put all that energy into it, thus constricting the tube.

ramses

Fri Feb 08 2008, 08:13PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

but the tube is rigid- it doesn't compress. but on second thought, I burned myself on the coil after one shot... anyway, i fixed the tube by heating up the projectile with a lighter and dropping it down the tube a few times. Problem solved, except it is a horrible duty cycle...

anyway, please answer the questions in my post on the 7th... Thanks, Ramses

FastMHz

Fri Feb 08 2008, 11:53PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I attempted to fire aluminum slugs out of my CG once and never did have success...as a matter of fact, at 3.5kj the aluminum didn't even move. That you got your copper to move at all is much better than I even got. It's almost like the round coil cancels out the induction effects, and just won't work

WaveRider

Sat Feb 09 2008, 09:30AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Hi Ramses.. Let me have a "shot" at helping you..

First: Induction launchers work on current and time scales that are at least an order of magnitude different from reluctance coilguns. For example, where a reluctance gun requires a coil current of 200-300A over 1-3ms, an induction gun will require 3000-15000A over 10-100us to generate significant acceleration. The time scales must be very short because otherwise the magnetic field begins to penetrate the Al or Cu projectile (as the current in the armature dies away). But it is this reaction current that interacts with the magnetic field and accelerates the projectile. When it dies away, there is no more acceleration.

Second: The external coil does not "compress". When fired, it tends to expand..often violently exploding outwards under extremely high currents (as can-crushers often see). The armature will experience a compressive stress. In an induction gun, one can think of the projectile being "squeezed" out of the high-field zone within the coil into low-field zone outside; a bit like squeezing a slippery bar of soap in your hand...

One way to develop an induction accelerator is the use a multi-turn shorted wire coil armature wound on an iron core. This is the "linear induction motor" idea that others have floated on the HV forum. I have not explored this idea in depth yet, but it seems like it could mitigate the need for high-speed switches, extreme voltages and currents. This is similar to what drives "maglev" trains and advanced conveyor systems...

Cheers!

ramses

Sat Feb 09 2008, 09:20PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

tried a 10 turn can crushing coil at 815v that's ~600j, or 1/3 max energy. the back emf diodes blew in half. since they were only rated for 400v and a total of 400a surge for 8.3ms. I assume that they ended up conducting backwards, absorbing the full 600j and blowing up. now I see that using under-rated components can have it's drawbacks.

I found a large diode looking thing in my basement with a part number h2612-21 on it. google says that it is either a car made by honda, or the h2612-12 version is a 10kv, 330ma diode.

Would a 1kv, 50a rectifier from ebay work if i used the + and - ends as a diode? from what I see, that puts 4 50a 1kv diodes in a "cube" for 2kv at 100a.

Wave Rider- the coil would "compress" slightly for 2 reasons: i was using the coil with a ferrous projectile at the time, and the ends of the coil become the poles of the magnet, and they attract.

Fastmhz- I don't believe that i would ever criticize you, but the fact that you were using a SCR with a turn on time of 1-3ms, I was using a mechanical switch. the SCR is more efficient, but the turn on time takes away from the massive, short spike.

edit: I had another thought: with the linear induction motor idea, 1, the core will saturate, and 2, the core will be attracted to the coil while the shored coil (armature) is repelled.

edit: I found a 3 phase 60a rectifier on ebay. if I use the output, I should get a 2*3 configuration for the diodes.

And i was experimenting with a quick and dirty trigatron. I connected one output of a piezoelectric to one output of the bank and had the other piezo output spark to the other output of the bank and that allowed it to conduct all but the last ~60v. Then it stopped working. I think the copper vapor from the main electrodes condensed on the block of wood i mounted them to and prevented the spark from making it between the electrodes. could the pulse from the piezo break down the diodes and let them conduct all of the bank energy?

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob