Caution Using an Induction Furnace

GreenTwin · Aug 12, 2024

This page shows what is probably a typical small induction furnace section.

https://inductiontech.com/products/induction-melting-furnaces/compact-power-cube-furnace/

GreenTwin · Aug 12, 2024

Some furnace data and general discussions here:

https://www.foundrymag.com/melt-pou...efficient-induction-melting-inductotherm-corp

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GreenTwin · Aug 12, 2024

I think building your own induction furnace would be like building your own automobile.
I could build my own auto, but I would not want to use it for anything other than slow-speed offroad use.

Much better to purchase a well-designed, well-finished auto, and enjoy driving.

I like to design and build things, but at some point it becomes more practical and less expensive to purchase a manufactured unit.

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GreenTwin · Aug 12, 2024

Here is a small induction melter for sale (not my photos).
Could probably be purchased in the $25k range (+ -).
I am not sure if the chiller is built into this unit, or would be a separate unit.
This is for melting 100 lbs.
As you can see, this is a very complex and expensive way to melt 100 lbs of metal, unless you were running a high-volume commercial operation, where speed was of the essence.
From a physical standpoint, it requires quite a bit of floor space, and no doubt a 3-phase electrical service.

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GreenTwin · Aug 12, 2024

Here is a pretty nice tilting oil-fired furnace, which would be a pretty economical way to melt metal for a small scale (perhaps backyard) operation.
I bit on the large side for hobby use, but not abnormally large.
Uses a packaged oil burner unit, often seen in commercial oil heating units.
Apparently hydraulic tilt.

Not my photos.

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GreenTwin · Aug 13, 2024

Here is a chart from an induction furnace manufacturer.

My oil burner operates at 120 kW (it can produce more power, that is just where I operate it).

So looking at the analogous 110 kW unit below, and assuming 208V, 3-phase, which is what I have, then we would have 305 amperes, which exceeds my 200A service.
My electrical service would not provide 200A continuous either, and so that would be another problem.
At most you coud draw 160A from my service, but it also depends on the transformer sizes on the pole, and the utility company always undersizes those by a great deal.

So in summary, an oil fired furnace is a very economical way to melt iron on a tight budget, and also a very reliable way to melt iron, with basically no maintenance costs over time.

If I happen to win the lotto, I will look at an induction furnace.

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abby · Aug 13, 2024

I think owning an induction furnace is not for the hobby model engineer , the economics just don't make any sense.
It's like having a 3D printer , handy, but Shapeways quality and service are superior and there are several other firms offering competition so prices are reasonable too.
However some time ago I looked at the possibility of a vacuum brazing rig using induction heating.
With laser cutting of steel so cheap I believe that brazed steel fabrications can economically replace many of the castings that I used to produce for various scale locos.
ALDI supermarkets, do you have ALDI in the states?had a plug in induction hob for about 30 quid (British pounds).
I bought one with the intention of removing the hotplate and fitting a coil but as always there are so many other jobs that come first.
Anyway a piece of steel sheet placed on the hotplate glowed red in under a minute so I reckon it could work.
Dan.

ddmckee54 · Aug 14, 2024

Like Abby says, the economics don't make sense for a hobby caster.

The infrastructure needed to support something like the 110KW heater listed by GreenTwin IS NOT CHEAP. Something like that would probably require a 400A service. An off the cuff guestimate would put its' minimum full load at about 270-ish amps. Multiply that by 1.25 to meet code, and you're at about 340A just for the load of heater. #400MCM THHN cable is rated for 380A so that's about what you'd need, the cost of that crap is 10-12$ per foot, for each phase. If you go for the full 400A then you need #500MCM - which is about 12-15$ per foot. Also your electric company would notice a load like that and bill you at that rate, year round. Not just when you use it, because they've got to be able to support that usage rate at any time.

Then there's the cooling, that 110KW heater needs 20L/min of cooling. That's over 5 gallons a minute for those of us on this side of the pond. While not a huge flowrate it'll still add up, to your water bill if you live in town, and if you've got your own well - can it flow that much?

For jewelry sized castings an induction heater is feasible, because you can get by with a few kilowatts. For the bigger stuff there's just too much infrastructure required for a hobbyist.

TSutrina · Aug 14, 2024

Clearly a hobbyist will not consider anything above unless your casting huge things. Most home circuit breakers will be in the range of 220 VAC single phase and a 30 or 50 Amp breaker with the smaller one the most common. Assume 75% efficiency: power ~ 5 Kw and 8.25 Kw. Drop this down further just to deal with heat losses like the water cooling the induction coil also cools the metal. That can be substantial.

ddmckee54 · Aug 14, 2024

TSutrina:

I agree, when I came up with the 270A that was for a 100% efficient resistive load, just to give a minimum number. If you want to be generous and go 90% efficient that number goes up to 300A and everything else goes up accordingly.

The costs that I listed were just for the copper conductors, you add in the conduit, installation/labor, distribution panel or panels and it really adds up.

Don

100model · Aug 14, 2024

GreenTwin said:
I could build my own auto, but I would not want to use it for anything other than slow-speed offroad use.

That is a great example to compare to those cheap induction melters on ebay. They are the same ones that are shown in the three videos in this thread. They are very cheap compared the induction furnaces from inductotherm. That cheapness comes at a cost as there is no controls. Many have told me if you buy one make sure to have plenty of spare power transisitors as they will fail from time to time. To use the car as an example building a car without a transmission will be very rough on the engine but a transistor without controls will fail a lot sooner than a car engine.

100model · Aug 14, 2024

abby said:
had a plug in induction hob for about 30 quid (British pounds).

I did the same thing with a inverter microwave oven. the inverter runs at 30Khz so it is suitable for induction melting or heating small things. If you modify anything be prepared to fry the power transistor. How do I know that...... In Australia so many throw out those inverter microwave ovens so there is plenty of them to experiment with. I assume that you feel comfortable experimenting with 240 volt appliances?

TSutrina · Aug 15, 2024

The Ignoble Troll said:
I feel like those coils should be hydrotested regularly. It takes such a short time to hydrotest something, especially a critical part thats exposed to corrosion.

I help packaged a piece of equipment that measured corrosion in pipe for oil refineries. the problem mentioned here also occurs there. What was done was to attach in this case titanium wires in an array and measure the resistance between them and compare to the original. Very hot pipes.
Your test is to measure resistance in each turn of the inductor coil and compare them using a bridge approach used decades ago before all of this fancy equipment. Need to amplify the difference.
I would change the cooling fluid to something that is far less corrosive and have a heat exchanger to the cooling water or a radiator. Then the total system can be pressurized to a fixed value (air bulb expansion chamber needed as small as possible. and set a trip off for a pressure drop which indicates a loss in fluid. That would be my approach. The insulation needs to contain the volume leaked without contacting the crucible. ( the gap between turns for example)
I also recall when working with similar inductors that were not in a hot environment was to put regular wire in plastic or rubber tubing and put cooling fluid through the tubing. Then for high frequency litz wire could be used.

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