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New Coilgun Project

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BigBad

Sun Dec 16 2012, 11:32PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Actually, when you're just at saturation with iron you can theoretically get upto 2000g.

So I make that about 22.5mm of active acceleration length to give 30 m/s.

So I'm thinking that running at saturation is not a big limitation.

The inefficiencies appear when you try to go beyond the saturation point.

Yandersen

Mon Dec 17 2012, 06:21AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

2000g and not going over saturation? What are the projectile' and coil' dimensions? I'm interested. I thought my coilgun is the most efficient by the moment, even though the field at the projectile' tip is around 5T (according to FEMM) and acceleration is something like 3000g. Going to beat my 20% of efficiency? Try! :)

BigBad

Mon Dec 17 2012, 11:58AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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If you use more or less the same coil winding geometry as you are, and drive them less strongly, but break up the same coil into multiple phases (like 3 phase) and drive them correctly. (And butt neighbouring coils together.)

You'll get less acceleration, but the efficiency will go up markedly.

To get 5T you're getting 5^2 losses. If you go down to 2T you'll get 2^2 losses- for 2/5 the acceleration; so a factor of 2.5 less coil losses.

You can largely control which subcoils on the same phase takes the power with capacitors and via inductance, the coils with the projectile have different inductance than the empty ones, so you just need to make sure that that active coil gets the juice.

You can wind different coils on the same phase in parallel, the projectile should lower the inductance on the ones it goes through (if it doesn't, then you haven't saturated it hard enough).

Yandersen

Mon Dec 17 2012, 04:41PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

I was dreaming about a cap-driven HV halfbridge multistage design with current control, optocable wires sensing the little space between coils, but is is just too much comlex for me. My design has simple automatic blind triggering allowing totally no gap between coils and gives a boost in kinetic energy in a 1J per 1cm of barrel length ratio. I was thinking about buying smaller caps for first two stages, as their efficiency is low due to longer pulse time, but just don't want to spend any more by now.
And I have negative exp with optic sensors inbetween coils - false triggering from coils' induced currents. Optocables I thought about but never tried. And many small stages means many optosensors and control circuits - it may be incredible amount of electronics also resulting in a bulky design. If anyone can build that one, it will probably be Saz. :)

BigBad

Mon Dec 17 2012, 05:19PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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I come at coilguns from quite a different angle.

At the end of the day this is just an electric motor. Why do electric motors even need closed-loop control circuitry?

I mean induction motors don't. Nor do ordinary synchronous motors.

I agree that there's a class of synchronous motors that have closed loop control. But why not build from the class of synchronous electric motors that don't need that?

Ash Small

Mon Dec 17 2012, 05:33PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

BigBad wrote ...

At the end of the day this is just an electric motor. Why do electric motors even need closed-loop control circuitry?

Are you suggesting that the 'slug' should 'complete the circuit' as it passes, for example, brushes at the beginning of each coil, so that the coil starts conducting as the 'slug' enters it, and stops conducting when the 'slug' is fully in the coil, so that you need no other switching circuitry at all?.....Sounds interesting.....

BigBad

Mon Dec 17 2012, 05:41PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Mmm. Just mocked up the magnetics of a coilgun using permanent magnets and some washers and a gram weight neo cylinder I had kicking about to simulate the projectile.

It looked initially very promising, but then I noticed the lateral instability. I think the iron's permeability is destabilising. It might be possible to fix that passively with quadrupole magnets every so often; that's what they do with particle accelerators.

And longitudinally there seems to be a stable zone, so it may be possible to fire open loop.

BigBad

Mon Dec 17 2012, 05:49PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Ash Small wrote ...

Are you suggesting that the 'slug' should 'complete the circuit' as it passes, for example, brushes at the beginning of each coil, so that the coil starts conducting as the 'slug' enters it, and stops conducting when the 'slug' is fully in the coil, so that you need no other switching circuitry at all?.....Sounds interesting.....

No. Think about a conventional AC synchronous motor; the magnet is stable, but there's nothing obvious there to stabilise it (but there is something not so obvious).

BigBad

Mon Dec 17 2012, 05:55PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Earnshaw rears its ugly head though. The projectile must be unstable in at least one direction, so you probably want it to be unstable longitudinally, and then you can use a feedback loop to control its speed, and then it will be stable laterally.

So you probably need at least one feedback loop for the whole gun (unless you use the electrodynamic maglev tricks to get lateral stability- actually that is a natural advantage of induction motors here, the propulsion tends to self-center as well as being stable longitudinally).

BigBad

Mon Dec 17 2012, 06:27PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

If you measure the phase of the current relative to the voltage, it should/ought/may be possible to work out what the projectile is doing; that's what normal synchronous motors do.

/trainofthoughtspammingyourthread=sorry

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