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High power EM projectile launcher

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DerAlbi

Sun Jan 24 2016, 06:33PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Well you obviously have a special feeling for such things - i admire that. I know that k is the shared flux and stuff but how it really affects energy transfer and why its kÂ² and not k, I wouldnt dare to predict. But my speciality is circuit design, not the phyiscs behind it...
I am also not sure about my model... since the inductor changes i used the complete formula you stated once in the Coilgun-thread. But i dont know if that actually conserves or creates energy because it is going into mechanical work (due to changing k) so it feels wrong to care abut energy conservation in the changing inductor.
However the efficiency seems to be about right. Whats not right is the current waveform. Maybe Kizmo shares an oscilloscope picture of the current to show you what i mean. Edit: i remembered unspecified waveforms, that were maybe not even loaded with a disc. We have to wait for Kizmo to do measurements.
Honestlsy, i used LTSpice only as nonlinear time domain solver for your equations. i am not sure what i am actually simulating.

Uspring

Mon Jan 25 2016, 11:58AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

DerAlbi wrote:

I know that k is the shared flux and stuff but how it really affects energy transfer and why its kÂ² and not k, I wouldnt dare to predict.
Yes, flux and Faradays law is involved.

Imagine a primary coil and a short circuited secondary coupled to it. For simplicity assume them to be without resistance. Then apply some voltage V for some time to the primary. That will generate according to Faradays law a flux phi in it:

phi = Integral(V*dt)

There will also be some flux generated from the current induced in the secondary but that affects only the current in the primary but not the validity of Faradays law. We can calculate the current in the primary by using the coupled primary inductance, i.e.

Icoupled = Integral(V*dt) / Lcoupled, so

Icoupled = phi / Lcoupled

The energy in the system is now

Ecoupled = 0.5 * Lcoupled * Icoupled^2

Note, that there is no flux through the secondary by Faradays law, since it is short circuited and therefore no voltage across it. Now move the secondary far away, so that it is not coupled anymore. The fluxes through both coils don't change during the movement, since there is no voltage across them. This implies a zero current in the secondary (no flux) and in the primary we have:

Iuncoupled = phi / Luncoupled

The energy in the system is after the movement:

Euncoupled = 0.5 * Luncoupled * Iuncoupled ^2

Putting these equations together, we get:

Euncoupled / Ecoupled = Lcoupled / Luncoupled

This allows us to calculated the electrical energy loss in the system from the change of the primary inductance. This loss is equal to the mechanical energy the disk acquires. With some more math the force can be derived. Applying the equation for k and the and the inductances, we get:

Euncoupled / Ecoupled = 1 - k^2

The theoretical max efficiency is then:

Eff = (Ecoupled-Euncoupled) / Ecoupled = k^2

I am also not sure about my model... since the inductor changes i used the complete formula you stated once in the Coilgun-thread. But i dont know if that actually conserves or creates energy because it is going into mechanical work (due to changing k) so it feels wrong to care abut energy conservation in the changing inductor.

I'm not sure about this myself. The I*dL/dt term is sort of a back emf effect. My hunch is, that your simulation is correct. Anyway, since the efficiency is less than 10%, this effect is not so large, so the error due to a wrong simulation should also be not so large.

Kizmo

Mon Jan 25 2016, 01:01PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Soon we will get much better velocity measurements! Im constructing laser and phototransistor based light gate setup where i can use oscilloscope for measuring time between two light beam crossings.

No more guess work from blurry 1200fps video.

Just for lolz we placed old and crappy basket ball on the disk to see if it would launch sky high. It didnt, the ball exploded and disk went pretty much straight through it

Mighty bang!

Uspring

Tue Jan 26 2016, 04:40PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

If there are sightings of silvery disks flying around Northern Europe, I know now, where they come from.

Just be careful, the coil can explode.

DerAlbi

Tue Jan 26 2016, 05:35PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Explode?? Shouldnt it just "implode" at best? (since its not the electrical weak point)

hen918

Tue Jan 26 2016, 06:01PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

DerAlbi wrote ...

Explode?? Shouldnt it just "implode" at best? (since its not the electrical weak point)

No, the huge magnetic fields act to push the windings apart. What you end up with cannot be called anything other than an explosion.

DerAlbi

Tue Jan 26 2016, 06:31PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Aha

Is that due to the disk then? Because all i know a coil it self contracts, but does not expand. And why would the force of the disk exceed the contraction?

dexter

Tue Jan 26 2016, 06:56PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195

DerAlbi wrote ...

Aha

Is that due to the disk then? Because all i know a coil it self contracts, but does not expand. And why would the force of the disk exceed the contraction?

Newton's third law: For every action there is an equal and opposite re-action.
now the force acting upon the disk is not on all pints perpendicular to the disk, this mean there is a big vertical component and a small horizontal component. Both are felt by the coil in the opposite direction (Newton's third law) and depending on the coil/ disk geometry and power levels the coil might explode :)

Kizmo

Tue Jan 26 2016, 06:59PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Uspring wrote ...

If there are sightings of silvery disks flying around Northern Europe, I know now, where they come from.

Just be careful, the coil can explode.

I am relatively careful. The coil is constructed in a way that i would be extremely impressed if it fails mechanically. Basicly it is wound with two parallel strips of copper sheet metal, 1.25mm by 35mm. Turn to turn insulation is done with duct tape and entire thing is bonded together with 2 component polyurethane based car body adhesive. The base is made from heavy duty birch plywood and the coil sits partially inside of a machined cavity.

EDIT:
Here is a website that shows failed coil. Doesnt look exploded to me.

klugesmith

Tue Jan 26 2016, 08:19PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Hi guys. It's nice to see active discussions and confirmations about instrumentation and modeling of inductive-repulsion shooters.

Kizmo wrote ...

Soon we will get much better velocity measurements! Im constructing laser and phototransistor based light gate setup where i can use oscilloscope for measuring time between two light beam crossings.

If your work coil has a hole through the middle, like Albi's, then the rising projectile can pull a short, lightweight stick/string/ribbon. The tail end can produce a nice linear stroke to measure, without any tumbling etc., in a place easy to shield from the magnetic field pulse. Good place for some cheap optointerrupters. You could sum the outputs of 5 or 10 or 20 to make a stairstep waveform for your oscilloscope. Could get even better spatial resolution by going analog. Progressively expose a long, narrow light source or detector. Or move an illuminated spot or slit (or projectile) that's optically imaged onto a position-sensitive linear detector.

Kizmo wrote ...

I am moving towards using very large triggered spark gap switch so almost any voltage from 1kV and up will be doable. Any amount of current is doable.

Is there any benefit from it being electrically triggered? People who shrink coins etc. generally migrate to mechanically triggered switches or spark gaps (non-sticking). Easy to match the performance of any trigatron or solid state switch, except for controlling the exact time of the shot.

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