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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am hoping someone with a good theoretical understanding of this problem can point me in the right direction:
I have had no problem with quickly melting small pieces of metal such as nuts and bolts.
I want to heat a large brick of iron for a forging application. I built a bigger coil that is about 3.5" ID/4" OD. It has 4 turns. I used the same 2.6uf celem capacitor and the same inverter circuitry. I found that the inverter waveform was not as square as with the smaller coil. The current when the coil was empty was also less than with the smaller coil. The big problem is when I put a large brick of steel in the coil the current waveform drops to almost nothing. When I get the input voltage up to 240vac I get about 10A with heats the brick, but not to a workable temperature.
Is this over- or under-coupling? Do I need a lower capacitance value? Do I need to increase the coil size further? Do I need less turns on my primary coupling transformer to the resonant tank to induce a higher voltage?
With regards to the last suggestion, I tried this and did get higher currents. The quenching phenomenon was still quite significant.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The series feed circuit you're using does that. A big lump of iron will decrease the current draw, and I'm not exactly sure why. I do know that once you get past the Curie point, the current draw shoots up and it starts to heat quicker.
This suggests that it's the permeability of the iron to blame, increasing the inductance of the coil and hence changing the impedance match, and probably lowering the Q too.
Fewer turns on the coupling transformer, or fewer turns on the work coil, or looser coupling to the workpiece, would probably help.
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Induction theory help
