Noob question - why are coilguns often high voltage,low current instead of high current,low voltage? / Electromagnetic Projectile Accelerators / Forums

Forums

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

« Previous topic | Next topic »

Noob question - why are coilguns often high voltage,low current instead of high current,low voltage?

Move Thread

LAN_403

Yanom

Thu Jun 07 2012, 01:08AM

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

I'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?

rp181

Thu Jun 07 2012, 02:07AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

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.

Thomas W

Thu Jun 07 2012, 02:15AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

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

Dr. Brownout

Thu Jun 07 2012, 02:37AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

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

rp181

Thu Jun 07 2012, 04:03AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

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.

Barry

Wed Jun 13 2012, 08:21PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Yanom wrote ...

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

Yandersen

Fri Oct 05 2012, 03:57PM

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

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.

Sulaiman

Sat Oct 06 2012, 08:22AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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.

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