Induction Heating

Dropping in some pictures today, uh summer cleaning I suppose. Plus it's been about half a year since I added something and that update message was starting to get annoying...

Circuits (click to enlarge)

Here's the current project exactly as it is, exactly as I'm typing this. The oscillator (detailed on the previous page) is on the right, the MOSFET half bridge in the middle, power transformer (100VAC 10A "iso"lation transformer, made from microwave oven transformers), rectifier and capacitors at bottom and the output circuits (DC blocking capacitor bank and matching coil) on the left. The work coil isn't shown, but would go between the free end of Lmatch and ground.

Work Coil

...This is the work coil right now, fitted with some (uselessly thin) tubing for water cooling. 5.4μF on there, as I recall. The coil is 4uH, so resonates somewhere around 34kHz - a bit off in practice due to the inductive loading coil and the work. The capacitors are 0.22uF 250V polypropylene, rated for heavy snubber duty. . .sorely inadequate for heating! At 30kHz, I can only go up to 100Vp-p or so before things start getting dangerous, and I'm not about to burn $40 of caps trying. Next installment will deal with building my own caps..

Heated Nickel

Okay, so let's see what else I've been doing. Oh yeah, heatin' stuff! Above is a U.S. 5-cent piece, otherwise known as...a nickel... heated to about 1400°F, a dull red heat, by the 2" work coil (1uH, so around 60kHz with the same tank cap). Ok, the above picture doesn't show it well. How about a piece of...

Heated Titanium

...Titanium? You can clearly see the edges are hotter than the center, evidence of skin effect. I think this was around 1500°F. With the help of some ferrite rods I've focused the field tighter and had a piece up to 1800°F or so.

Solder Melt

And what picture show would be complete without proof of my first induction melt and cast? Just a parlor trick, I mean I'm using a folded piece of paper for a crucible for pete's sake, but as lame as it is, you simply can't stop smiling about actually melting and pouring something! On the plus side, it does show you all the molten metal physics going on. If you toggle the power, you can see the edges of the melt whirling and rising from being constricted by the field and the heat causing heavy convection currents.

Things I haven't melted yet - well besides my goal of steel, I haven't melted aluminum. Now aluminum has a pretty low melting point, you may observe. Indeed it does - but, it's soo conductive that repulsion from the field keeps turning it upright! I try to pin it under a rod, but that just doesn't work good enough. It's exactly like, but exactly the opposite, of putting a nail on the back of a speaker and watching it stand on end. Well not exactly, this is diamagnetism, not antiferromagnetism. Actually it's not diamagnetism either, it's Lenz's law in action. This has the effect of diamagnetism to changing fields though. :-)

Part Six


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