Omega Owners Forum
Chat Area => General Discussion Area => Topic started by: TheBoy on 30 March 2014, 19:32:00
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Mate of mine has a project where they need to test if they can get several hundred amps down a bit of coax for a very short period of time, and asked me if I knew of an automotive relay man enough. Starter solenoid was best I could think of, but as built into most starters, makes it a bit cumbersome/bulky.
Initial test is going to be at 12v (hence automotive relays considered), but ultimately will go up into the kV range if tests are positive, and this is when big money will need to be spent. But for now, we're looking for a cheap-as-chips solution to prove the cable can withstand the huge currents required.
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Come on think old school,like this
http://www.ebay.co.uk/itm/like/400398269358?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&device=t&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=108&ff19=0
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Come on think old school,like this
http://www.ebay.co.uk/itm/like/400398269358?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&device=t&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=108&ff19=0
We had, but thought that those crappy old solenoids, which didn't hold up too well at the time, only turned over poxy little engines, thus low rated. Do you know what they are rated at? Looking in region of 500-900A
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The biggest relay I have found is 100amp which I used in a home made split charge system on my camper van.
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Do they know what they are doing?
A relay or contactor will not give you a controlled pulse period, the currents they are talking will only give at a guess a matter of ms before the core is fried.
It needs a silicon device such as an IGBT
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Do they know what they are doing?
A relay or contactor will not give you a controlled pulse period, the currents they are talking will only give at a guess a matter of ms before the core is fried.
It needs a silicon device such as an IGBT
Mostly, yes. There is a concern about timing accuracy of relays or electro-mechanical switching, but at this stage, low cost experimentation if needed to see if the coax will handle that sort of current, before proper money is spent. 900A relays are readily available, but are approx. £2k a pop, too expensive for an early trial.
As I understand it, it needs a 20mS test at 500-900A for the first stage of the trial.
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Specialist equipment is available for testing switchgear.
http://www.trtest.com/page.php?cid=21 (http://www.trtest.com/page.php?cid=21)
http://www.megger.com/uk/Products/ProductDetails.php?ID=1563&Description=ingvar (http://www.megger.com/uk/Products/ProductDetails.php?ID=1563&Description=ingvar)
High currents can cause things to go pop in a quite exciting manner. Add in high voltages and it gets properly dangerous.
Kit like this would usually be hired when required.
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I doubt you'll find anything mechanical that can provide a pulse that fast at that current, and I'd be very worried about the contacts welding, especially if your current source can deliver significant continuous power (i.e. you're not just discharging a big cap). Solid state will be the way to go, I'm sure.
You might want to google "can crusher" designs - e.g.:http://www.powerlabs.org/pssecc.htm (http://www.powerlabs.org/pssecc.htm)
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.. and if it only needs to conduct for 20ms with a low repetition rate, some fairly modest semiconductor devices might well do the job provided they are switched hard enough to avoid too much dissipation.
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.. and if it only needs to conduct for 20ms with a low repetition rate, some fairly modest semiconductor devices might well do the job provided they are switched hard enough to avoid too much dissipation.
As I understand it, this test doesn't involve any duty cycle, its a one off test to prove the cable doesn't fail, as the cable manufacturer (understandably) has no specifications related to this type of use.
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Sounds like an ideal test to do in your new garage. What could possibly go wrong? ::) :o :o :o :o
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Sounds like an ideal test to do in your new garage. What could possibly go wrong? ::) :o :o :o :o
It's ok, he's borrowed a pressure washer, and presumably a hose, so he won't need the fire brigade this time at least. Smart puppy see. ;)
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.. and if it only needs to conduct for 20ms with a low repetition rate, some fairly modest semiconductor devices might well do the job provided they are switched hard enough to avoid too much dissipation.
As I understand it, this test doesn't involve any duty cycle, its a one off test to prove the cable doesn't fail, as the cable manufacturer (understandably) has no specifications related to this type of use.
It will fail. Why would anyone expect otherwise :-\
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.. and if it only needs to conduct for 20ms with a low repetition rate, some fairly modest semiconductor devices might well do the job provided they are switched hard enough to avoid too much dissipation.
As I understand it, this test doesn't involve any duty cycle, its a one off test to prove the cable doesn't fail, as the cable manufacturer (understandably) has no specifications related to this type of use.
It will fail. Why would anyone expect otherwise :-\
It is believed it won't...
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.. and if it only needs to conduct for 20ms with a low repetition rate, some fairly modest semiconductor devices might well do the job provided they are switched hard enough to avoid too much dissipation.
As I understand it, this test doesn't involve any duty cycle, its a one off test to prove the cable doesn't fail, as the cable manufacturer (understandably) has no specifications related to this type of use.
Bank of big capacitors that you charge to the required pulse energy, then you might well get away with using a relay / starter solenoid to discharge them into the load? You'll need some way of capturing the voltage and current waveforms to verify what actually went into the load.
Or.. for a one-off, find a test house who have a facility for this type of testing and pay for the test instead of the hardware?
Not sure where you'd go, but there'll be someone who can do it, I'm sure.
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I'd have thought the co-ax core would act like a rather large fuse and simply blow at 500 amps !! even for only 12V for 20ms that's one hell of a lot of energy .. some welders produce less !!
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I'd have thought the co-ax core would act like a rather large fuse and simply blow at 500 amps !! even for only 12V for 20ms that's one hell of a lot of energy .. some welders produce less !!
You obviously havent seen the energy readout on a rapid car charger. At peak I saw 395V and 125A :o From that point on I made a conscious decision to never sit inside while it is charging on a Rapid.
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It does rather depend what type of coax we're talking about. The radio contest group of which I am a member uses this : http://www.rfparts.com/old_site/heliax_LDF550A.html (http://www.rfparts.com/old_site/heliax_LDF550A.html).
It's affectionately known as "donkey's dork", due to the fun and games involved in unrolling it and wrestling it off the ground and up a 100' lattice tower. ;D
I don't think you'd "blow" that in a hurry. :o
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The biggest relay I have found is 100amp which I used in a home made split charge system on my camper van.
I have fitted 200amp ones in my split charge systems in my mobile workshop
http://www.ebay.co.uk/itm/12-VOLT-200-AMP-HEAVY-DUTY-MAKE-BREAK-WINCH-CAMPER-SPLIT-CHARGE-RELAY-/321101707045?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ac3273725 (http://www.ebay.co.uk/itm/12-VOLT-200-AMP-HEAVY-DUTY-MAKE-BREAK-WINCH-CAMPER-SPLIT-CHARGE-RELAY-/321101707045?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ac3273725)
or you could try
http://www.ebay.co.uk/itm/500-amp-MEGA-FUSE-12v-24v-32v-500a-500-Brown-We-stock-All-Sizes-and-Fuse-Holder-/121208863346?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item1c389c7a72 (http://www.ebay.co.uk/itm/500-amp-MEGA-FUSE-12v-24v-32v-500a-500-Brown-We-stock-All-Sizes-and-Fuse-Holder-/121208863346?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item1c389c7a72)
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Hang on. I've had a great idea. Guffer... When is your milk float available for some "experimentation"? :-X
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Hang on. I've had a great idea. Guffer... When is your milk float available for some "experimentation"? :-X
Ferk o.... :-X...erm....no ;D
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I'd have thought the co-ax core would act like a rather large fuse and simply blow at 500 amps !! even for only 12V for 20ms that's one hell of a lot of energy .. some welders produce less !!
You obviously havent seen the energy readout on a rapid car charger. At peak I saw 395V and 125A :o From that point on I made a conscious decision to never sit inside while it is charging on a Rapid.
Cables are a tad thicker than co-ax last time I saw a milkfloat being refilled ... :)
I'm well used to high power circuits .. an aircraft external power cart gives 40KVA at 115 v and 400 cps at 3 phase ... but the cables are about an inch thick for each phase plus another the same size for the 0v line... slightly bigger than co-ax ?? :)
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Or.. for a one-off, find a test house who have a facility for this type of testing and pay for the test instead of the hardware?
Not sure where you'd go, but there'll be someone who can do it, I'm sure.
In effect, this is what he does. I've given the simplistic view of what he needs to achieve to see if the coax will survive the high currents, and due to low budgets (education), hopefully portrayed the need to do the initial tests with low cost, off the shelf components. If things are looking positive, budgets can be applied for, and a bucketload of 1kA thyristers, or industrial power switches can be bought.
And the capacitors needed, they are friggin expensive, and they need a roomful. For now, he's hoping a big car battery will be able to deliver the current needed for the test.
I don't begin to understand half the stuff he has to do, but I know it is researching beyond the limits of what we currently know about physics. Hence, he has stuff in deep mines helping search for pesky particles, and this experiment is to create magnets so powerful that it can affect matter at the atomic level.
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Why coax out of interest?, the Andrews LDF stuff that Kevin mentioned is good gear, for RF, the smaller spec, 250, 450 is solid inner core, cant remember about 550 as not used it for a while, but once you go 750 or above the core isnt solid, its hollow, as a cost and wieght saving, at RF levels makes no difference as it more about keeping the inner and outer dielectric size the same for impedeance reasons, the point beng it probably no good going for a higher spec RF cable for proper DC / AC current drains.
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Or.. for a one-off, find a test house who have a facility for this type of testing and pay for the test instead of the hardware?
Not sure where you'd go, but there'll be someone who can do it, I'm sure.
In effect, this is what he does. I've given the simplistic view of what he needs to achieve to see if the coax will survive the high currents, and due to low budgets (education), hopefully portrayed the need to do the initial tests with low cost, off the shelf components. If things are looking positive, budgets can be applied for, and a bucketload of 1kA thyristers, or industrial power switches can be bought.
And the capacitors needed, they are friggin expensive, and they need a roomful. For now, he's hoping a big car battery will be able to deliver the current needed for the test.
I don't begin to understand half the stuff he has to do, but I know it is researching beyond the limits of what we currently know about physics. Hence, he has stuff in deep mines helping search for pesky particles, and this experiment is to create magnets so powerful that it can affect matter at the atomic level.
OK, so if capacitors are out, you need to produce an accurate pulse, which means solid state switching. In some respects, you are in a similar situation with the switching devices as with the cable, in predicting what will survive. I wouldn't mind betting that a parallel combination of cheap MOSFETs would work at low voltage. They are normally limited by junction temperature rather than rated current for short pulses, so you need to calculate how much that pulse will heat up the junction, and parallel devices as required. At a couple of quid a go, you can afford to use a few. The downside is that you need to ensure that current is shared between the devices reasonably equally, and the gate capacitance goes up, meaning you need a better driver to switch them quickly. Switching speed is key to avoiding heat dissipation in the switching devices, of course.
Your current will also be limited by resistance, so at 12V you'd only be able to test a very short length of cable, and need to be able to pretty much bolt your circuit onto the battery terminals.
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IIRC, LDF-550 is hollow cored, but my point is that it'd survive a lot more than what most people think of when coax is mentioned (i.e. the stuff hanging out the back of your TV). ;)
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Im not dissagreing with you Kevin, its good gear, but at near DC levels solid core is better at current than hollow. Possible another issue would be how long does it need to be, on the bigger Tramsmiter Site, talking BBC, IBA type, long lengths of Coax are pressurised by Gas or Air by a pressure pump, as so to keep moisture, temp at bay, so if long lengths are involved here, what effect would that have on the 'so called texting'?
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Im not dissagreing with you Kevin, its good gear, but at near DC levels solid core is better at current than hollow. Possible another issue would be how long does it need to be, on the bigger Tramsmiter Site, talking BBC, IBA type, long lengths of Coax are pressurised by Gas or Air by a pressure pump, as so to keep moisture, temp at bay, so if long lengths are involved here, what effect would that have on the 'so called texting'?
Yep, all agreed. :y
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Or.. for a one-off, find a test house who have a facility for this type of testing and pay for the test instead of the hardware?
Not sure where you'd go, but there'll be someone who can do it, I'm sure.
In effect, this is what he does. I've given the simplistic view of what he needs to achieve to see if the coax will survive the high currents, and due to low budgets (education), hopefully portrayed the need to do the initial tests with low cost, off the shelf components. If things are looking positive, budgets can be applied for, and a bucketload of 1kA thyristers, or industrial power switches can be bought.
And the capacitors needed, they are friggin expensive, and they need a roomful. For now, he's hoping a big car battery will be able to deliver the current needed for the test.
I don't begin to understand half the stuff he has to do, but I know it is researching beyond the limits of what we currently know about physics. Hence, he has stuff in deep mines helping search for pesky particles, and this experiment is to create magnets so powerful that it can affect matter at the atomic level.
Right, I am now confident that he/they are out of thier depth....big time.
20mS through a mechanical contact....no chance and certainly not at the currents talked about.
Thyristors.....nope, as you cant turn them off.
As for will the coax take it, yes it may well.
Yes the losses are such that the core will get hot but if the duration is short enough then it wont have heated to the point of failure in the short time.
The bigger concern would be the connectors.
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This experiment is to create magnets so powerful that it can affect matter at the atomic level.
Thats already done, pop down your local hospital for a demo.
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This experiment is to create magnets so powerful that it can affect matter at the atomic level.
Thats already done, pop down your local hospital for a demo.
Yup, but think on a larger scale ;)
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This experiment is to create magnets so powerful that it can affect matter at the atomic level.
Thats already done, pop down your local hospital for a demo.
Yup, but think on a larger scale ;)
In which case hes a scientist and not an engineer lol....
When you can go to some hospitals and find 7 Tesla fast switching magnets.....whcih are leading edge with billions of dollars of research behind them.....
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This experiment is to create magnets so powerful that it can affect matter at the atomic level.
Thats already done, pop down your local hospital for a demo.
Yup, but think on a larger scale ;)
In which case hes a scientist and not an engineer lol....
When you can go to some hospitals and find 7 Tesla fast switching magnets.....whcih are leading edge with billions of dollars of research behind them.....
He's one of the engineers who have to build the shite that the scientists come up with ;D. I suspect by its nature there is some crossover, where the scientistics have to be (almost) engineers, and the engineers are infected with the scientists :P
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I would have thought the place to start is by doing some calculations.
R = V / I, so to use 12v at 900A requires a maximum resistance of R = 0.013ohms
Standard coax has a DC resistance of 6.7ohms/km or 0.0067ohms/m, so you need 2m for the experiment.
Power per second = I^2 * R, so 900 * 900 * 0.013 = 10.53kw
But as you only need a pulse of 20ms, Power used = 10530 * 20 / 1000 = 210w. This is 1w per cm of the coax length.
I can't see why this would be a problem. Longer pulses, like the relay contacts welding together may require a visit from the local fire service at 10.53kw / second. ::) :o :o :o
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Given the (required but not achievable using a relay/contactor) short pulse duration, as the length increases then the capacitance will become an issue to as the system is scaled up for longer lengths.
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I would have thought the place to start is by doing some calculations.
R = V / I, so to use 12v at 900A requires a maximum resistance of R = 0.013ohms
Standard coax has a DC resistance of 6.7ohms/km or 0.0067ohms/m, so you need 2m for the experiment.
Power per second = I^2 * R, so 900 * 900 * 0.013 = 10.53kw
But as you only need a pulse of 20ms, Power used = 10530 * 20 / 1000 = 210w. This is 1w per cm of the coax length.
I can't see why this would be a problem. Longer pulses, like the relay contacts welding together may require a visit from the local fire service at 10.53kw / second. ::) :o :o :o
It wont get near that level of current though due to the internal resistance of the battery plus connection resistances. :y
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It wont get near that level of current though due to the internal resistance of the battery plus connection resistances. :y
Yep, it's going to take more than a single car battery. :y
Now, a submarine battery... http://www.relocationspecialists.org/bat/cell.html (http://www.relocationspecialists.org/bat/cell.html)
The video's worth a look. 8)