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Registered Member #57984
Joined: Thu Nov 19 2015, 09:44AM
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To V2006.
I don't argue that supercaps have more energy capacity than common electrolytic ones. But their intrinsic characteristic is very low voltage which prevents them from being used in coilguns. We can of course connect them in series like you suggest, but such decision is not reasonable for the cost issues - the species you linked to are about 40 S per unit. To have 25...30 volts (lower voltages are impractical because of parasitic resistances presenting in any construction) we should use 10 pieces - about 400 S only for the power supply of our hypothetical coilgun. It is simpler to connect LiPos in series and get comparable power at higher volteges like guys from Arcflashlabs did. Besides, supercaps still need external accus to support their charge on a desirable level. Anyway, all these speculations (interesting of course) are out of the scope of my investigation - I suggested a conventional coilgun structure equipped with electrolytic capacitors.
To hen918 Supercapacitors are much smaller and lighter than ordinary electrolytic capacitors. I also doubted the properties of supercapacitors. Until I saw it:
Spot welder using 500Farad super capacitor , Ultra capacitor as spot welder
With lithium batteries it was already full of experiments. And with series-connected supercapacitors (in the amount of 20 - 30 pieces) no. The world has changed since the collapse of the USSR. When I have a box of supercapacitors, I will make an electromagnetic accelerator of the most incredible power.
Registered Member #57984
Joined: Thu Nov 19 2015, 09:44AM
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OK, I'm sure you will create an extraordinary supercapacitor coilgun which will shake the world. I wish you good luck. But I won't discuss this theme any more as it is out of topic of this thread.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Looks like nice work you did, Eugen.
Regarding supercapacitors, I will join the chorus saying that they're great for storing energy, but inferior to electrolytics for delivering power. (look up Ragone plot).
In another recent 4hv thread: we saw that internal resistance makes it impossible to discharge a supercap faster than time constant of about 1 second. ( Consistent with RC numbers given in this thread. Series or parallel banks of multiple caps have the same RC product as an individual capacitor. ) And that's with a short circuit, which delivers zero power to the load (because zero voltage), and is abusive to the capacitor. Of course this sub-forum is full of stories about abusing capacitors.
To deliver the most power from a supercapacitor to a resistive load, the load R needs to match the capacitor's internal R. Then when switch is closed, initial load voltage is 1/2 of initial capacitor voltage. Half of the stored energy goes to the load and half is lost inside the capacitor, warming it by a fraction of a degree. To get more of the energy to the load, discharge needs to be slower. Say, 10 seconds. Appropriate for accelerating a vehicle that makes frequent stops.
It's slightly different in coilguns, where inductance and stored energy in the coil are significant. That doesn't change the fact that aluminum electrolytic caps can always deliver more power than supercaps of similar size and weight. At lower powers, appropriate for both kinds of capacitor, the supercapacitor can run much longer. Coilgun pulses are too short for supercaps to be efficient, unless you want to get many shots out of a single charge.
Registered Member #57984
Joined: Thu Nov 19 2015, 09:44AM
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In general, from the point of view of my calculation, it's no matter what power source is used in a specific accelerator. It may be conventional eletrolytics, LiPos and other accus, or supercaps. The only influence is the more capacitance it has - the closer a system is to "overdamped" type and more exact are the formulae. Supercaps and LiPos can be suggested as caps with very large capacitance - that's the only difference. I just tried to emphasize that geometrical properties of the coils play major role in limiting the velocity of projectile - such that it may be assessed by the geometry itself with minimal additional information. A limitation concerning mass of the power source is only one example of how the demands layed on a portable system could shrink a range of results we got. Many other limitations may be taken into account, too, but the principle of the calculation will stay unchanged.
In your model max velocity seems to be limited by the slow decay of current after the coil is shorted. If current and with it, the magnetic field lasts too long, you will have suckback. Decay time is about L/R in your circuit, where L is the coils inductance and R its resistance. Thinner coils (D-d small) have a higher resistance or more precisely a lower L/R, which makes them suitable for higher velocities. By choosing turn off earlier, i.e. before the projectile is in the middle of the coil, that can be improved somewhat.
Much better would be a adding a resistance in the discharge path of your circuit. That would decrease decay time and at the same time allow for a less resistive, i.e. thicker coil. That will also increase efficiency.
Registered Member #57984
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to Uspring:
In your model max velocity seems to be limited by the slow decay of current after the coil is shorted. If current and with it, the magnetic field lasts too long, you will have suckback. Decay time is about L/R in your circuit, where L is the coils inductance and R its resistance. Thinner coils (D-d small) have a higher resistance or more precisely a lower L/R, which makes them suitable for higher velocities.
Yes you are right, the decay of current is another limiting factor. But it depends strongly on a specific circuit utilized in a coilgun (see below), so it isn't subject to simple analysis. May be my next investigation will be dedicated to this effect. As for the simple "freewheeling" scheme (without an additional resistance and other measures) you wrote about, it is used very rarely because current indeed decays very slowly here.
By choosing turn off earlier, i.e. before the projectile is in the middle of the coil, that can be improved somewhat.
The problem is that the accelerating force has its maximum near to the middle of the coil, too. So the early turn-off the coil would substantially decrease efficiency of our accelerator. More promising is early turn-on - not when a projectile enters the coil, but in advance, especially for high initial velocities.
Much better would be a adding a resistance in the discharge path of your circuit. That would decrease decay time and at the same time allow for a less resistive, i.e. thicker coil. That will also increase efficiency.
In general, current -damping schemes may be divided into 3 groups depending on what measure is used to absorb inductive energy of the coil after turn-off:
1) Simple resistive (or R+D) - its just what you wrote about.
As I show here, damping resistor is set by a fairly simple equation. The point is that it must have considerable (in comparison to the coil's) resistace to enforce the decay of current, which automatically means high voltage spike on a power switch - the faster current decays, the higher the voltage is. As an example, in this construction its author had to use high-voltage IGBTs while the power source is only 50 V battery, and still he wrote that he faced a problem of the switch failure because of the inductive spike.
2) Varistor circuit.
In this scheme a special component -varistor - is connected in parralel to the switch or to the coil. It has nearly constant breakdown voltage, so it ensures nearly linear fast current decay. A kind of such circuit is a scheme where only MOSFET is used (without any additional damping circuitry) and all the energy is absorbed by its internal Zener diode. Very careful calculation must be provided here to be sure that the energy doesn't exceed the limits of a specific transistor. As a rule, only the most powerful switches in TO-247 cases may be used in such configuration.
3) Halfbridge circuits.
The inverse initail voltage is applied to the coil in this case, so the current decreases as fast as it has increased. The main feature of halfbridge is its ability for recuperation of energy back into the capacitor. I examined this question in details here and here. But the halfridge is not an ideal from the point of view of fast current decay - its optimal current form is close to triangle, so we must choose the coils attentively in accordance to this circuit and conduct FEMM or other modelling to be sure that no suck-back effect occurs.
As I show here, damping resistor is set by a fairly simple equation. The point is that it must have considerable (in comparison to the coil's) resistace to enforce the decay of current, which automatically means high voltage spike on a power switch - the faster current decays, the higher the voltage is. As an example, in this construction its author had to use high-voltage IGBTs while the power source is only 50 V battery, and still he wrote that he faced a problem of the switch failure because of the inductive spike.
I think, you are choosing the damping resistor RD too large. Select it in a way, that the initial current through it will cause a voltage drop of about U0. That will cause a decay time of the same duration as the charge up time and a transient across the transistor of 2*U0. The current pulse length is then twice the charge up time (=on time of transistor). The current pulse length can be chosen to fit into the interval between the projectile entering the coil and its middle position.
In order to achieve a sufficiently high current into the coil for fast projectiles and corresponding short pulse lengths, the inductance of the coil needs to be made accordingly small. Large velocities seem feasible.
Registered Member #57984
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That will cause a decay time of the same duration as the charge up time and a transient across the transistor of 2*U0. The current pulse length is then twice the charge up time (=on time of transistor). The current pulse length can be chosen to fit into the interval between the projectile entering the coil and its middle position
Unfortunately, such a simple approach cannot yield high efficiency acceleration. The problem is that we must tune the current in such a way that it coincides with the form of pulling force, which is highly assymetrical. The example below is from Barry's site, but can also be obtained by FEMM for any another coil:
We can see that maximum force is situated near the center of the coil (or, more exactly, accords to the moment when the projectile is nearly fully pulled into the coil). That is why the current's maximum must be somewhere about that, too. So an ideal current form should be like sawtooth with long front and very fast end,nearly coinciding with the moment when the projectile's center reaches the center of the coil.
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Maximum speed in multistage accelerator with identical windings
