Bigger caliber coilguns more efficient? / Electromagnetic Projectile Accelerators / Forums

Forums

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

« Previous topic | Next topic »

Bigger caliber coilguns more efficient?

1 2 next

Move Thread

LAN_403

Yanom

Sun Dec 02 2012, 02:22AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

So basically, I was told that the more copper in a coil, the better it is and therefore it's more efficient for coilguns. This makes sense - if you make the wires thicker, you have less resistance. However, adding more and more and more copper makes the coil thicker, and when the coil diameter gets to like 3x the gun caliber or more, it's less effective to add copper because now there's a lot of wire far from the bullet, where it's not doing much good.

So what I thought of is this - take the same wire from that ridiculous coil and wrap it around as a coil for a coilgun of much bigger caliber. Now it's not so ridiculous, and the wires are closer to the bullet because the diameter is bigger.

So, it seems to me that I've increased the efficiency (by bringing the same wire with the same power supply closer to the bullet) by making the gun bigger.

does that make any sense?

Yandersen

Sun Dec 02 2012, 07:52AM

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

Yes, it is. The bigger caliber the higher efficiency for the same velocity, generally saying.
The less layers in coil the less energy you have to push into it to create the same bullet acceleration force. But heat losses are increased also.
Interesting thing I've recently discovered (and it was proved in practice) about coils of the same geometry being fed from the same caps: no matter the wire gauge it was wound by, coil will create the same pulling force, and only the difference in their operation is the pulse time (the thicker the wire the smaller the inductance so the pulse time shortens).
So I would recommend to minimize number of layers. Though, first stages still need more copper to minimize heat losses as their pulse time is longer than one of the subsequent stages. Better to start from choosing the wire gauge, and then determine the number of layers which will give required inductance value. Then, building each subsequent stage layers number should keep decreasing.

Yanom

Sun Dec 02 2012, 02:03PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

so gun efficiency is ultimately limited by the amount of wire you have?

Yandersen

Sun Dec 02 2012, 06:08PM

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

No. High acceleration kills efficiency - that's a major factor, that's why multistage coilguns are more efficient. The amount of wire you have doesn't mean anything directly. The way you wind it does. Shorter coils and projectiles increase efficiency. The highest you can get is with length equal to diameter - simply because projectile can not be shorter otherwise it may jam in the barrel. Shooting with balls from bearing is not a good idea, because projectile' iron must be as close to the walls of the barrel as possible.
But what is most important for efficiency is the coil power' driving. Ideally, current should rise immidiately when the projectile approaches the coil, then it must stand constant while projectile moves through the coil and current must be immidiately dumped back to the power source as soon as projectile reaches the middle of the coil. Assuming the coil operates this way, it is possible to convert a significant part of the magnetic field energy into projectile kinetic energy (100% will never be converted - the more layers coil has the less energy it may add to the projectile assuming the same initial energy, but it may be up to the 50% or something - I have achieved 20% in practice). But this operation is not possible when dempher diode is used in parallel with the coil, because current will run through the coil for an infinite time. Coil must be either discharged on a cap or at least heat resistor or zener diode. Not removing magnetic field energy immidiately after zero point will result suckback force to be almost equal to the acceleration one, which will give few percent efficiency at most just because of current decay which results in a little difference between acceleration and suckback. Easy solution is to drive coil from the non-polar cap. Cap discharges to the coil, then coil discharges back to the cap, reversing it's polarity, and, assuming thyristor commutation, all unused energy will be captured back in cap. I do this way and have promising results - 10% of efficiency for the first coil and up to the 31% for some of the subsequent stages (9.4mm caliber, speed is 50m/s).
Efficiency-wise it is best af all not to go over saturation. While below, pull force (so the kinetic energy too) is proportional to the energy coil driven by. Over saturation, pull force is linearly proportional to the current resulting in increase in kinetic energy to be proportional to the square root of the energy coil driven by. In order to be able to accelerate unsaturated projectile by a short barrel I will recommend, again, to shorten the projectile, because pull force is approximately proportional to the square of the front area, so smaller length will result in smaller mass and higher acceleration.

Yanom

Sun Dec 02 2012, 09:40PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

Yandersen wrote ...

Efficiency-wise it is best af all not to go over saturation.

How can I know if it's getting saturated or not?

and, how is it that high acceleration kills efficiency? Simply because you're burning a lot of power to get that?

Yandersen

Mon Dec 03 2012, 07:59AM

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

To determine saturation run simulation in FEMM and check the peak value of flux density - if it is over 2.2T then saturation exists. While field intensity is below 2.2T everywhere around the projectile, pull force is proportional to the square of the current, just as a heat losses, meaning increasing current will not cause drop in efficiency. Howether, the more iron is oversaturated, the more linearly pull force reacts on current increase, while heat losses are always proportional to the square of it. If field intensity is dozens of Teslas all over the projectile volume, then doubling the current will cause pull force to be doubled and heat losses to be quadred. Kinetic energy and force are linearly corellated.

To double the acceleration you need to double the pull force by doubling the current. All coilguns operate over saturation limit, so see the description above.
In other words, if you put two coils of the same size instead of one, then achieving the same speed will be possible by dissipating 4 times less heat.

Yanom

Mon Dec 03 2012, 06:49PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

after a coil is shut off, does a bullet's saturation go back to zero? So that the next coil in the line recieves an unsaturated bullet to work with?

Yandersen

Mon Dec 03 2012, 07:40PM

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

It takes time for bullet to remagnetize from 2.2T back to almost zero, and this time is significant. In multistage case, all stages must maintain the same direction of magnetic field, otherwise efficiency drops significantly (proved in practice).

Oh, and about the coil shape. I run some FEMM simulations and figured out that it was right about 3x coil's outer diameter to inner - this shape will dissipate less heat creating the bigger force comparing to other coil shapes. I was also told that coil's length must be equal to the inner diameter. This ideal shape require the minimum possible amount of wire (this is resistance) to produce the desired inductance. As force is proportional to inductance (as square root), then the same is true for force coil produce - the desired force is achieved with minimum amount of heat-dissipating wire.

Yanom

Tue Dec 04 2012, 01:14AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

wait, so LESS wire is better?

if i make the wire thicker, it will require less voltage and therefore less power to produce the same current value.

Yandersen

Tue Dec 04 2012, 03:46AM

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

To maximize efficiency you need to achieve highest pull force with minimum amount of copper. The heat power dessipation and pulling force will not change if you use the different wire gauge to wind the coil of the same shape - pulse time will, as well as energy lost in total.

Imagine two coils of the same shape, no external iron, same caps o charge both of them; the difference is the wire - say, one is 2 times thicker than another. Make a table to compare coils parameters (resistance, turns, inductance and so on). This will show you that some parameters (as amperturns which is pull force, and R*I*I) will stay constant. This will give you some understanding of coilgun main part - the coil. :)

1 2 next

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