Permanent Magnet Rail Gun (PMRG)
Marko, Sat Oct 27 2012, 04:25PMHello everyone
Seeing that little amateur work has been done lately in the railgun field, I decided to do some small scale research myself. In short, I decided to ignore many of the commonly established railgun concepts and pursue my own ways. The most important establishments I've made so far are:
- An amateur rail gun is obviously a low speed device. A tipycal case of 1cm wide projectile moving at speed of sound in 1 tesla magnetic field only produces about 3 volts of back EMF. Only the work done against this back EMF is the energy transferred to the projectile! Hence, the commonly used power supplies involving 300V+ electrolytic caps can be expected to have extremely poor efficiency at these low speeds.
- A solid metal projectile is clearly a no-go. Brushes are essential, though they still seem to be lossier than I desire. For an efficient low-speed railgun, the resistance of the whole circuit has to be well under a miliohm which is extremely hard to acheive. My last resort weapon are going to be liquid metal wetted brushes.
- In order to increase projectile EMF at low speeds, I used neodymium magnets for augmentation. The magnetic path is enclosed by an iron core to get about 1 Tesla field in the air gap. Their usefulness is questionable though, at high currents (10 000A+) their effect fades and may even get demagnetized at some point.
- Power supply is an extremely difficult part due to extremely low internal resistance required. I have still not decided which way to take, though it's clearly not going to be tens of kilojoules of 400V lytics. Efficient shooting requires large amount of charge (50Coulombs or so) at low voltages, implying farad range capacitances.
One possibility would be to use many parallel car audio capacitors, which are in range of farads, up to 25V and about 2 miliohm internal resistance. Depending on brush resistance I acheive, it may pay out to use two in series (at cost of efficiency). Only a fraction of energy stored in the caps would be used per shot, making this an expensive approach unless I can find some cheap source of these caps.
Another possibility, which I will more likely go for, would be to use a transformer driven by IGBT bricks to convert 400V+ capacitor voltages to low voltages suitable for the railgun. For example, the supply would take 500V at 500A to produce 25V at 10 000A. Losses would be greatly reduced due to comparatively low currents that are now taken from capacitors. The transformer output would be rectified by a whole bunch of schottky diodes, and the secondary may have to be resonated in order to remove the negative effect of leakage inductance.
UPDATE
Obviously, I want a lot more destruction for this system, and a power supply with much better characteristics than a lead acid battery is required for this. After talking with Steve for a while, we reached an interesting idea: a fairly big iron transformer is used to drive the gun directly, with 400V electrolytics being discharged into primary by a big SCR. The transformer would have a third winding which would be used to presaturate the core in opposite direction before firing to get 2x the volt seconds in the pulse. Pics of that coming some time later - accepting all suggestions as well!
Marko
Raw materials:
The mess of drilling:
Ready to glue the magnets, paper insulation placed underneath the rails:
Magnets glued:
Almost done, rails polished:
Thyristor switch:
Projectile construction:
Damage to apple by lead acid battery:
Re: Permanent Magnet Rail Gun (PMRG)
Steve Conner, Sat Oct 27 2012, 05:05PM
Well done! What are you using for the switch in your test setup? Some sort of MOSFET brick?
Steve Conner, Sat Oct 27 2012, 05:05PM
Well done! What are you using for the switch in your test setup? Some sort of MOSFET brick?
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Sat Oct 27 2012, 07:35PM
Hi Steve
The brick is actually 6 parallel thyristor modules that I previously used for trans-cranial magnetic stimulation experiments. I didn't bother to disassemble them, even though a single one would probably do for this test.
My guessimation is that the current was less than 1000A, leading to projectile velocity of some 50m/s. BTW, the battery has a rating of 850A written on it - is this short circuit current, or some other value? If the rumor that starter batteries tend to have 10-20 miliohms internal resistance, I'd guess it may be short circuit current?
I have some 21000uF, 100V caps around, but am unsure whether to even bother trying using them. They store way too little charge and are likely to get discharged very early in the firing...
Marko
Marko, Sat Oct 27 2012, 07:35PM
Hi Steve
The brick is actually 6 parallel thyristor modules that I previously used for trans-cranial magnetic stimulation experiments. I didn't bother to disassemble them, even though a single one would probably do for this test.
My guessimation is that the current was less than 1000A, leading to projectile velocity of some 50m/s. BTW, the battery has a rating of 850A written on it - is this short circuit current, or some other value? If the rumor that starter batteries tend to have 10-20 miliohms internal resistance, I'd guess it may be short circuit current?
I have some 21000uF, 100V caps around, but am unsure whether to even bother trying using them. They store way too little charge and are likely to get discharged very early in the firing...
Marko
Re: Permanent Magnet Rail Gun (PMRG)
..., Sat Oct 27 2012, 08:35PM
The 850A is usually 'cold cranking amps' or the likes, which means it can deliver 850A for 30s without the terminal voltage dropping below ~7.2v at 0° Fahrenheit
The short current will be much more than that, probably on the order of 2-10kA depending on charge, temperature, health, etc
..., Sat Oct 27 2012, 08:35PM
The 850A is usually 'cold cranking amps' or the likes, which means it can deliver 850A for 30s without the terminal voltage dropping below ~7.2v at 0° Fahrenheit
The short current will be much more than that, probably on the order of 2-10kA depending on charge, temperature, health, etc
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Mon Oct 29 2012, 03:22AM
I have always believed this to be the solution, using banks of HV caps for a low energy transfer, has always seem to be the weak point in everyones attempts. (including really smart people like Sam Barros).
Ive alwayts wanted to try this with transformers, but i dont have the time or money, however the US Navy wants railguns in the next few decades to be practical.
Patrick, Mon Oct 29 2012, 03:22AM
Marko wrote ...
Another possibility, which I will more likely go for, would be to use a transformer driven by IGBT bricks to convert 400V+ capacitor voltages to low voltages suitable for the railgun. For example, the supply would take 500V at 500A to produce 25V at 10 000A. Losses would be greatly reduced due to comparatively low currents that are now taken from capacitors. The transformer output would be rectified by a whole bunch of schottky diodes, and the secondary may have to be resonated in order to remove the negative effect of leakage inductance.
Another possibility, which I will more likely go for, would be to use a transformer driven by IGBT bricks to convert 400V+ capacitor voltages to low voltages suitable for the railgun. For example, the supply would take 500V at 500A to produce 25V at 10 000A. Losses would be greatly reduced due to comparatively low currents that are now taken from capacitors. The transformer output would be rectified by a whole bunch of schottky diodes, and the secondary may have to be resonated in order to remove the negative effect of leakage inductance.
I have always believed this to be the solution, using banks of HV caps for a low energy transfer, has always seem to be the weak point in everyones attempts. (including really smart people like Sam Barros).
Ive alwayts wanted to try this with transformers, but i dont have the time or money, however the US Navy wants railguns in the next few decades to be practical.
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Mon Oct 29 2012, 03:43PM
Hi guys
Well, hmm, it'd be great if a lead acid battery can provide 10kA, though I'm somewhat skeptical about that...
The next thing I have to do is to set up some measurements. This involves
1.) Measuring the input current to the railgun with a suitably large shunt
2.) Measuring the voltage at the ends of tails
3.) Measure the projectile velocity
4.) Record everything on a microcontroller, fpga or oscilloscope and compare data
So far I don't have much idea what contributes the most to the resistance in my circuit, though I'm afraid it's still the projectile-rail interface.
The projectile used was a piece of square wire bent into U shape, after which a piece of R>G-174 coax sheath was pulled over it. After shooting, signs of melting were visible and the braid has welded to the wire in some places. Obviously, significant heating still occured, despite it was far better than the solid projectile.
I would like to compare the current situation with the projectile brushes wetted with liquid metal. I'm certain there is going to be some improvement that way!
Only then I can decide what power supply voltages and currents I actually need.
Well, I intend to use a setup pretty similar to a DRSSTC, except the output will be low voltage and rectified into DC. I think it would not be too ahrd to modify a driver of a suitably large brick DRSSTC for this application.
Well, the navy railgun accellerates projectiles to many thousand m/s so the situation is somewhat different there... the EMF on their projectile is high enough to allow for use of high voltage caps (IIRC, they use 4-5kV).
What I'm more interested about is why they only attempted using a fairly simple armature system which does seem to erode rails greatly upon firing. Still, circumstances there are far different from an amateur model and fiber brushes or liquid metal may just not work at these extreme speeds.
Marko
Marko, Mon Oct 29 2012, 03:43PM
Hi guys
Well, hmm, it'd be great if a lead acid battery can provide 10kA, though I'm somewhat skeptical about that...
The next thing I have to do is to set up some measurements. This involves
1.) Measuring the input current to the railgun with a suitably large shunt
2.) Measuring the voltage at the ends of tails
3.) Measure the projectile velocity
4.) Record everything on a microcontroller, fpga or oscilloscope and compare data
So far I don't have much idea what contributes the most to the resistance in my circuit, though I'm afraid it's still the projectile-rail interface.
The projectile used was a piece of square wire bent into U shape, after which a piece of R>G-174 coax sheath was pulled over it. After shooting, signs of melting were visible and the braid has welded to the wire in some places. Obviously, significant heating still occured, despite it was far better than the solid projectile.
I would like to compare the current situation with the projectile brushes wetted with liquid metal. I'm certain there is going to be some improvement that way!
Only then I can decide what power supply voltages and currents I actually need.
I have always believed this to be the solution, using banks of HV caps for a low energy transfer, has always seem to be the weak point in everyones attempts. (including really smart people like Sam Barros).
Ive alwayts wanted to try this with transformers, but i dont have the time or money
Well, I intend to use a setup pretty similar to a DRSSTC, except the output will be low voltage and rectified into DC. I think it would not be too ahrd to modify a driver of a suitably large brick DRSSTC for this application.
however the US Navy wants railguns in the next few decades to be practical.
Well, the navy railgun accellerates projectiles to many thousand m/s so the situation is somewhat different there... the EMF on their projectile is high enough to allow for use of high voltage caps (IIRC, they use 4-5kV).
What I'm more interested about is why they only attempted using a fairly simple armature system which does seem to erode rails greatly upon firing. Still, circumstances there are far different from an amateur model and fiber brushes or liquid metal may just not work at these extreme speeds.
Marko
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Mon Oct 29 2012, 04:11PM
Would a low strength/low melting point metal be useful for your armatures interface? like solder, i would sure hate to use toxic mercury...
Patrick, Mon Oct 29 2012, 04:11PM
Would a low strength/low melting point metal be useful for your armatures interface? like solder, i would sure hate to use toxic mercury...
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Mon Oct 29 2012, 04:28PM
Hi Patrick
My plan is to use this alloy, due to it's good balance of cost and melting point:
The whole railgun would be wrapped with a coil of wire and induction heated to some 50 deg C before firing, in order to keep this alloy in molten state.
A Gallium-indium-tin alloy would be superior because it's liquid at room temperature, but it's cost and shipping concerns make it a bit prohibitive for me at this time.
My primary concern though is possible amalgamation of gallium with copper, resulting in increase in melting point or even compromised mechanical properties of the copper rails. I know it does bad things to aluminum, but no idea yet for copper. Ideally, the alloy should wet both metals well and at same time keep it's low melting point.
It seems that I'll have to do an experiment to find out more about this!
Marko
Marko, Mon Oct 29 2012, 04:28PM
Hi Patrick
My plan is to use this alloy, due to it's good balance of cost and melting point:
The whole railgun would be wrapped with a coil of wire and induction heated to some 50 deg C before firing, in order to keep this alloy in molten state.
A Gallium-indium-tin alloy would be superior because it's liquid at room temperature, but it's cost and shipping concerns make it a bit prohibitive for me at this time.
My primary concern though is possible amalgamation of gallium with copper, resulting in increase in melting point or even compromised mechanical properties of the copper rails. I know it does bad things to aluminum, but no idea yet for copper. Ideally, the alloy should wet both metals well and at same time keep it's low melting point.
It seems that I'll have to do an experiment to find out more about this!
Marko
Re: Permanent Magnet Rail Gun (PMRG)
..., Mon Oct 29 2012, 04:54PM
Worth noting with French's metal is that it has cadmium and lead, so it may not be the best thing to be vaporizing. I was actually researching low melting point metals last week looking for some non-toxic solder that melts at 50-100c, and the only two that were free of cadmium, and mercury were Fields metal (mp 62c) and alloys of Indium/Tin (basically the same thing as fields metal, minus the bismuth ex mp 110c) I didn't really look into the gallium based alloys since they had a lower melting point than I was looking for, and like you said are expensive!
I am not sure if your application could handle the >100c melting point temperatures, but it is worth mentioning that bismuth has terrible electrical/thermal properties, being about 1/10th as conductive as indium and 1/80 as conductive as copper.
..., Mon Oct 29 2012, 04:54PM
Worth noting with French's metal is that it has cadmium and lead, so it may not be the best thing to be vaporizing. I was actually researching low melting point metals last week looking for some non-toxic solder that melts at 50-100c, and the only two that were free of cadmium, and mercury were Fields metal (mp 62c) and alloys of Indium/Tin (basically the same thing as fields metal, minus the bismuth ex
mp 110c) I didn't really look into the gallium based alloys since they had a lower melting point than I was looking for, and like you said are expensive!I am not sure if your application could handle the >100c melting point temperatures, but it is worth mentioning that bismuth has terrible electrical/thermal properties, being about 1/10th as conductive as indium and 1/80 as conductive as copper.
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Mon Oct 29 2012, 06:31PM
Oooo, keepig it near its mealting point upto the second of firing, then you really pulse the inductive heat. This kind of thinking may lead to a break through... youve clearly put a lot of thought into this... that Frenchs metal looks like a good starting point, its cheap enough, yet will stay solid when your device is not in use.
EDIT: mercury isnt so bad, just volatize some of that, felt hatters were likely nuts to begin with.
EDIT 2: I was being facitous.
EDIT 3: Isnt there a U-boat missing of the norway coast? with 4000 liters of mecurcy in its keal?
Off topic but still interesting, Action of 9 Febuary, 1945.
The first time a submerged ship shot another submerged target and killed it, and has never been done since.
(65 tons of mercury on board)
This is one of those historic martial events like Major Phil Handly's 20mm auto-cannon kill of a MiG 19 from a F-4 at Mach 1.2, Using a high speed deflection shot-- the first and only time in history a supersonic gun kill was made.
Patrick, Mon Oct 29 2012, 06:31PM
Oooo, keepig it near its mealting point upto the second of firing, then you really pulse the inductive heat. This kind of thinking may lead to a break through... youve clearly put a lot of thought into this... that Frenchs metal looks like a good starting point, its cheap enough, yet will stay solid when your device is not in use.
EDIT: mercury isnt so bad, just volatize some of that, felt hatters were likely nuts to begin with.
EDIT 2: I was being facitous.
EDIT 3: Isnt there a U-boat missing of the norway coast? with 4000 liters of mecurcy in its keal?
Off topic but still interesting,
Action of 9 Febuary, 1945. The first time a submerged ship shot another submerged target and killed it, and has never been done since.
(65 tons of mercury on board)
This is one of those historic martial events like Major Phil Handly's 20mm auto-cannon kill of a MiG 19 from a F-4 at Mach 1.2, Using a high speed deflection shot-- the first and only time in history a supersonic gun kill was made.
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Tue Oct 30 2012, 10:56PM
Hi guys
Sorry to hear that the french's alloy contains cadmium and lead... I just looked it up myself. If the system works, there would be little or none of it vaporized per shot (probably the most at the moment projectile leaves the barrel). i'm not particularly afraid of metallic lead either, I already have few hundred kg of it laying around the garage! But I'm unfamiliar with cadmium, and I wouldn't want it to get onto everywhere and into environment.
I've also found field's metal on ebay, only a bit more expensive but alsoa higher melting point, would need more heating to get going.
I also hoped that some of you guys would be able to tell me more about chemical and surface properties of these alloys, like ability to wet copper, and also the possibilities of troublesome amalgamation that may occur over time (gallium spectacularly attacks aluminum, for example).
I want to know if I wet my rails with liquid metal, that it won't form an alloy with copper over time and lose it's ability to melt (then I'm screwed!)
Cheers,
Marko
Marko, Tue Oct 30 2012, 10:56PM
Hi guys
Sorry to hear that the french's alloy contains cadmium and lead... I just looked it up myself. If the system works, there would be little or none of it vaporized per shot (probably the most at the moment projectile leaves the barrel). i'm not particularly afraid of metallic lead either, I already have few hundred kg of it laying around the garage! But I'm unfamiliar with cadmium, and I wouldn't want it to get onto everywhere and into environment.
I've also found field's metal on ebay, only a bit more expensive but alsoa higher melting point, would need more heating to get going.
I also hoped that some of you guys would be able to tell me more about chemical and surface properties of these alloys, like ability to wet copper, and also the possibilities of troublesome amalgamation that may occur over time (gallium spectacularly attacks aluminum, for example).
I want to know if I wet my rails with liquid metal, that it won't form an alloy with copper over time and lose it's ability to melt (then I'm screwed!)
Cheers,
Marko
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Mon Nov 05 2012, 09:15PM
Hi guys
Update with some pictures and future prospectives!
Marko
Marko, Mon Nov 05 2012, 09:15PM
Hi guys
Update with some pictures and future prospectives!
Marko
Re: Permanent Magnet Rail Gun (PMRG)
..., Mon Nov 05 2012, 11:40PM
I know that indium (and most alloys of it) will wet copper, I have used it for soldering to copper in the past. That said, it requires flux, any oxidation on the copper prevents it from sticking. I wonder if it would be worth it to pre-wet the rails with the metal? I am envisioning a pre-firing procedure where you stand the gun on end, put a plug in the breech, pour in some flux and your solder of choice, and then push/pull the plug down the rails to give them a nice even, fresh, coat of solder. Then load in the projectile, maybe with a little plug of solder in front of it for good measure. Free added benefit of having the barrel lubricated
..., Mon Nov 05 2012, 11:40PM
I know that indium (and most alloys of it) will wet copper, I have used it for soldering to copper in the past. That said, it requires flux, any oxidation on the copper prevents it from sticking. I wonder if it would be worth it to pre-wet the rails with the metal? I am envisioning a pre-firing procedure where you stand the gun on end, put a plug in the breech, pour in some flux and your solder of choice, and then push/pull the plug down the rails to give them a nice even, fresh, coat of solder. Then load in the projectile, maybe with a little plug of solder in front of it for good measure. Free added benefit of having the barrel lubricated
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Tue Nov 06 2012, 01:56AM
Hi Peter
Yes, I originally intended to have entire rails pre-wetted; I wouldn't expect the projectile to do this well during firing! And yes, I also thought some flux would be necessary for this to go well. I'm not sure if normal high temp soldering flux would do - is there any alternative you would recommend for lower temperatures?
By the way - I also know a possible alternative to liquid metal, and that is those electrically conductive greases used by linemen for power line connections. I don't know what properties of this stuff are nor where could I get some. I'm afraid it could actually be more expensive than low melting point alloys!
However, I'm currently concentrating on power supply for this cannon, as well as measurements. It's crucial to get some idea of the contact resistance and how it changes with various types of brushes. It'll take a while before I reach the liquid metal wetting phase!
Marko
Marko, Tue Nov 06 2012, 01:56AM
Hi Peter
Yes, I originally intended to have entire rails pre-wetted; I wouldn't expect the projectile to do this well during firing! And yes, I also thought some flux would be necessary for this to go well. I'm not sure if normal high temp soldering flux would do - is there any alternative you would recommend for lower temperatures?
By the way - I also know a possible alternative to liquid metal, and that is those electrically conductive greases used by linemen for power line connections. I don't know what properties of this stuff are nor where could I get some. I'm afraid it could actually be more expensive than low melting point alloys!
However, I'm currently concentrating on power supply for this cannon, as well as measurements. It's crucial to get some idea of the contact resistance and how it changes with various types of brushes. It'll take a while before I reach the liquid metal wetting phase!
Marko
Re: Permanent Magnet Rail Gun (PMRG)
Shrad, Tue Nov 06 2012, 01:01PM
why not try liquid tin like the one you'd use for printed circuit board chemical tinning?
Shrad, Tue Nov 06 2012, 01:01PM
why not try liquid tin like the one you'd use for printed circuit board chemical tinning?
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Tue Nov 06 2012, 01:44PM
Hi Shrad
Well, pure tin solder has a melting point at over 200C which would destroy my insulation, glue, and possibly the magnets. I need stuff with melting point reasonably close to room temperature in order for this idea to be worth of hassle.
Marko
Marko, Tue Nov 06 2012, 01:44PM
Hi Shrad
Well, pure tin solder has a melting point at over 200C which would destroy my insulation, glue, and possibly the magnets. I need stuff with melting point reasonably close to room temperature in order for this idea to be worth of hassle.
Marko
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Tue Nov 06 2012, 05:22PM
tin is one of those scary metals, tin whiskers have killed other peoples work on many occasions. At high temp and high velocity i wouldnt want to tempt fate either.
Patrick, Tue Nov 06 2012, 05:22PM
tin is one of those scary metals, tin whiskers have killed other peoples work on many occasions. At high temp and high velocity i wouldnt want to tempt fate either.
Re: Permanent Magnet Rail Gun (PMRG)
Shrad, Wed Nov 07 2012, 07:51AM
I was talking about chemical tinning agent which could be applied to the rails for wetting, but if you say it's not suitable.... ;)
I think you're stuck to indium or even lower melting point metal, but price is not the same
Shrad, Wed Nov 07 2012, 07:51AM
I was talking about chemical tinning agent which could be applied to the rails for wetting, but if you say it's not suitable.... ;)
I think you're stuck to indium or even lower melting point metal, but price is not the same
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Wed Nov 07 2012, 05:49PM
cheap, low melting point, non-poisoinus, seems to be his most immeadiate requirements.
Patrick, Wed Nov 07 2012, 05:49PM
cheap, low melting point, non-poisoinus, seems to be his most immeadiate requirements.
Re: Permanent Magnet Rail Gun (PMRG)
Josh Campbell, Thu Nov 08 2012, 01:09AM
Difficult task... it seems gallium is the best choice given the cost and it's ability to wet metals even with a bit of oxidation in the way. This is primarily the case be cause gallium is corrosive to all metals except tungsten and tantalum. Which presents another issue. In fact liquid metals can dissolve most metals. That might make their use as a room temp wetting agent tricky. However the level of corrosiveness on say, oxygen-free hard tempered copper may be low enough that the effects of corrosion are tolerable.
Interesting table that shows how high melting points are grouped around tungsten (W) which has the highest melting point.
Will be interesting to see what you come up with in testing.
Josh Campbell, Thu Nov 08 2012, 01:09AM
Difficult task... it seems gallium is the best choice given the cost and it's ability to wet metals even with a bit of oxidation in the way. This is primarily the case be cause gallium is corrosive to all metals except tungsten and tantalum. Which presents another issue. In fact liquid metals can dissolve most metals. That might make their use as a room temp wetting agent tricky. However the level of corrosiveness on say, oxygen-free hard tempered copper may be low enough that the effects of corrosion are tolerable.
Interesting table that shows how high melting points are grouped around tungsten (W) which has the highest melting point.
Will be interesting to see what you come up with in testing.
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Thu Nov 08 2012, 02:47AM
im going to class tomarrow, ill stop buy theh chemistry lab prepareres room and ask about low melting point metal and wetting, do your rails have to be copper ? aluminum, gold and silver are probably prohibitively expensive or prone to liquidification.
EDIT: have you been using the car battery? i think from a new battery you can get dangerously high currents, much more than it takes to get starter motor to turn.
Patrick, Thu Nov 08 2012, 02:47AM
2bytes wrote ...
In fact liquid metals can dissolve most metals.
i Have always wanted to build an AIM-9 Sidewinder missle with 40 lbs of mercury as the warhead, just to piss off the russians.In fact liquid metals can dissolve most metals.
im going to class tomarrow, ill stop buy theh chemistry lab prepareres room and ask about low melting point metal and wetting, do your rails have to be copper ? aluminum, gold and silver are probably prohibitively expensive or prone to liquidification.
EDIT: have you been using the car battery? i think from a new battery you can get dangerously high currents, much more than it takes to get starter motor to turn.
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Thu Nov 08 2012, 07:10PM
Hi guys
Yes, one of my concerns is that gallium based liquid metal may alloy with the surface of copper and remain solid and unmeltable at practical temperatures - or even worse, it may embrittle copper like it would aluminum.
Gallium-indium-tin would be perfect, though too expensive to ship. I'm most likely going to use french's metal - need to find a way to limit the release of cadmium though...
Well, yes, and this battery was fairly new, though I'm not convinced I can get the currents I require with a reasonable number of batteries. The battery is rated to drop half it's voltage at cold cranking current, which yields about 8 miliohms of internal resistance. Even if the "hot cranking" situation is 2x better it's all still way too much resistance. I need less than a miliohm over entire current path!
Has anyone actually measured short circuit currents of some of the best lead acid batteries? That'd make some very valuable data for me.
Marko
Marko, Thu Nov 08 2012, 07:10PM
Hi guys
Yes, one of my concerns is that gallium based liquid metal may alloy with the surface of copper and remain solid and unmeltable at practical temperatures - or even worse, it may embrittle copper like it would aluminum.
Gallium-indium-tin would be perfect, though too expensive to ship. I'm most likely going to use french's metal - need to find a way to limit the release of cadmium though...
EDIT: have you been using the car battery? i think from a new battery you can get dangerously high currents, much more than it takes to get starter motor to turn.
Well, yes, and this battery was fairly new, though I'm not convinced I can get the currents I require with a reasonable number of batteries. The battery is rated to drop half it's voltage at cold cranking current, which yields about 8 miliohms of internal resistance. Even if the "hot cranking" situation is 2x better it's all still way too much resistance. I need less than a miliohm over entire current path!
Has anyone actually measured short circuit currents of some of the best lead acid batteries? That'd make some very valuable data for me.
Marko
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Thu Nov 08 2012, 08:12PM
Patrick, Thu Nov 08 2012, 08:12PM
Marko wrote ...
Has anyone actually measured short circuit currents of some of the best lead acid batteries? That'd make some very valuable data for me.
i have 2 brand new large group size batteries, how can i do this test for you? i have a USB Oscope good for 2MHz, and bolt stud SCR's a low value resistor for high power would be needed too. i coulduse tham single, parallel or series.Has anyone actually measured short circuit currents of some of the best lead acid batteries? That'd make some very valuable data for me.
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Thu Nov 08 2012, 09:24PM
It's not the easiest test to do. You need a high current shunt and some way of breaking the circuit after you fire the SCR. A big fuse could be used to break the circuit. The fuse may interfere with the measurement, though. You could also simply have a plug plug or big switch which you pull manually a fraction of second you fire the scr... or just use the switch/plug alone without the scr. This would be the simplest and most accurate method, but I wouldn't really call it safe and good for the battery.. your contacts may weld or some other horror may happen.
I'd probably simply press two big pieces of copper braid together as a crude switch, which can be pulled apart quickly. To my understanding of physics, a few seconds of short circuit should not damage the battery by thermal means, but I may be wrong, no idea about possible negative battery health consequences!
Marko
Marko, Thu Nov 08 2012, 09:24PM
It's not the easiest test to do. You need a high current shunt and some way of breaking the circuit after you fire the SCR. A big fuse could be used to break the circuit. The fuse may interfere with the measurement, though. You could also simply have a plug plug or big switch which you pull manually a fraction of second you fire the scr... or just use the switch/plug alone without the scr. This would be the simplest and most accurate method, but I wouldn't really call it safe and good for the battery.. your contacts may weld or some other horror may happen.
I'd probably simply press two big pieces of copper braid together as a crude switch, which can be pulled apart quickly. To my understanding of physics, a few seconds of short circuit should not damage the battery by thermal means, but I may be wrong, no idea about possible negative battery health consequences!
Marko
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Thu Nov 08 2012, 10:42PM
what if i usea cap bank to simulate a ultra low resistance, high current temporary load. i have a bunch of 3300uF 20 volt caps that could be banked.
or better yet i could get 000 awg welding cable and then 10 awg wire and make the resistance shunt do the melting/switching.
Patrick, Thu Nov 08 2012, 10:42PM
what if i usea cap bank to simulate a ultra low resistance, high current temporary load. i have a bunch of 3300uF 20 volt caps that could be banked.
or better yet i could get 000 awg welding cable and then 10 awg wire and make the resistance shunt do the melting/switching.
Re: Permanent Magnet Rail Gun (PMRG)
Marko, Fri Nov 09 2012, 04:51PM
A few 3300uF caps would have internal resistance much higher than the battery itself. The idea may work if you could find farads worth of caps wth sub-miliohm esr, not something very practical.
I'd probably just make some sort of a switch that can be quickly connected and disconnected manually (no scr's) and use a DSO to capture the shunt voltage O_O
Marko
Marko, Fri Nov 09 2012, 04:51PM
Patrick wrote ...
what if i usea cap bank to simulate a ultra low resistance, high current temporary load. i have a bunch of 3300uF 20 volt caps that could be banked.
or better yet i could get 000 awg welding cable and then 10 awg wire and make the resistance shunt do the melting/switching.
what if i usea cap bank to simulate a ultra low resistance, high current temporary load. i have a bunch of 3300uF 20 volt caps that could be banked.
or better yet i could get 000 awg welding cable and then 10 awg wire and make the resistance shunt do the melting/switching.
A few 3300uF caps would have internal resistance much higher than the battery itself. The idea may work if you could find farads worth of caps wth sub-miliohm esr, not something very practical.
I'd probably just make some sort of a switch that can be quickly connected and disconnected manually (no scr's) and use a DSO to capture the shunt voltage O_O
Marko
Re: Permanent Magnet Rail Gun (PMRG)
Patrick, Fri Nov 09 2012, 06:04PM
maybe a heavy duty steel spring and lever that breaks and makes quickly.
Patrick, Fri Nov 09 2012, 06:04PM
maybe a heavy duty steel spring and lever that breaks and makes quickly.
Print this page