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Just finished my 3.5kj cap bank & RL test results

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LAN_403

FastMHz

Fri Mar 24 2006, 01:56AM

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

Thanks for all the fantastic feedback everyone. I did fire the coil today at 1260v successfully. I placed a hard disk ring on top, then a soda can, then a brick on the soda can. I fired it and the can became a disc...and to my surprise, I can launch these flattened can discs even though they have no hole in the middle...I thought a hole was required

Well as soon as the weather gets better I'm going to launch some stuff outdoors and use my chrony to see what kind of output I'm getting.

After firing 3 times in a row, the coil was nice and warm, but not hot. The system is working very well, and no blown up SCRs either. I have yet to fire it at 1400v yet because my transformer doesn't put out enough voltage to charge beyond 1260v...so does this mean I need a microwave oven transformer?

GimpyJoe

Fri Mar 24 2006, 04:19AM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

MOT and a variac should work nicely

FastMHz

Fri Mar 24 2006, 06:27PM

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

Alright everyone, some numbers for you to crunch. I myself find the efficiencies hard to believe, but the spreadsheet used the same 1/2MVsquared formula across the board...I'll get some footage of things launching 'cause you've got to see it to believe it. This thing is lethal!

EDIT: Bad Data Removed, see post below

Wilson

Sat Mar 25 2006, 02:12PM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

Thats an excellent efficiency. By aluminium 'disc', i presume that you don't have an iron rod protruding from you coil like in classic thompson guns?

Could we get some pics? I will be building one of these soon, from a 3kJ cap bank of computer grade caps and i would love to have a look at someone else's success b4 i start :)

Thanks,

Wilson

FastMHz

Sat Mar 25 2006, 06:56PM

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

Nope, no central rod, and no hole in the projectile - just a 3.5" 5/16" 62g aluminum platter I cut out of scrap:

Wilson

Sun Mar 26 2006, 12:19AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

cool stuff :P

Have you tried launching the discs laterally by sandwiching it between two coils?
You'll probably get much more height due to aerodynamics if you can get it to launch in the right direction

FastMHz

Sun Mar 26 2006, 11:24PM

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

I was right to be suspicious of those efficiencies of 25%. Like any good scientist I wanted to repeat the test and see if I got the same results, so I set up everything and got ready to chrono the launches. The speeds were much lower than I got the day before which was strange...I couldn't figure out why the chrono would mess up...

Then I discovered that I made an error when setting up for testing the first time and used reflected light instead of a direct source. What happened was the projectile projected a shadow on the reflector for the chrono, and this shadow would trigger the second sensor even though the projectile was only half way to it...so the speeds were just about doubled...

The result is that the sub 10% efficiencies posted earlier are accurate because I used direct light when taking those measurements, and the two 25% ones are not. Today I'm going to do some extensive testing, and also see what difference wiring the banks in parallel for 700v @ 14400uf instead of 1400v @ 3600uf makes. I'll also see about using the dual coil approach Wilson suggests.

[Mod edit: merged double post. To go back and add images to a post, stick them on the attachments board and then link to them when you edit.]

I just ran a bunch of tests and repeated them with acceptable deviation in the results. Here is a spreadsheet showing the average results for varying setups:

1 Layer = Single layer coil
2 Layer S = 2 layers wired in series
2 Layer P = 2 Layers wired in parallel

Wilson

Mon Mar 27 2006, 02:45AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

Interesting...the trend of results seem to hint that an Al disc with 2 parallel coils would yield a higher efficiency.

FastMHz

Mon Mar 27 2006, 07:15PM

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

That seems to be the case, and it makes sense too because in parallel the current is doubled and the pulse is therefore shortened. These things launch so fast that a long pulse is wasteful since the projectile is long gone. I'm unable to do a 3 layer parallel for fear of destroying my SCRs with excess current. At some point adding layers would be ineffective because the bottom layers are further from the disc and have less magnetic coupling.

Wilson

Wed Mar 29 2006, 01:34AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

what SCRs are you using atm? puk or stud?
I've bought 4 studs for use in my launcher, put i'm not sure if their 3kA surge rating will be sufficient for a 2-3kJ bank.

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