Advice on designing and building an induction furnace for small-scale iron melting - Page 2

# Thread: Advice on designing and building an induction furnace for small-scale iron melting

1. Lindsay reprinted Frank Curtis's High Frequency Induction Heating - as early as page 6 he is going into the large power requirements

2. Originally Posted by oughtsix
Look at all the sub \$2K induction heaters on ebay. Look at the size of the coils. How are you going to shove 60lbs of iron in a coil that size and heat it?

You have to tune the circuit for the coil you are using. You can probably spend a year trying to calculate the parameters for the coil that will be big enough to melt 60lbs of iron with any sort of efficiency. Like 9100 - Bill said above the characteristics of the coil change as the metal goes from below the curie point to above curie. Curie is the temperature at which a metal looses its magnetic properties. So to melt a metal with an induction heater you have to a coil that is a compromise between the below curie and above curie efficiencies. This is what proper induction heater design is about, designing a circuit and coil that are efficient enough to get the job done without having to use a massive power supply to over come the inefficiencies of your design.

Your table napkin math is assuming that you can come up with a design that is 100% efficient. Real life is far from 100% efficient... like I pointed out above you might be able to design a coil that has decent efficiency below curie but as soon as your metal reaches the curie temperature its power transfer efficiency is going to be so low that the heating will all but stop. So you are going to need a MUCH large power supply than your simple calculations suggest.

Attachment 257088 Attachment 257092

This is my Induction heater. It is computer controlled and monitors the intersection of the area under the voltage wave form and the area under the current wave form which is the actual power going into the metal. The closer the temperature gets to curie the lower you have to adjust your frequency to compensate for the loss in magnetic strength of the metal. At curie you have to make a big jump in frequency to continue to induce energy into the metal.

Bottom line... you are tilting at windmills!
As I pointed out in my original post I know that my rough calculations are not representative of real world. Based on the tone of your post I feel like i should apologize for asking the question. I, amazingly enough, did notice the small coils on the cheap induction devices linked and did have some "vague notion" that some other coil arrangement would be necessary for my 60 pound crucible. See paragraph three of the original post.

The reason for asking a question is to learn.

Thanks,
Denis

3. Hot Rolled
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My CFEI Induction furnace runs at 100 kW (150 amps per phase off 415 volt three phase. It 'self tunes' by pinging the coil and measuring the resonant frequency, which obviously varies with the charge - I'd guess 60 lbs of iron would be possible - and it runs at about 3 kHz

Not only do I need to give it vast amounts of electricity for the induction driver, it has an additional 15 kW chiller for the water cooling passing through the coils - it's good kit but I've not yet re-commissioned it since I moved here, but it's 'on the list'

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More pictures

5. oughtsix and 9100 you are working with very small induction heaters. The rules are the same, the parameters are different for large melting equipment.

dgf - resend the the email address, it didn't make the trip. And your pm inbox is full.

Tom

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Originally Posted by dgfoster
As I pointed out in my original post I know that my rough calculations are not representative of real world. Based on the tone of your post I feel like i should apologize for asking the question. I, amazingly enough, did notice the small coils on the cheap induction devices linked and did have some "vague notion" that some other coil arrangement would be necessary for my 60 pound crucible. See paragraph three of the original post.

The reason for asking a question is to learn.

Thanks,
Denis
I certainly didn't mean to offend you. I think the stupidest thing you could have done is not asked the question! I have worked with induction heaters a bit... I am no expert... I was fortunate to have 2 experts in the field working with me. My intention was to share some of my limited knowledge to help you understand what you do not know... maybe discourage you from spending a lot of time and a lot of money on a project that isn't very likely to be successful... maybe to point you in the right direction of what information you will need to research to continue.

The mini induction heater I built was just a test unit to prototype the computer control... proof of concept. The application was heat treating. The power going into an induction heater is not directly proportional to the power going into the metal being heated. The efficiency of the induction heating determines the actual energy transferred into the metal. The efficiency of the induction heating changes with a lot of variables like the temperature of the metal in the coil and the mass of the metal in the coil. Shifting the frequency of the induction heater can compensate for changes in the temperature/mass of the metal affecting efficiency. For critical applications... like uniformly heat treating heavy duty axles... very complex models are created to determine if every part of the axle shaft has been heated to the proper depth for a strong hard surface but still maintain a softer less brittle center. It is impossible to perfectly model such a process because the variables are too numerous.

By measuring and recording the efficiency of the induction heater periodically (1000 times a second) the actual energy absorbed by the metal can be closely approximated and a much more uniform heat treating over entire length of the axle can be assured. The power going into the coil can also be constantly altered along with the frequency to insure that a uniform depth and hardness are obtained.

I honestly know almost nothing about melting iron in an induction heater... but did experiment enough to see the big obvious changes in the efficiency of the induction heater as the metal approached curie.

7. You might look around. Electronics is not the only way to go.

I think the fellow with the Youngstown Museum had a motor-generator type furnace in your power range. That might be more mechanical, and involve less power electronics. Having done power electronics for most of my career, I can tell you that it is not trivial, and there are details galore that determine whether it works or fails. of course both types can fail, but.....

Unless you want to learn a complicated specialty, get a commercial unit. the motor generator based type may be the most rugged and practical for you, but the electronic ones can work also. Problem is there are many that are just outside your needs, while the M-G types were made to do what you want done, in sizes .suitable

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If you're just trying to 'clean up' the process, why not preheat using resistive heating, then melt using diesel?

Large resistive heaters are easy to source and do not require a complex power supply. I know a guy who does 'industrial' furniture (I have bones to pick about that phrase but let's not get stuck in the weeds here) and does all his melting with a resistive furnace, but I believe he only does aluminum.

My first job out of college was for a large induction furnace manufacturer, and all I can say given that experience is:
1) I would not attempt to build my own furnace (at the very least, check the used market).
2) I'm happy to let other people deal with molten metal for me. Never enjoyed working in a foundry, and I've enjoyed working in just about every other industrial environment I've been in.

Regardless of your goals, if you want any sort of efficiency with a melt of almost any size (even 60 pounds), you need a ton of power. I'd follow up and confirm that's available before you get all worked up planning a furnace.

9. I think I will revisit the cupola for your application. A number of papers have been written lately about the furnace and controls. The attached reference shows a cupola more to your size. Assuming you have a way of handling the molten iron, this furnace could probably run by one person.

Cupola furnace - Wikipedia
cupola furnace sizes - Google Search

Tom

10. I appreciate the suggestions of running a cupola. That option has been thoroughly considered previously. Trust me that there are several factors that make it impractical for my circumstances.

It is also, sadly, sounding impractical to set up an induction furnace.

Denis

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Talk to a manufacturer of furnaces and see what the requirements are for their furnace so you then know what is required on the power side.

12. I believe you are correct. Stay with what you have. However, you did not state the reasons to depart from the oil fired furnace. Is there an opportunity to improve here?

Tom

13. Oil-fired furnaces are somewhat noisy (a neighbor issue), require tending and modestly tricky tuning by eye and ear (an art not entirely science) and, most importantly, destructive to the best refractory materials available. The combo of fuel, oxygen and heat is much more damaging than oxygen and heat alone by a factor of ten or more. In addition rapid temperature cycling from 3000+F to 70 over and over is a torture test for refractory materials.

Being able to to just plug in the heater and coming back a few hours later, say, would be a luxury. With a well-tuned and well-insulated diesel furnace you can melt in an hour to 85 mins, but you have to work at it adding metal, skimming, tuning, listening, tuning, etc.

Denis

14. The noise could be attenuated to some degree. The temperature cycling is just something you will have to live with. You probably know a lot more about refractories than I do. The tuning I don't understand. Is this varying fuel/air mix, the blower pressure or what?

Have you considered an electric arc furnace. Unfortunately these have to be tuned by adjusting the electrodes. check with member Miguels244. He was looking for design information last fall on melting of steel using electric arcs.

Tom

15. I worked a little on arc furnaces. This book is old but the principles are still the same. The ones I worked on were three phase but it would work with single phase power. The carbon arc rods were motor driven in until they touched the iron, then pulled away when the arc was established. The current was from rod to rod, no connection to the iron. A single phase unit might to establish a common to the iron. I can show you a circuit that Lincoln Welding used on a wirefeed system that would work with these rods and be simple to build.

Bill

Title Electric Melting Practice
Author A. G. E. Robiette
Publisher C. Griffin, limited, 1935
Original from the University of California
Digitized Oct 5, 2007
Length 324 pages

16. Originally Posted by 9100
I worked a little on arc furnaces. This book is old but the principles are still the same. The ones I worked on were three phase but it would work with single phase power. The carbon arc rods were motor driven in until they touched the iron, then pulled away when the arc was established. The current was from rod to rod, no connection to the iron. A single phase unit might to establish a common to the iron. I can show you a circuit that Lincoln Welding used on a wirefeed system that would work with these rods and be simple to build.

Bill

Title Electric Melting Practice
Author A. G. E. Robiette
Publisher C. Griffin, limited, 1935
Original from the University of California
Digitized Oct 5, 2007
Length 324 pages
Hmmm, maybe I should be taking a look at this idea. How complicated is the power supply to power the arc?
I’ll try to do a little reading along this line. I would be very interested in any experience/insights you might have.
Thank you.

Denis

17. Originally Posted by dgfoster
Hmmm, maybe I should be taking a look at this idea. How complicated is the power supply to power the arc?
I’ll try to do a little reading along this line. I would be very interested in any experience/insights you might have.
Thank you.

Denis
Super cheap used hardbound

Electric Melting Practice - AbeBooks

18. Originally Posted by dgfoster
Hmmm, maybe I should be taking a look at this idea. How complicated is the power supply to power the arc?
I’ll try to do a little reading along this line. I would be very interested in any experience/insights you might have.
Thank you.

Denis
My direct experience was rewinding and making a set of split core current transformers for the arc furnaces at the foundry next to the Van Deventer overpass, I believe originally called Century Foundry. They used three carbon rods about 6" in diameter and ran at 12,000 to 20,000 amps each. When they struck an arc, the flexible cables to them writhed like Medusa's hair.

As to power supply, they ran on simple 60 cycle three phase power. I don't remember the voltage but such arcs typically run about 45 volts. They had current transformers on all three legs, each controlling a motor that ran the rod up and down. The current for each phase would be returning through the other two so the phases interacted to balance themselves. On single phase it would be possible to have one rod contacting the metal and the other pulled back to put all the voltage on its arc. A common reference would be necessary but shouldn't be a problem since it would not be carrying much current.

The control circuit in the Lincoln welder had a DC motor generator with the generator field controllable to the desired voltage. A DC motor was connected between the generator and the wire feed. When the arc voltage exceeded the generator output, the motor (geared down, of course) fed the wire faster to shorten the arc. When the arc voltage was lower, the motor would slow or reverse. Starting, the motor would feed the wire in until it touched, then back off to the desired voltage.

Doing it with AC would be a little trickier, but you could do it with a two phase servo motor.

Remember that the reference source needs to be able to conduct backflow.

Bill

19. Dennis, look at this video. Gives an idea of what a arc furnace might look like.

Electric Arc Furnace (aka Carbon Arc Furnace)

Tom

20. That is great but way over designed for most purposes, especially for aluminum that you can melt with your gas stove. As you know, melting iron is a whole different level.

One thing they mentioned in passing, one of the limiting factors of fuel air heat is that for every cubic inch of oxygen you burn, you are blowing 4 cu in of cold nitrogen on the metal. The other methods eliminate that.

Personally, I would look at two arc rods if you don't have three phase so you don't have all that current going through the crucible.

Arc lamps often simply let the top rod loose so it falls against the lower rod, then yank it back a preset distance with a solinoid in series with the rod. If the arc fails, it will restrike the same way. To compensate for burn off they shut off for a second every so often and restrike.

I will post a picture of the small furnace I made for melting silver. I used light weight fire brick, which cuts readily with a band saw, just don't breath the dust, and cut out V shaped sections to make the insulating walls. I don't know if it is available for those temperatures.

The use of a welder is a good idea if you can come up with a big enough one. They typically have some version of a cruciform core transformer that inserts leakage reactance in the circuit to limit peak current when touching up.

The three furnaces in Century Foundry had no interlocks so all three could be struck at the same time. I'll bet the power company loved them.

Bill