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

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Navrit Bal
Thu Nov 25 2010, 10:53PM
Navrit Bal Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
So 210 + (3x40) = 330 milliOhms but I imagine the other components would bring it up to around 350milliOhms. Does that sound reasonable?

Since that is getting quite low I was messing around with Barry's RLC simulator, would be be better to use a 14 SWG coil with 114 turns, 100uH and estimated 200milliOhms overall

1290725284 3323 FT99411 Snap018

or a 18 SWG coil with 124 turns, 73uH and estimated 250milliOhms overall?
1290725284 3323 FT99411 Snap021


As I understand, there is a relationship between the number of turns and current^2 for the magnetic field, however which one would be better?
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Alex Smith
Fri Nov 26 2010, 09:24AM
Alex Smith Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25
The field strength is proportional to number of turns multiplied by the current, google Amperes law, but to be honest I really don't understand it :S

^If thats true then the difference between both of those coils is negligible so just choose for convenience or pulse length etc.
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Barry
Fri Nov 26 2010, 03:27PM
Barry Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Navrit Bal wrote ...

Would be be better to use a 14 SWG coil with 114 turns, 100uH and estimated 200milliOhms overall or a 18 SWG coil with 124 turns, 73uH and estimated 250milliOhms overall?
Magnetic field intensity is the simple product of current and number of turns. So when I read the peak current from your charts above, the comparison becomes:

1,480 amps x 114 turns = 168K amp-turns
1,360 amps x 124 turns = 168K amp-turns

Since the peak magnetic field is the same, all you have to do is choose between 100uH@200mOhm and 73uH@250mOhm.

My recommendation is to always use lower resistance circuit. Coilgun performance is sensitive to the last few milliohms.

Don't use cellotape as an insulator. It's too thin, easily scratched, and you cannot rely on dielectric insulating properties. Very unsafe.

You can use simple connections like alligator clips for the charging circuit and other non-critical areas. But the high-current discharge path must have firmly bolted or soldered connections throughout. If not, it will not-so-politely indicate the weakest connections by briefly illuminating them for you. cheesey The principal at work is that heat dissipation is equal to I^2R. Do the math. The "current-squared" is enormous in a coilgun. For example, if you have an errant connection with say 100 mOhm resistance in a circuit with 1480 amps, the heat would briefly become 218,040 watts at the connection. Poof! lol

Cheers, Barry
Speaks about himself in third person when the other two people in his head have taken over.
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Alex Smith
Fri Nov 26 2010, 06:24PM
Alex Smith Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25
Barry wrote ...

But the high-current discharge path must have firmly bolted or soldered connections throughout. If not, it will not-so-politely indicate the weakest connections by briefly illuminating them for you. cheesey

I got some green sparks - its was AWSOME!
But it is really hard to solder the thicker gauge wires so bolting it is then?

Also, can I ask what impact a change in inductance would have (if any) on the performance of a coilgun?

Cheers Alex
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Saz43
Fri Nov 26 2010, 08:04PM
Saz43 Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Alex Smith wrote ...

Also, can I ask what impact a change in inductance would have (if any) on the performance of a coilgun?

Cheers Alex


Higher inductance increases the pulsewidth (you can see this effect when playing with Barry's sim), which may be a good or bad thing depending on how long you want your pulse to be. More inductance will also decrease current, but most likely at the gain of more wire turns, which *can* balance it out in terms of performance (see Barry's comment on amp-turns). Also, the inductance will unavoidably increase as your projectile enters the coil.
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Navrit Bal
Fri Nov 26 2010, 08:10PM
Navrit Bal Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
Could you secure the ends of the coil by using a screw and a nut, then soldering/brazing them together? My thinking is that it would provide a much larger contact area which you could connect things to with relative ease and safety compared to the bare ends of the copper wire.

I would also like an answer for how inductance affects coilguns. I will use insulation tape before anyone else tells me not to use cellotape cheesey

The crocodile clips were only used for my 20V charge/discharge tests, I didn't intend on using them for anything else.
I will use the thicker wire for the coil and the 18 SWG wire for the connection between components, apart from the triggering circuit as there is no need.

Out of interest, what kind of kind levels would be produced at 218kW ?
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Navrit Bal
Fri Nov 26 2010, 08:25PM
Navrit Bal Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
If the projectile (iron/steel) basically filled the coil, roughly what would be the peak inductance?

Also look at this titanium coated silicon steel screwdriver bits Link2 this happens to be very close to my arbitrary 41mm coil length, I could quite easily saw these to nearer 41mm. Isn't silicon steel very magnetically permeable and therefore an excellent projectile?
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Barry
Sat Nov 27 2010, 02:00AM
Barry Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Navrit Bal wrote ...

Could you secure the ends of the coil by using a screw and a nut, then soldering/brazing them together?

I would also like an answer for how inductance affects coilguns.

Out of interest, what kind of kind levels would be produced at 218kW ?
Yes, securing wires together by tightly bolting them works quite well. Most likely the additional soldering or brazing is not needed. It can be useful to have a convenient disconnection point like this. Use common sense for a good connection: generous surface area and clean bare ends on the wires.

Increasing inductance will lengthen the discharge time. The pulse width is directly proportional to the LC time constant (simple product of inductance times capacitance). Try it in the RLC simulator. Note that a longer discharge time will also lower the peak current, too, by way of spreading out the energy over time. In effect, the area under the curve drawn by current vs time will be proportional to the total energy discharge from the capacitors.

The "levels produced by 218kw" are plenty high enough to vaporize blobs of metal in a poor connection and shoot them out in random directions. The total energy expended melting things is impossible to predict since we don't know the time duration. But obviously the energy cannot be more than what was stored in the capacitor. Anyway, much smaller capacitors than yours are fully capable of making big gouges in say a screwdriver across its terminals.

Cheers, Barry
Music is like candy. To enjoy it, you must first throw away all the rappers.
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Navrit Bal
Sat Nov 27 2010, 01:35PM
Navrit Bal Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
Good, I will use the bolting method. Any opinions on the silicon steel projectile?

Also, before I order anything, wouldn't this diode Link2 be excellent for my anti-parallel diode ,across the coil of course, because it has an ultra fast response time (35ns), can handle 540A over 8ms (i intend on using around 4ms) and finally is rated for 1kV, not that I will go anywhere near that. The description in the pdf also says it was made for use as a free-wheeling diode.

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Artikbot
Sat Nov 27 2010, 01:56PM
Artikbot Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137
So, you're on 450V right?

Link2

Look at this bish. It's worth about 1GBP less and has more surge current. On the downside it's rated at 600V. But it's still far from your voltages, so it's not really a disadvantage.
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