Making an electric furnace.

[Dazza]

This furnace is for melting aluminium, I was just about ready to start construction, useing gas LPG/propane as the fuel, then I came across a few backyard hobbyist foundrys, that use electric powered furnaces.  So now I think an electric furnace is the way to go, but I need some help please .

This is what I have for the elements

Nichrome 80,    28 Gauge B&S   13.4R/per meter,  Max 1200,C

I want to use two heating elements, each elements will be 2 m long, which will be coiled and stretched out to the length I need.

2 meters of resistance wire = 26.8R  240VAC  9A  2160W, so 2 elements will be needing 18A  4320W

I need a way to control the power, does anyone have suitable schematics for this?

Also how do I configure the two elements so they both are running at 2160W.

Any help is appreciated .

[Ldanielrosa]

An appliance repair shop may be willing to sell you an "infinite range control" for an electric stove.  This is a self heating thermal switch which is rated for 240V and at least 10A, usually more.

Run the elements in parallel if they are identical.

What are you planning to use for a crucible?  What kind of flux?  Casting in sand?

[Ante]

Hi Dazza,

A standard triac circuit would work fine for this. Since the load is resistive the interference will not be too bad. A triac circuit with zero crossing control will run without interference though.

[Dazza]

Hi Ante, Ldanielrose,

Thanks for your reply ,
 
Ldanielrose, I have experience casting with oil based sand and die casting, large floodgates and irrigation valves, I'm going to give Green sand ago to start with.  I'm going to use a steel crucible.  I won't be using any flux unfortunately.

I would really appreciate some help, modifying this circuit that I found to suit my application, with the limited choice of components I have available.  The only (DIAC) I have available to me locally is (BR100-03) and for the TRIAC (BT139-600  400V 15A) can I use two of these TRIACS in parallel?  When playing with mains power I am very reluctant to use trial and error.

Any advice is very much appreciated, Thank you.
 

[Dazza]

Attached are datasheet for components mentioned above, and PDF file containing information about the 220V_240V LIGHT DIMMER.

Thanks again for any help.

[Ante]

You can build two identical circuits one for each element and why not a tandem pot for adjusting the power?

[Dazza]

Yes, good thinking Ante I'll do that.  I have a couple of questions, as usual .

In the components list for the capacitors, it says to use dipped polyester, are these Green caps metallised polyester.  In the pic they are blue, I have a heap of recycled Green caps some of which are brown, I am assuming they are from different manufacturers, or do the different colours indicate different voltages, I'm having a real problem with reusing these capacitors, how do you tell what voltage they are?

[Dazza]

Ok I'm nearly done with the electronic side of things.  I have almost got it right, the only real problem I have had so far is choosing the right size heat sink.  I gave it a test run connected to an electric heater, with the heater set at 750W and the light dimmer set to maximum, I could hold my fingers on  the heat sink for about five seconds, when I reduce the power a little with the light dimmer switch, the temperature of the heat sink reduced accordingly.

At around 1400W, I could hold my fingers on the heat sink for about two seconds, I think this is its limits.
So if I double the heat sink size I should be right, which is a shame because I think I've made a nice neat little unit, and I could of given the furnace a test run with a single element to see how it went, before adding a second element and making a second light dimmer and putting them into a single unit.

I left out the choke and C1, as they are for reducing radiofrequency interference, because I wouldn't have a problem with that, although I was worried about leaving C1 out in case it served another purpose as well.

I don't know how much heat is normal, for a TRIAC with this circuit and the application I am using it for, does this seem like its working the way it should?  Is this amount of heat normal for a TRIAC?

[Ante]

Hi Dazza,

Sorry I missed your last post; I suppose you have worked out the marking on the caps by now. You can

[Dazza]

No worries Ante, I have pretty much got those capacitors sorted out now . I decided it would be a good idea to place this project inside a insulated  box for testing and safety, considering my lack of experience and I was playing with 240V , but it turned out so well I was hoping I could fit the right size heat sink in their, and use it for curing the refractory of the furnace. I started with a very small heat sink and tested it on my fan 75W 240V to get an idea on how much heat the TRIAC will need to dissipate, then I tried the largest heat sink I had, and that's about the biggest I can fit in their, I don't think it would be very practical to try to modify the box to fit a bigger heat sink, after all it was only really meant to be used for testing and getting an idea of the size heat sink I would need. No change of plans I'm still going for 2160W for each element, I'm working my way up to it,  just using a little trial and error to see what this project can handle .  Yes I change the fuse to a 10A, the  TRIAC is a 600V 15A BT139-600.

I think I will just have to bite the bullet, and scrap the case and start over , I have an old computer power supply box that should be just the right size to accommodate two of these light dimmer switch.  I didn't use it in the first place because it wasn't insulated.

The heater that I have, has three settings 750W, 1250W, and 2000W, the higher settings is pretty close to what this circuit needs to handle, so it is good for testing, I will need to allow some headroom for the element as they will draw more current when they are cold.

Ante can you offer me any safety advice on the construction?

This is how I am going to go about it.  Use a computer power supply case, two separate light dimmer circuit with their own heat sinks, two mains power wall socket's mounted side-by-side on the box for the controlled output supply, one mains power supply lead to supply power to both circuits, a transformer and cooling fan. The in going and outgoing earth wires connected to the metal case.

Thanks for the help Ante I really appreciate it .

[Ante]

Hi Dazza,

As long as you use those yellow and green wires (I presume that

[Alun]

I hope you earthed the heat sink too especially as you were touching it.

[Dazza]

Thanks Ante , I probably wouldn't of taken on a mains power project like this, if I didn't have somewhere someone to ask advice .

About the cooling fan(DC 12V 0.13A), is there a way I can use it without a transformer, reduce the voltage down enough to use a bridge Rectifier and voltage regulator

[Dazza]

Hi Alun,

Funny you should mention that , the TRIAC wasn't insulated from the heat sink with my first test run.

I thought it over before I touched it to see how hot it was getting, and came to the conclusion that it should be LIVE .
Being an adventurous kind of guy, I touched it quickly with the back of my hand, I knew I would get a tingle, for some reason I just couldn't resist .

That just wasn't very smart was it .