charging an inductor

[spuffock]

I have an air cored inductor ,2.5millihenry, resistance 1.2 ohms. It is wound from 0.56mm enamelled copper wire. I need to store a kilojoule in it, for at least a microsecond.
How should I best go about charging it ?

 

[Ratchit]

spuffock,

I have an air cored inductor ,2.5millihenry, resistance 1.2 ohms. It is wound from 0.56mm enamelled copper wire. I need to store a kilojoule in it, for at least a microsecond.
How should I best go about charging it ?

The energy stored in a coil is ½LI² . Therefore, ½*2.5E-3*I² = 1000 joules ===> I = 894.43 amps . So send 894.43 amps through your coil, and it will store 1 kilojoule in its magnetic field. It will also continously dissipate 894.43²*1.2 = 960 kilowatts from its resistance.

Ratch

 

[JimB]

Putting a slightly different twist on this, you do not say what form of energy should be stored in the inductor.

Let us assume that the inductor has a mass of 50 grams and that you want to put 1kJ energy into it,
mechanically E = ½MV², so if you can get its velocity up to 200m/s it will have an energy of 1kJ.

200m/s is equivalent to 720km/h, so, just take it for a ride on an aeroplane, a nice trip with Mr Boeing will do the the world of good.

Much safer than all those sparks and balls of molten copper proposed by Mr Ratchit!


JimB

 

[MrAl]

I have an air cored inductor ,2.5millihenry, resistance 1.2 ohms. It is wound from 0.56mm enamelled copper wire. I need to store a kilojoule in it, for at least a microsecond.
How should I best go about charging it ?

Hi there,


Very interesting, and i'll try to be just a little more practical than the other two replies so far which were cute btw <little chuckle>.

The main constraint here is the time we'll be allowed to charge and discharge...if we take too long we melt the wire. The other problem is the series resistance is rather high at 1.2 ohms, which poses the problem of needing a very high voltage to charge even if we had all day to charge it. Since the current will be high too, this means we'll need a high voltage power supply with the ability to put out 900 amps while maintaining much over 1000 volts, so we're talking about a 100KW or better power supply here, or at least a capacitor bank that can put out 900 amps for 2ms or more at very high voltage like 1500 volts.

Taking all these into consideration without going into exact calculations (yet), you'll need a power supply that can put out at least 1740 volts at 900 amps for at least 2ms, and you'll have to time the charge for exactly 2ms (say plus or minus 5 percent, with a starting coil temperature of 25 deg C), and however you discharge you'll have to do that within another 2ms. If you can do that you'll prevent the wire from melting, but you'll also have to wait for the coil to cool down before you can begin another charge/discharge cycle. The time required to cool down will depend on the thermal characteristics of the coil and any air flow.
Note that if you use a power supply that can put out a voltage higher than 1740 volts you can charge faster and that's good, but if you try to use a power supply that puts out less than 1740 volts (estimated) you very well might melt the wire before you get to use it for anything.

If you can do all that above then you are there, else you cant do it.

The above assumes you will discharge into a short circuit. If you can discharge using a negative power supply that will mean you can discharge faster which will allow the constraint on the high voltage requirement to relax a little, but it will never happen with less than about 1200 volts no matter how we discharge it.

The kind of power supply we are talking about here is really a high voltage capacitor bank, charged with a high voltage transformer that charges the caps within a minute or so, and has an SCR type firing circuit to route the energy at the time needed. These would usually be custom built by say yourself at home, and we're talking about 2 foot by 2 foot by 1 foot in size roughly. You really need to have some experience with these kinds of power supplies however because obviously they are very dangerous.

I am kinda hoping you made a mistake in the specification of this problem somewhere

 

[spuffock]

OK, I have 4*40uf 5kv capacitors to hand, and a minute to charge them, which won't be a problem. Theres 2kJ. The problem will come with the switching, especially the dump. I was thinking of triggered spark gaps, but really need a bit more info. I think I may be able to make something that fires with a standard photoflash trigger, experiments will follow.
I may have to beef up the coil, but then I dont get as many turns in, so need more current.
Its all diminishing returns, I may have to get the big capacitors out...
In a previous life, I worked with a capacitor bank of a megajoule at about 100kv, and have been tutored to exhaustion about the safety procedures for such. I think I remember all of it.
JimB : In this place we also have a blowlamp......