Solid core inductance

[[527]phil]

Hello everyone, first off I want to say that I've searched all over the place, and I even asked my mentor/physics teacher about this. For my senior project I'm doing high voltage electro magnetics, and I'm going to build a coil gun. I've built them before but they weren't very efficient. I've been trying to find a formula to calculate the inductance of a coil wrapped around a solid core. I've only been able to find this one.
But the only problem I'm really having is determining the permeability of the medium. I'm using the core of a gutted transformer from an ATX power supply. I checked the markings on the outside but came up empty. I can't just calculate the inductance as if it was an air core coil because the final data would be incorrect. Can anyone tell me how to find the permeability of any given material, or if I could but a core with a known permeability. If not then I guess I have to make an air core coil.

[Richard Wallace]

You've asked a good question. And it is understandable that you are having difficulty finding guidance on inductor design.

An answer will require a little more information. To begin, what voltage and frequency do you plan to apply to this inductor? Was the transformer from the old power supply operated at the same frequency? What are the dimensions of the transformer core? Do you know what the core is made of?

And how will the coil gun work?

[[527]phil]

First off this inductor is for a dc/dc boost converter. I plan on bringing 9v up to about 200. I haven't decided on a final frequency yet, but it would probably be something around 1.5KHz. I'm not sure what the original frequency of the power supply is because I don't own an oscilloscope.The core is shaped kind of like El , kinda hard to illustrate. It's 1.5 inches long, 2 inches tall, and half an inch thick. And I have absolutely no idea what it's made of. But judging from THIS website it is probably a ferrous alloy powder E core.

A coilgun is the not so correct name for an electromagnetic projectile accelerator. Basically a bank of capacitors is charged to a high voltage and/or high amperage. Then the current is forced all at once through a coil of wire, kind of like a solenoid. The magnetic field caused by the coil attracts a ferrous projectile that is inside a tube that runs through the coil. The current is then shut off once the projectile reaches the middle of the coil and the projectile leaves the front of the barrel.

[Kevin Sevcik]

Richard,

A coil gun is a kind of high-velocity, low usefulness gun. You have an inductor, you something on the order of a Farad of charged capacitance through it, and you get a massive magnetic pulse to hurl something out of it. Usually magnetic, for some reason, but I imagine a brass or copper pellet would be likely to work better unless you could manage to instantly kill the magnetic field before it starts slowing down your iron pellet on the exit. Eddy currents are always repulsive, after all, though I'm admittedly unsure a pellet would be large enough to develop sufficient current.

[yongkimleng]

Originally Posted by [527]phil

A coilgun is the not so correct name for an electromagnetic projectile accelerator. Basically a bank of capacitors is charged to a high voltage and/or high amperage. Then the current is forced all at once through a coil of wire, kind of like a solenoid. The magnetic field caused by the coil attracts a ferrous projectile that is inside a tube that runs through the coil. The current is then shut off once the projectile reaches the middle of the coil and the projectile leaves the front of the barrel.

You're making something I can only dream of! Haha..
In gaming terms its called a Railgun I suppose. I voted for single coil.. coz I thought additional accelerator stages can be easily added and controlled timing to the microsecond with a microcontroller

Your coil is hollow and the medium is air.. so I guess the permeability of the medium would be some kinda constant of air? Just a quick guess..

[Al Skierkiewicz]

Phil,
It sounds like you are remanufacturing the transformer core from an ATX power supply into a DC converter power supply. If that is the case, there are too many unknowns at this point for you to accurately make calculations. If you don't have access to a scope then you likely don't have access to other instruments either. An easy way to back into this calculation would be to make a coil that would fit over the core. Measure the inductance with the coil and no core and then measure with the core. The difference would give you a rough idea of the permeability of the core. The ATX power supplies normally run at higher frequencies than what you are designing. Likely 100kHz or higher but the core might still usable at 1.5 kHz. In transformer and inductor design there are other factors involved. The object is to know the action of the core but also allow it to operate in a range of currents that does not saturate the material. This occurs when the current/magnetic field causes the core to reach a point where no additional current will cause a higher magnetic field.
The type of core you describe is a typical "E" design for obvious reasons. The core is made this way so that manufacturing costs are lower. If it is a laminated design (many thin strips of iron) instead of powdered iron, it is better suited for your lower frequency design. (Be aware that a design that stresses a powder core can cause it to crack.) The space between the "E" and the "I" part of the core is a magnetic loss and takes away from the permeability of the core material and as such needs to be included in the calculation.
So now the question is where to start. Check the name on the power supply and see if they have a website. You might be able to find a spec sheet on your power supply that will list the operating frequency. It may also list the components. Then search the net for manufacturers of core material looking for something designed for switching power supplies in your operating frequency range. Manufacturers will usually supply all the data for their products on their websites and may even show design applications that would fit your project. Then it is just a matter of winding the core. Make sure the wire you use will withstand the voltage you expect across the output.
As a word of caution, a bank of capacitors can store a huge amount of energy. At 200 volts, a discharge could be dangerous, even fatal. Be sure to include in your design, bleeder resistors to discharge the caps when the power is removed and never work on the capacitor bank until you are sure that the caps have been discharged.

[[527]phil]

Thanks for your assitance so far, but please read all the posts before responding, I explained how a coilgun works already. And i've made these before, however they were highly inefficient, and kevin I disagree. I've been able to shoot projectiles through tin cans and drywall sheets, at the moment the technology isn't advanced enough to be usefull, but they could be in the future. Also copper does not work, the projectile has to be steel, or another material that can be attracted to a magnet. The discharge from the capacitors at 200 volts not lethal, but it does hurt a little.

I just want to state again that i need to find the permability of the inductor, that's it at this point. There are only a few unknowns, none of which effect the equation for the indutance. I need to find the inductance of the inductor, NOT the fireing coil. So please only respond if you can help me find the permability of the E core.

[Richard Wallace]

Phil,

We might be able to offer better advice if we know what tools and instruments you have available. Al's suggestion is probably the best approach: (1) wind a test coil (loosely) on the center leg of the E, (2) measure the inductance of that test coil on the core, (3) remove the coil and measure the inductance again in air, and (4) calculate the relative permeability of the EI core as the ratio of the two inductance measurements.

This leaves open the question of how to measure the inductance of your test coil. Again, how best to do that will depend on what tools and instruments you have available.

[Al Skierkiewicz]

Phil,
The energy stored in a capacitor is equal to 1/2 * C (in mfd) * V^2 (in kV). working for a 1 farad stack at 200 volts this works out to 20,000 joules. If you don't think that is enough to be letal than you need to stop and get better advice than we can give you here. The 200 volts is enough to penetrate into muscle tissue and cause permanent damage. Even a small bank of capacitors at 200 volts is enough to be lethal across certain parts of the body.

[[527]phil]

200 volts at 500 micro farads is not enough to be lethal. trust me I've shocked myself with them before. I only have a bank of 2 capacitors, now if they were a full farad each THEn i could see taking some precautions, but this is not as dangerous as working at Sequa deli . Also, I think I may be able to secure the use of an oscilloscope so i will be able to determine the native frequency of the transformer. How would i go about measuring the inductance of the core. Also, i do not have to construct an air core inductor and measure it, being the value for the permeability of air is readily available on wikipedia.

[Al Skierkiewicz]

Originally Posted by [527]phil

200 volts at 500 micro farads is not enough to be lethal. trust me I've shocked myself with them before.

Phil,
You need to trust me on this one. 200 volts is considered HIGH voltage. A wall outlet produces a little over 200 volts, peak. If you have been shocked before, consider yourself lucky, very lucky!
You need to measure the inductance in an air core first and then slip the core into the coil to measure the difference. You get an idea of what the permeability of the core is by solving the inductance equation for permeabilty when you know the inductance in both cases. I suggest a trip to the library for a copy of the ARRL Amateur Radio Handbook. You will find some discussion on inductance and measuring techniques.

[[527]phil]

I've decided to buy a core from that company that i linked to before. I can buy a core in a variety of KNOWN permeabilities, which will let me calculate the inductance accurately. But i will definatly get a copy of that book and check out those techniques!!

[sciguy125]

Ugh... Thanks, I was just starting to get over the nightmares from electromagnetics... Now I'll be woken up in the middle of the night by some sort of vector calculus fourier series.

[John Gutmann]

Have you thought about using AC current and having it shoot like an aluminum ring? Or do you have to shoot something that is ferrous like a magnet?

[X-Istence]

Originally Posted by Al Skierkiewicz

Phil,
You need to trust me on this one. 200 volts is considered HIGH voltage. A wall outlet produces a little over 200 volts, peak. If you have been shocked before, consider yourself lucky, very lucky!
You need to measure the inductance in an air core first and then slip the core into the coil to measure the difference. You get an idea of what the permeability of the core is by solving the inductance equation for permeabilty when you know the inductance in both cases. I suggest a trip to the library for a copy of the ARRL Amateur Radio Handbook. You will find some discussion on inductance and measuring techniques.

200 volts is considered high? Pfft, 240 volts is the standard in european countries, with 440/480 used for Dryers. Enough to kill you? Sure, likely hood? Almost none. Ive shocked myself with 240 and 110. 240 just bites more.