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| Small radius heat induction coil connected via coaxial cable? |
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| Plasmateur:
Hello, I am in need of something I believe to rather specific. A small radius heat induction coil. I currently have access to a 13.56MHz, 300W of power with a tuning circuit. I am able to match for certain working coil configuration, however I can only match when the radius and/or turns of the working coil are not within the parameters I need. I'm a little confused by the matching network as well. It employs two tunable capacitors. The first one from the source to ground and the second on in series with the working coil. I'm confused because the working coil is shorted to ground, so I'm not sure how the math works out for matching the impedance of the load. Does the load factor in radiation resistance? I need to use coaxial cable for this application in short pulses as well. I was able to heat a 1mm diameter tungsten wire to white hot conditions in about 1 second without damaging the coaxial cable I had previously used - but the working coil that I constructed isn't of ideal dimensions. Is there something commercially available for this application? What If I terminated the working coil into a 50Ohm power resistor? Would this make the match easier to achieve? Thank you. |
| GeoffreyF:
Impedance at frequency is not the same as DC resistance. What would appear to be a DC short is common to see in RF circuits. Impedance will change when there is a metal object for the coil to heat. Why do you believe your inductive heating coil is appropriate for the RF source that you have? If you are asking these kinds of questions, you are quite possibly out of your depth for working on a device that heats things up. |
| coppercone2:
what are you doing with it/ i am curious because i have been thinking about building one for a long time but i think its because its just a popular project. i had a few control system ideas i wanted to build and test at least, let alone get into power electronics, but i keep asking myself why do this... i just see people doing brazing and heating bolts with them, but a torch is fine for home use. casting is not really too appealing either because the complex shapes are either useless or cosmetic without really advanced finishing equipment, and even simple shapes need a well equipped machine shop to clean up. and if you use softer metals you don't get strength increases from extrusions and stuff. i thought alloy chemistry might be useful but you need mad equipment to study crystal structures and i don't know if anything remotely useful would ever come out of it. for reference a 2000c commercial vacuum induction kiln that is capable of casting 30cc of metal has a power consumption of like 5kw. scary electronics. for some reason the whole metal casting and alloy making scene makes me feel a tad nihilistic. unless its custom vacuum tubes. maybe casting custom alloys for vacuum tube use in tiny amounts. or special wires. |
| Plasmateur:
--- Quote from: GeoffreyF on January 03, 2019, 03:20:56 pm ---Impedance at frequency is not the same as DC resistance. What would appear to be a DC short is common to see in RF circuits. --- End quote --- Right, I'm aware impedance vs resistance. I'm trying to understand what you're saying here. If one end of the working coil, or "heating coil", is connected directly to ground, it is not correct to say shorted to ground? What's the best terminology to use then and how would that be described on a circuit diagram? --- Quote ---Impedance will change when there is a metal object for the coil to heat. --- End quote --- I'm aware of this as well. --- Quote ---Why do you believe your inductive heating coil is appropriate for the RF source that you have? --- End quote --- Where did I say that? I have an RF source with a matching network. The matching network doesn't appear to be appropriate for the particular coil I need to fabricate. The matching network on hand works for other coils I fabricate which are outside the dimensional parameters I need. I need to have higher frequencies (MHz) than what are typically used in inductive heating applications (kHz) because I am attempting to heat wire - meaning smaller skin depth. --- Quote ---If you are asking these kinds of questions, you are quite possibly out of your depth for working on a device that heats things up. --- End quote --- And making these statements to someone who is trying to gain a better understanding of any topic isn't helpful. Why would anyone bother to learn new subject matter if they are initially "out of their depth" when first encountering it. What an absurd thing to say. I've already used this device to heat wire, but under non-ideal working coil dimensional parameters. The wire heats up when I achieve a match - the reflected power is minimized. I'm trying to gain a better understanding of the theory behind the matching network and how it relates to what I'm doing. Should I just stop attempting to learn? |
| Plasmateur:
--- Quote from: coppercone2 on January 03, 2019, 08:44:57 pm ---what are you doing with it/ i am curious because i have been thinking about building one for a long time but i think its because its just a popular project. i had a few control system ideas i wanted to build and test at least, let alone get into power electronics, but i keep asking myself why do this... i just see people doing brazing and heating bolts with them, but a torch is fine for home use. casting is not really too appealing either because the complex shapes are either useless or cosmetic without really advanced finishing equipment, and even simple shapes need a well equipped machine shop to clean up. and if you use softer metals you don't get strength increases from extrusions and stuff. i thought alloy chemistry might be useful but you need mad equipment to study crystal structures and i don't know if anything remotely useful would ever come out of it. for reference a 2000c commercial vacuum induction kiln that is capable of casting 30cc of metal has a power consumption of like 5kw. scary electronics. for some reason the whole metal casting and alloy making scene makes me feel a tad nihilistic. unless its custom vacuum tubes. maybe casting custom alloys for vacuum tube use in tiny amounts. or special wires. --- End quote --- I need to heat a small wire to white hot temperatures in a vacuum for an experiment I am attempting. Way outside the realm of brazing or soldering or joining different metals together. I was already able to do this with only 300 watts of power @ 13.56MHz, but the heating coil dimensions aren't ideal for the application. I don't believe I will be able to use a helical coil. The wire I am heating will be electrically attached to an oscilloscope further down the line. Off axis components of the H field from the induction coil relative to the wire it is attempting to heat might cause current to run down the length of the wire and back into the oscilloscope. At high power, this might cause some problems! |
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