I am doing some experiments with pulsed magnetic fields. I am using an audio amplifier as a power source, and a simple software as my frequency generator. I use mostly square waves and sometimes sine. Both straight forward, simple waves - sort of like a clean audio test signal of different shape. Nothing fancy. I can even hear them when I run them through my coil and when it is noisy around here, I can just put on headphones and hear clearly whether the coil is working.
I am terribly uneducated when it comes to all things electromagnetic (I only know a bit about audio and that electricity going through a wire will create a magnetic field)… so I need some help. On the upside, I am a fast learner, given right direction.
So far I have been experimenting with a coil I built, that is 4" in diameter, about 1" wide and has about 1lb of 22 gauge insulated copper wire on it (I think that is some 300ft but I don't remember any more) - as a result, it is 8 ohms resistance which was done on purpose as my amplifier is rated at 8 ohms at max power. In any case, the coil works quite well, down to about 300 Hz, and up to 5000 Hz although it tends to get hot around 300 Hz after 10 minutes or so. Haven't tested it lower than that, except today at 20 Hz when it overheated in a second and I had to shut it off. Since I am using an audio amplifier as a power source, I made sure that the coil is 8 Ohms resistance, as that's what the amplifier requires.
Now, I have two questions:
If I need a coil that can take low frequencies, like 20 Hz, the one that I have does not seem to like it, as it immediately gets extremely hot - within seconds literally. It is well varnished (I did it by hand, on every turn) and quite solid, but I imagine that such low frequency makes it vibrate enough to produce high heat. I tried even at very low power setting, such as 1/10 of the max that I normally use, but still it gets very hot very quickly and I am sure that if I let it run, it would melt in about a minute or two. Would using thicker copper wire be better for lower frequency? If so - instead of using 22 gauge wire, what should I use to be able to run that coil as if it is on higher frequency? And if thicker wire would work better, would that somehow weaken the magnetic field? (right now I can run 1000 Hz through it no problem, for hours on end, at max power which is around 1000 W maybe even a bit more). I imagine that since my power amplifier is rated at 8 Ohms I would just use thicker wire but with lower number of turns to achieve the same resistance. If it matters - I am using a Mackie professional power amplifier, the type that is used for concerts. It is rated at 1500 W when in bridged mode (both channels bridged together into one).
How can one achieve an even higher power of the magnetic field using, say, an audio amplifier as a power source? I imagine, getting a more powerful amplifier would be a good start - perhaps a 2000 W or more. But then, how do I build a coil that will not overheat?
Any general advice you can give me in that respect is much appreciated. I think, but I am not sure, that thicker wire is better suited for lower frequencies. Having lots of power, as in - strong magnetic field, is essential. I just don't know how to do it. I understand that there is a practical limit at which coil will necessarily melt, but I don't need to go that far.
One more question: I made an observation that - the lower the frequency the stronger the magnetic field. Am I correct there? Namely, I run 1000 Hz, and I can hear some of the iron stuff on my desk vibrate - from my Luxo desk lamp to the box cutter knife, but it does not attract anything, and can run like that for hours… but at 20 Hz, that same knife stuck to the coil at 1/10th of the power (remember - the coil has no core - it is pure copper)! It was scary for a moment - I imagine that's what happens when someone brings metallic objects into an MRI machine room…