Bigger caliber coilguns more efficient?
Yanom, Sun Dec 02 2012, 02:22AMSo 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?
Re: Bigger caliber coilguns more efficient?
Yandersen, Sun Dec 02 2012, 07:52AM
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.
Yandersen, Sun Dec 02 2012, 07:52AM
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.
Re: Bigger caliber coilguns more efficient?
Yanom, Sun Dec 02 2012, 02:03PM
so gun efficiency is ultimately limited by the amount of wire you have?
Yanom, Sun Dec 02 2012, 02:03PM
so gun efficiency is ultimately limited by the amount of wire you have?
Re: Bigger caliber coilguns more efficient?
Yandersen, Sun Dec 02 2012, 06:08PM
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.
Yandersen, Sun Dec 02 2012, 06:08PM
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.
Re: Bigger caliber coilguns more efficient?
Yanom, Sun Dec 02 2012, 09:40PM
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?
Yanom, Sun Dec 02 2012, 09:40PM
Yandersen wrote ...
Efficiency-wise it is best af all not to go over saturation.
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?
Re: Bigger caliber coilguns more efficient?
Yandersen, Mon Dec 03 2012, 07:59AM
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.
Yandersen, Mon Dec 03 2012, 07:59AM
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.
Re: Bigger caliber coilguns more efficient?
Yanom, Mon Dec 03 2012, 06:49PM
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?
Yanom, Mon Dec 03 2012, 06:49PM
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?
Re: Bigger caliber coilguns more efficient?
Yandersen, Mon Dec 03 2012, 07:40PM
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.
Yandersen, Mon Dec 03 2012, 07:40PM
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.
Re: Bigger caliber coilguns more efficient?
Yanom, Tue Dec 04 2012, 01:14AM
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.
Yanom, Tue Dec 04 2012, 01:14AM
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.
Re: Bigger caliber coilguns more efficient?
Yandersen, Tue Dec 04 2012, 03:46AM
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. :)
Yandersen, Tue Dec 04 2012, 03:46AM
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. :)
Re: Bigger caliber coilguns more efficient?
Yanom, Tue Dec 04 2012, 04:03AM
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
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?
Thanks for the help by the way.
Yanom, Tue Dec 04 2012, 04:03AM
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. :)
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.
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?
Thanks for the help by the way.
Re: Bigger caliber coilguns more efficient?
Yandersen, Tue Dec 04 2012, 04:20AM
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.
Yandersen, Tue Dec 04 2012, 04:20AM
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.
Re: Bigger caliber coilguns more efficient?
PlayNice, Mon Dec 10 2012, 11:28PM
I sure learned a lot from reading this thread. So max power is 1 layer, and make the length of that one layer the diameter of the projectile right?
PlayNice, Mon Dec 10 2012, 11:28PM
I sure learned a lot from reading this thread. So max power is 1 layer, and make the length of that one layer the diameter of the projectile right?
Re: Bigger caliber coilguns more efficient?
Yanom, Tue Dec 11 2012, 02:26AM
The length of the coil is the length of the projectile. Exactly.
When you put the projectile inside the coil, the ends of the projectile should be flush with the ends of the coil.
Yanom, Tue Dec 11 2012, 02:26AM
supak1 wrote ...
I sure learned a lot from reading this thread. So max power is 1 layer, and make the length of that one layer the diameter of the projectile right?
I sure learned a lot from reading this thread. So max power is 1 layer, and make the length of that one layer the diameter of the projectile right?
The length of the coil is the length of the projectile. Exactly.
When you put the projectile inside the coil, the ends of the projectile should be flush with the ends of the coil.
Re: Bigger caliber coilguns more efficient?
Yandersen, Tue Dec 11 2012, 12:00PM
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
Yandersen, Tue Dec 11 2012, 12:00PM
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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