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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
Yandersen wrote ...
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. :)
we're assumming that both coils have the same amount of copper here? When I talked about making the wire thicker, I assumed that the length of wire would stay the same, so I would now have more copper overall
wrote ... To maximize efficiency you need to achieve highest pull force with minimum amount of copper.
so... why is this so again? Energy efficiency is the most pull force per energy expended, it's cost efficiency that has to do with the wire. It seems to me that more copper is better - this is why we have coils instead of singular loops of wire. Right?
Two coils of the same size, difference in wire diameter is 2. Thick-wired one will have 4 times less turns (wire is 4 time shorter), each turn has 4 times less resistance. So resistance of the thicker coil is 16 times less. Inductance will be 16 times less too, as it is a function of turns, which is 4 times less. Assuming caps used for both coils are the same, then pulse time of the thick-wire coil will be 4 times less, so current is 4 times higher. Square it and multiply by resistance and you will see that power dissipation will be equal for both coils. But due to 4 times shorter pulse time thick wire coil will dessipate 4 times less heat. Okay? Pull force is proportional to amperturns. Thick-wire coil has 4 times less turns but 4 times bigger current. So pull force, as well as heat power dessipation is the same for both coils, see? So those two parameters do not depend on wire gauge - only cap energy and coil shape. As bullet energy proportional to force and distance, it will be the same for both coils too (assuming the pulse time is ideally suitable for bullet' initial speed). The ratio between pull force and power dissipation depends only on coil shape and external iron if presented. But for bare coils the ratio is most beneficial for coils of the Ideal Shape. Do you understand it now?
If you take thicker wire and wind more copper, this way increasing the coil's size, you will dessipate less heat, yes, but force will drop too, resulting in less increase in bullet's kinetic energy, and it will be more significant than little decrease in heat dessipation. If you wind less layers, force will increase, but heat dessipation too, and increase in heat dessipation will be higher than increase in kinetics. In other words, if you need highest power possible, wind 1 layer. If you want to maximize efficiency - wind coil of inner diameter equal to length and outer diameter equal to 3 inner diameter.
Projectile may be bigger than a coil, but not vice-versa. It may even be better - just separate projectile-length coil onto few small ones, divide power equally and you will get the boost in efficiency. ;P
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Bigger caliber coilguns more efficient?
