Induction heating/furnace

Home enthusiasts discuss their Foundry & Casting work.

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reubenT
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Joined: Wed Dec 24, 2008 11:04 pm
Location: Spencer TN USA

Induction heating/furnace

Post by reubenT » Sun Aug 26, 2018 7:48 am

It seems a home made induction furnace might be well within the scope of home construction. Details found at http://www.richieburnett.co.uk/indheat.html

That's just the basic unit explained without the refractory and cooling. In practice one would pump coolant through the copper pipe and have it embedded in the outside of a melting pot made of insulating refractory.

Now starting from scratch with everything might seem daunting, but I've collected enough junk of all sorts I think I have about everything to make it, including the insulating refractory, the high power IGBT block (from electric forklift control board) the square wave generator and oscilloscope to watch wave forms which is the easiest way to tune it. And just picked up copper pipe and a few fittings to connect to a hose. I have endless water from a spring gravity flow, so wasting water is not an issue. I will just run continuous flow through it without needing a circulating pump and cooling the coolant. The only little hitch in simplicity may be resonant frequency drift, but being it's a short term use device with constant attention needed anyway, watching the display while in operation and adjusting the drive frequency as needed will be possible. How much drift and how easy that will be waits for actual experience. Then there's the options of simple half wave driving, half bridge driver, or full bridge driver. Will experiment and see what it takes to get results. I will start simple with single IGBT and half wave control. Power it with the AC welder with bridge rectifier to get DC. But I have 3 projects demanding attention, might be a week or several before I can get to it. But making my own steel parts is getting toward being higher priority enough to actually do it soon. I've done plenty of aluminum casting so far, built cupolas for iron but haven't tried them out yet. Always too many projects to get em all done.

Harold_V
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Re: Induction heating/furnace

Post by Harold_V » Sun Aug 26, 2018 4:06 pm

Thanks for sharing. I appreciate the link, which helps one understand the principle of operation.
The unit I'm working with is a second generation machine (motor/generator), which is considerably more efficient than the first generation equipment, although only about half as efficient as the third generation equipment (solid state). It is very demanding of power (I have three phase delta, 240 volt service, 400 amps). The unit was built by Ajax Magnethermic, back in '62, and is rated at 50 kw.. It operates at 3,000 Hz and 400 volts, single phase. I have a pair of furnaces, one of which will be lined to melt approximately 100 pounds of iron (steel), with the other lined to melt about 75 pounds. Coil size dictates capacity. The unit has switching provisions, so two furnaces can be connected to the power supply at all times. Only one can operate, however. Everything in this system is water cooled, including the capacitors and high frequency transformer.

Good luck with your venture. I'd appreciate reading more when you get started.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

reubenT
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Joined: Wed Dec 24, 2008 11:04 pm
Location: Spencer TN USA

Re: Induction heating/furnace

Post by reubenT » Sun Aug 26, 2018 9:23 pm

Back in '62 they didn't have high power transistors, they had just begun to experiment with silicon transistors then, now they are common and not too expensive. I got a few next door to free from old fork lifts at the scrap yard. I wonder if the higher frequency would be more efficient. I'm sure there's info somewhere as to what frequency causes the hottest eddy currents in steel. The ones described are running at 170-200 KHz range, and keeping it running in resonance helps maximize efficiency. I could experiment with different frequencies. Being I'm way out from large cities and in a valley between mountains I think I can get away without causing too much RFI. Might need a low pass filter on it to help out. AM radio starts at 500 KHz. But requires a large resonance tuned antenna to radiate efficiently, so I hope it will be ok with no antenna in spite of the high power. Well, not so high power when it comes to radio transmission. Those big stations run 100 KW. I'm looking at less than 10 KW to start with, and no more than maybe 5 lb steel in the experimental version. Enough to make many of the small parts I need. 100 lb of molten steel would be a mighty hot bowl of soup, probably need a reflective foundry suit to get anywhere near it. And definitely an overhead lift to handle the weight safely an get it poured. When I get that much to deal with I will make a swinging boom overhead lift to cover the foundry pouring area.

Harold_V
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Re: Induction heating/furnace

Post by Harold_V » Mon Aug 27, 2018 1:50 am

There's a relationship between heat size and frequency, although I'm not versed enough to discuss it intelligently. I know that Ajax produced the 50 kw unit in both 3,000 and 10,000 Hz. Higher frequency, as I understand it, allows for melting smaller bits, which makes sense considering they make induction units that melt as little as a troy ounce of gold or silver (used in the manufacturing of jewelry). The frequency must be extremely high.

If you aren't aware, I did a thread on the progress of restoring my unit. If you'd like to take a look, here's a link. viewtopic.php?f=24&t=105394
In the thread I describe the issues I've faced, and how they've been resolved. There's a few pictures, including one of the reassembled power supply.

I haven't updated the thread for a long time, and I've done a lot more work, primarily rebuilding the two furnaces I have. I'll get to the update in due time, but right now I am too busy to do so. Lots to do before our rainy season begins in earnest.

You likely know that there are line frequency furnaces on the market, but I suspect they are quite large in capacity.

If you're interested, I have a chart that relates furnace size to frequency. It was published many years ago by one of the major builders. I can send you a copy via email if you so desire. If you'd like to get it, send me a PM with your email address and I'll get it off to you. It might help you decide on the frequency you'd need for your project.

You commented on handling 100 pounds. As you suggested, I have provisions for a gib crane, which is yet to be built. When I poured the footings for my shop, I poured a length of 6" well casing in, which was buried in about 4' of concrete. The floor, which is 6¼" thick, lends further support. The shop has 12' ceilings, and the pipe extends to within about 10" of the ceiling, so there's enough (head) room to handle pretty much anything I'd encounter. The gib crane will swing a 10' radius arc, which will cover all critical areas I have designated for casting.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

rrnut-2
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Location: New Hampshire

Re: Induction heating/furnace

Post by rrnut-2 » Mon Aug 27, 2018 2:08 pm

Generally speaking, the smaller the furnace, the higher the frequency. We have one furnace that burns 1 gram samples: it runs at 30kHz. The 100lb furnaces run at 3kHz, 5000lb furnace at 500Hz. Lead/Lag is accomplished by switching in/out capacitors. Since the late '60's, the units that I am familiar with, used SCR's not transistors.

Jim B

Harold_V
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Re: Induction heating/furnace

Post by Harold_V » Mon Aug 27, 2018 3:31 pm

Jim has extensive experience in the induction world, and was the source of the 100 pound furnace I rebuilt. He isn't aware of the current state of the furnace, but he'd be well pleased to see how it looks. In due time, I'll get some pictures up so he can see what a furnace that was stored outside in the elements, looks like now.

Thanks again, Jim. You've been so helpful with my project that I don't know what I'd have done without you.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

rrnut-2
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Re: Induction heating/furnace

Post by rrnut-2 » Mon Aug 27, 2018 3:46 pm

You're quite welcome Harold.

Jim B

reubenT
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Re: Induction heating/furnace

Post by reubenT » Tue Aug 28, 2018 9:15 pm

Im sure yer aware that foundry floors should not be concrete. Molten metal and concrete don't mix very well I've heard, splatters bad I guess. A layer of leveled packed clay in the pouring area would work. Long as it's dry enough, when spilling molten metal on it it won't do anything. I didn't use concrete at all, building cheap pole barn shop, and knew I was going to do hot metal as well. Someday I will make a machine shop with concrete floor beside the big dirt floored shop, which is 30x60 and 12-14 ft walls.

Harold_V
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Re: Induction heating/furnace

Post by Harold_V » Wed Aug 29, 2018 2:01 am

Yep, I'm well aware of the problems of casting over concrete, but I must have a decent floor in my shop, as I have a lift truck and some heavy equipment (the induction power supply weighs in at 6,000 pounds, for example).

I plan to have a raised area filled with sand where I'll do my casting. So long as you don't have intimate contact of hot metal with the concrete, it's unlikely a spill would transfer enough heat to cause a steam explosion, and that's the problem with casting on concrete. I learned that lesson as a teenager, when I spilled some molten lead on the basement floor.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

rrnut-2
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Re: Induction heating/furnace

Post by rrnut-2 » Wed Aug 29, 2018 9:22 am

Our shop uses a "foundry mix concrete", it has a lot more rock in it than normal. Even with everyday use, it still has to be replaced every five years.

Jim B

Harold_V
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Re: Induction heating/furnace

Post by Harold_V » Wed Aug 29, 2018 3:56 pm

I've not heard of such a mix, but find that comment interesting.

Is a protective layer of sand, or other insulating media, used on the floor, Jim, or do they simply work directly on the concrete?

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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tornitore45
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Re: Induction heating/furnace

Post by tornitore45 » Wed Aug 29, 2018 4:19 pm

The relationship between furnace size and frequency is defined by the "skin effect".
The skin effect depth of penetration should be in the order of the size (radius) of the crucible.
While losses increase with frequency, there is no point in raising the frequency to the point that only the crust of the load is heated.
Deeper than skin depth the eddy currents are much reduced.
Is all a matter of getting the best coupling of energy from source to load.
Since Iron is magnetic, skin effect phenomena appear at much lower frequencies that RF in Copper.
Mauro Gaetano
in Austin TX

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