Induction furnace

[brainstatic]

Hello All,

I am new to posting to this forum. However, I have been a scientific lampworker for nearly ten years now. If you wish to make an electric furnace for the purpose of melting metals then you should contact these companies (they are both owned by Amaco): Amaco and the Brickyard.
This link is for the electric elements. The ones you all probably will want to use are the metal enameling replacement elements. I have one of these kilns, produced in 1948, and it reaches temperatures in excess of 2500 degrees C on 120VAC! The kiln has two elements of 500 watts each, connected in series for the high temp setting and in parallel for low (or the other way round, I cannot remember). A pyrometer and thermocouple show absolute temperature.
http://www.amaco.com/jsps/grouphome1...0Amaco%20Kilns
The elements can simply be stapled to kiln brick, which is fairly cheep when not precut, in a zig-zag or cyclic fashion.

For kiln brick for any style of refractory, contact the Brickyard http://www.brickyardceramics.com/

For the making of high temperature crucibles as described to me by a dear friend:

1. Choose a suitable clay (some experimentation may be necessary for your exact application, e.g., type of metal).
2. Form some of the clay into thin pieces (about 4mm thick), dry, and bisk fire in a kiln, or your furnace, to a cone of less than that of the final firing, see later steps. Large underground kilns have been used for centuries for this purpose, heated by wood or charcoal.
3. Break the bisk into small particles, fine grind with a hammer, or some other means.
4. Combine up to about 50% bisk back into the original clay, this should be as homogeneous as possible.
5. Form into a crucible shape, either with a wheel, or core-formed. I would not recommend hand-built techniques, as it should be a single piece of material, not joined parts.
6. Dry, and fire the second time at a higher temperature.

Notes: After the firing process is heated to the desired temperature, always cool the stoneware inside whatever it was fired in, and as slowly as possible till completely cool. The addition of bisk clays provides for more strength and higher temperature capabilities compared to that without.

The crucibles can be made into any size and thickness. As small comercially available crucibles are relatively cheap, however, the larger ones are a bit expensive.

Hope this helps...

[CypherNinja]

Just throwing out an idea here.....

An Argon 'flood' would probably be alot easier than vacuum. Argon is heavier than air, and tends to pool in low places (this is a serious safety concern in welding, use extreme caution when TIG welding in a confined space ).

All equipment and refills are readily available at welding supply places. You could rig a system to keep a slow steady stream flowing into the furnace and simply displace everything but the inert Argon. Figuring out a way to pour might be tricky, but the flipping method mention earlier sounds like a winner.

[nikolatesla20]

Um, since AL is non-ferrous, I don't think you can use inductive heating on it, since inductive heating relies on eddy currents from the magnetic field..since AL is not magnetic, it won't work - only for iron and steel...

-niko

[JBV]

Well steel and iron loose the magnetic properties after the curie temperature.. about 800 if i remember correctly... You can heat all conductive materials with induction!

[nikolatesla20]

Yes you are right, I was thinking about that too - when iron and steel get beyond a certain heat they lose their magnetic properties..so thanks you have answered it..

[JBV]

It works partially on eddy currents... but also on skin effect!

[JBV]

I have done some more research! I belive you could just bypass the output rectifier on a welding inverter and use it for induction heating! I don't know if the switching frequency is optimal for melting metal! But it should work!?

[peter.blais]

Actually it works on any conductive material. The steel and other magnetic materials heat *faster* initially due to being constantly remagnatised, but loose that effect over ~750C. somewhere around there.

Your basically relying on a huge field of electron charge, concentrated in the center of the coil, and running down the inside (or up)..> It sort of looks like a stretched out donut, with electrons running down the middle and around, up the outside.

Thats all I can remember from physics

[Mcgyver]

this subject is interesting - from googling, here's a description of power supply and frequency requirements - what's the frequency of the welder?

http://www.inductionatmospheres.com/powersupplies.html

would the basis of the circuit be a timer switching on and off some big power transistors? kind of like a pwm but more heavy duty?

[JBV]

You would want much lower frequencies for meliting metal 50 Hz to 20 kHz or so! Higher is good for hardening and heat treatment as i understand it! I don't know what switching frequency a welder use.. But the principle is:

AC > Rectifier > H-Bridge(PWM) > Transformer > Work coil

If you have 10A DC supply... 50% Dutycycle... 10:1 Output transformer and a 15 turn work coil you should get ~700A, including losses, running trough the workpiece

[JBV]

Built a H-Bridge for my induction heater project today It's built for 350VDC and up to 30A (To bad i only got 16A). The heatsink i will mount it to is 42cm long (16,5")...

Have begun designing the driver board with a microcontroller, with lcd and rotary encoder for settings! Cant wait to try it out

[Warpspeed]

There are two quite different mechanisms at work in induction heating. The first is hysteresis heating of any magnetic material. The higher the operating frequency, the greater the heat generated. But this only works for ferrous metals below the curie point. For non magnetic metals (or above the curie temperature for magnetic metals), the heating effect is created entirely through circulating eddy currents.

In effect, the metal being heated becomes a single shorted turn of a high frequency transformer. The difficult part is coupling enough energy into the work to be useful. In order for this to work reasonably well, the induction coil absolutely must be made part of a resonant tuned circuit.

The idea is that a resonant build up of energy vastly increases the circulating current in the work coil, way beyond what the power source would be able to create all by itself. Many hundreds or even thousands of amps are required to pass through the induction work coil in order to couple enough energy into the work.

So there is a lot more to this than just coupling up a few turns of copper to a welder and hoping for the best.

The first requirement is to build a tuned circuit that can carry all this power, possibly ten to twenty times the basic input power of the induction heater. Several hundred thousand VAs (reactive watts) of circulating energy may be involved, and a very special and expensive water cooled tuning capacitor will be required that can carry several hundreds of continuous amps without burning up.

So the starting point of your home made induction heater will be getting your hands on one of these special induction heater tuning capacitors to tune your work coil to the desired operating frequency.

The next problem is driving this tuned circuit in such a way that the driver will not self destruct if either the driver or tuned circuit get out of step with each other. This is a lot more difficult to do than many people realise.

An internet search will turn up many people that have tried to do this and failed. It just ain't that easy. Heating a few pennies to red heat, or the end of your pliers with a couple of hundred watts is dead easy. Feeding several (or many) kilowatts of power reliably into several pounds of metal is going to be a far greater challenge.

My interest in induction heating has just now been aroused, and I am prepared to have a go at this myself. It will take me a while to track down and obtain some of the rather difficult to find (and expensive) special parts that will be required to try out some ideas of my own.