Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
Yanom, Thu Jun 07 2012, 01:08AMI've noticed most people power their coilguns with 330V photoflash capacitors or the like. These caps are small capacitance (meaning low current output, right?) but high voltage. Why wouldn't you want to use a low voltage capacitor with high current output? I haven't built a coilgun yet, but I'm trying to learn how this all works. What's the rationale behind this?
Re: Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
rp181, Thu Jun 07 2012, 02:07AM
A couple things:
A higher capacitance doesn't mean a higher current, it means more energy. The current will only be high if the resistance of the coil is reduced, according to ohms law. The logic behing a higher voltage is that it allows a higher current to be drawn, a it can overcome the resistance of the coil. Utilizing a low voltage high capacitance source simply results in a longer pulse of a lower current. This will actually be detrimental, if the magnetic field is still present once the projectile passes through the coil, the projectile will be pulled backwards.
So a higher voltage allows more current to be drawn, in a shorter pulse of energy.
rp181, Thu Jun 07 2012, 02:07AM
A couple things:
A higher capacitance doesn't mean a higher current, it means more energy. The current will only be high if the resistance of the coil is reduced, according to ohms law. The logic behing a higher voltage is that it allows a higher current to be drawn, a it can overcome the resistance of the coil. Utilizing a low voltage high capacitance source simply results in a longer pulse of a lower current. This will actually be detrimental, if the magnetic field is still present once the projectile passes through the coil, the projectile will be pulled backwards.
So a higher voltage allows more current to be drawn, in a shorter pulse of energy.
Re: Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
Thomas W, Thu Jun 07 2012, 02:15AM
yes, while a lower capacitance means a lower current (although ESR does have a significant part in the current too)
i also have reason to believe the higher volts per turn increases several things allowing for a better efficiency, along with the fact that lower current and higher voltage means that you can use thinner wires ofcourse, saving money and allowing for a more compact coilgun.
pardon me if im wrong, its 3:15 am
Thomas W, Thu Jun 07 2012, 02:15AM
yes, while a lower capacitance means a lower current (although ESR does have a significant part in the current too)
i also have reason to believe the higher volts per turn increases several things allowing for a better efficiency, along with the fact that lower current and higher voltage means that you can use thinner wires ofcourse, saving money and allowing for a more compact coilgun.
pardon me if im wrong, its 3:15 am
Re: Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
Dr. Brownout, Thu Jun 07 2012, 02:37AM
Tom you are correct, really think about it in amp turns. If you have 100 amps going through a one turn coil you have 100 amp turns, of course you need big copper too, now if you have 100 turns with 1 amp you can use smaller lower cost wire and have the same 100 amp turns. and hence the same force as a one turn with 100 amps.
If you use FEA software its pretty easy to see what kind of pullforce you will have on you coil based on the projectile material and placement. I think FEMM will do this I use something different but the results should be pretty much the same.
--Brian
Dr. Brownout, Thu Jun 07 2012, 02:37AM
Tom you are correct, really think about it in amp turns. If you have 100 amps going through a one turn coil you have 100 amp turns, of course you need big copper too, now if you have 100 turns with 1 amp you can use smaller lower cost wire and have the same 100 amp turns. and hence the same force as a one turn with 100 amps.
If you use FEA software its pretty easy to see what kind of pullforce you will have on you coil based on the projectile material and placement. I think FEMM will do this I use something different but the results should be pretty much the same.
--Brian
Re: Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
rp181, Thu Jun 07 2012, 04:03AM
The issue of pullback still remains. Using a lower capacitance with a higher voltage is generally good for hobbyist use. With a lower current and longer pulse may require active switching to cut the pulse short.
rp181, Thu Jun 07 2012, 04:03AM
The issue of pullback still remains. Using a lower capacitance with a higher voltage is generally good for hobbyist use. With a lower current and longer pulse may require active switching to cut the pulse short.
Re: Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
Barry, Wed Jun 13 2012, 08:21PM
The important thing is to maximize the capacitor's stored potential energy, as measured in joules. So choose your capacitor according to PE = 1/2 C V^2, and buy as much as you can afford. It can be useful to compare capacitors by their "price per joule" and don't worry about the voltage rating.
The photoflash capacitors are popular because they have an excellent "price per joule". However, they are small and it would take a large number to build a high energy coilgun.
The physics here says that the coil's magnetic field depends on coil current (not coil voltage); that's why it's measured in amp-turns. So, by the time the energy reaches the coil it really doesn't matter what voltage your capacitors were charged to.
The goal is to maximize "amp-turns" and to have the right LC time constant for the desired pulse time. To get them both right, the coil must be designed to match to the capacitance. For a given number of joules of stored energy, there is an endless variety of coil+capacitor combinations, but they will all end up with the same timing and the same amp-turns.
Also we want to minimize the total resistance through the high-current path. Coilgun performance is sensitive to just a few extra milliohms. This generally leads to coils made of heavy wire and just one or two layers.
Anyway, the choice of capacitor voltage is a practical or financial decision, and not so much an engineering issue. For example, my most efficient coilgun, the Mark 3, is charged to 12vdc from a small battery.
Cheers, Barry
Barry, Wed Jun 13 2012, 08:21PM
Yanom wrote ...
Why wouldn't you want to use a low voltage capacitor with high current output? What's the rationale?
Why wouldn't you want to use a low voltage capacitor with high current output? What's the rationale?
The important thing is to maximize the capacitor's stored potential energy, as measured in joules. So choose your capacitor according to PE = 1/2 C V^2, and buy as much as you can afford. It can be useful to compare capacitors by their "price per joule" and don't worry about the voltage rating.
The photoflash capacitors are popular because they have an excellent "price per joule". However, they are small and it would take a large number to build a high energy coilgun.
The physics here says that the coil's magnetic field depends on coil current (not coil voltage); that's why it's measured in amp-turns. So, by the time the energy reaches the coil it really doesn't matter what voltage your capacitors were charged to.
The goal is to maximize "amp-turns" and to have the right LC time constant for the desired pulse time. To get them both right, the coil must be designed to match to the capacitance. For a given number of joules of stored energy, there is an endless variety of coil+capacitor combinations, but they will all end up with the same timing and the same amp-turns.
Also we want to minimize the total resistance through the high-current path. Coilgun performance is sensitive to just a few extra milliohms. This generally leads to coils made of heavy wire and just one or two layers.
Anyway, the choice of capacitor voltage is a practical or financial decision, and not so much an engineering issue. For example, my most efficient coilgun, the Mark 3, is charged to 12vdc from a small battery.
Cheers, Barry
Re: Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
Yandersen, Fri Oct 05 2012, 03:57PM
I've done some theory about High Voltage @ lower current vs. High Current @ lower voltage and come to the conclusion that whichever you choose will not have impact on efficiency, as I thought earlier, simply looking at the formula of heat losses Eh=R*I*I which bring me to the idea that current should be minimized. No, it is not that simple. The only benefit of HV path is the possibility of using thinner wire, which is just easier to wound around a barrel. The end result I've got from playing with formulas is that highering the voltage by 2 times lets using a wire 2 times thinner.
But there is a little hidden advantage HV can give. Current path is not limited by coil only, but it is going through the capacitors and connection wires. The higher the voltage the higher ESR and ESL caps could be allowed and longer the connection wires could be for the same efficiency coilgun systems.
By my personal opinion, 400V is good voltage for coilgun to operate at. Lower voltages will require wire thick like a barrel, higher voltages are the way too dangerous.
Yandersen, Fri Oct 05 2012, 03:57PM
I've done some theory about High Voltage @ lower current vs. High Current @ lower voltage and come to the conclusion that whichever you choose will not have impact on efficiency, as I thought earlier, simply looking at the formula of heat losses Eh=R*I*I which bring me to the idea that current should be minimized. No, it is not that simple. The only benefit of HV path is the possibility of using thinner wire, which is just easier to wound around a barrel. The end result I've got from playing with formulas is that highering the voltage by 2 times lets using a wire 2 times thinner.
But there is a little hidden advantage HV can give. Current path is not limited by coil only, but it is going through the capacitors and connection wires. The higher the voltage the higher ESR and ESL caps could be allowed and longer the connection wires could be for the same efficiency coilgun systems.
By my personal opinion, 400V is good voltage for coilgun to operate at. Lower voltages will require wire thick like a barrel, higher voltages are the way too dangerous.
Re: Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?
Sulaiman, Sat Oct 06 2012, 08:22AM
As far as the coil/projectile is concerned
there is little theoretical difference between
high voltage, low current, thin wire and low voltage , high current, thick wire
BUT
the switch (thyristor/transistor/mechanical etc.)
has to handle both the peak voltage and current
so a compromise between switch rating, cost etc. is made.
The cost/esr/esl/energy storage of the discharge capacitor
is also a compromise.
Sulaiman, Sat Oct 06 2012, 08:22AM
As far as the coil/projectile is concerned
there is little theoretical difference between
high voltage, low current, thin wire and low voltage , high current, thick wire
BUT
the switch (thyristor/transistor/mechanical etc.)
has to handle both the peak voltage and current
so a compromise between switch rating, cost etc. is made.
The cost/esr/esl/energy storage of the discharge capacitor
is also a compromise.
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