induction heating

[machworx]

Anyone out there know anything about how to build a small induction heater/furnace for melting small quanities of metal?

[Chris Smith]

The heater element in most coffee pots minus the thermal cut out switch can get quite hot! <p>Enough to melt aluminium I know, for a fact. <p>What you need is a way to control it, or it will run away and melt down, and possibly start a fire?<p> A variac is the way I control most things, and chopper light switches are the other, but the common light dimmer can only handle 600 watss so that wont do. You can beef them up however, with a larger triac. <p>The heater is way over a thousand watts, most in the 1500 range. <p>20 and 30 amp triacs are cheap.

[machworx]

Yes but that is resistive heat not inductive. Real precious & refractory metal foundaries use inductive heating to melt metal which as I understand it now uses eddy currents & hysteresis losses to heat the metal. ( this process is also used for non-contact heating of metallic objects) There is some relevance to the frequency of the power source with regards to how effectively it heats the metal.

[toejam]

the way i look at things, heat is heat.Can you explain the differance between resistive heat and inductive heat? Maybe we can get into microwave generated heat, and the relation of frequency of the power applied to the heat.A whole new science could evolve from this.

[Chris Smith]

The hot water heater we had over seas was 50 cycles, 240 volts, and after It burned out and I took it apart, it was nothing more than a standard coil configuration wrapped around a stacked sheet metal plate core. I believe it simply fought it self in harmony until things got hot.

[machworx]

Inductive heat relates to heat generated via induction . Similar to how a transformer works. Resistive heat refers to heat generated via current flowing through a resistance. Microwave heat comes about because of a harmonic vibration set up between a particular molecule ( typically water ) and the frequency of the electromagnetic radiation.

[CeaSaR]

A quick Google gave these 2 sites to start.<p>1.) Ameritherm Inc. - About induction heating.<p>2.) Lepel homepage with links to more info and articles.<p>Sounds like you really need to know your metallurgy. Good luck.<p>CeaSaR

[cato]

Thanks Ceasar thats a good link ( I've only looked at the first one). I too have wondered about inductions heating and, for example, what would have to be done to use a short wave transmitter to drive a work coil. I'd like to try to weld aluminum cans together to make antenna elements. I think the precision offered by induction is the way to go. But the output impedance of the shortwave transmitters I'm aware of is about 50 or 75 ohms. I'm guessing you need something much lower to drive an induction coil. Additionally, I have no idea how to determine how much current (in the coil) is reguired to heat a give load to the welding temperature. Any thouhts Machworx?

[machworx]

I suppose one could use some sort of frequency generator as a driver if you could flucuate the high current with the frequency. The higher the amps the better The one I want to build I would think would draw 230V AC @ 60 amps. The high frequency relates to skin effect heating VS a deeper heat when a lower frequency is used. Most furnaces I have read about are between 60 & 1000 HZ with an occasional at very high frequencies. So far as I have been told the metal in the crucible ( surrounded by a helical water cooled coil ) is magnetically coupled causing it to act like the secondary in a large transformer which is basically in short circuit mode.

[Chris Smith]

Buy a Tig machine

[L. Daniel Rosa]

Machworx, There are some factors that need to come into consideration. What size cricible(s) are you going to use. The coil shouldn't be much bigger than necessary to go around the crucible and some refractory. Also, what are you willing to pay for parts? This decision will probably determine the power of the device.<p>IMHO less than 1000VA will be a bit iffy and more than 4000VA will get expensive really fast and require a 30A(or more)/240V breaker on a dedicated circuit.<p>The frequency will probably end up in the high-audio to superaudio rnage just to balance switching losses against coil resistonce.<p>FWIW, small scale melting will probably be easier and cheaper with a resistive furnace. I have one in my garage that will melt aluminum, but not copper on 120V. All of the parts (nichrome element and an infinite range control) will happily run on 240V and may be able to melt copper at that point.

[machworx]

No I want an induction type furnace cause who knows what other metals I might want to cast? The price of the componets shouldn't be a problem if I construct this myself. Best I can tell right now I need between 15 & 50 KW to melt approx 25# of metal in a reasonable amount of time. I'm not currently sure about the possible material choices for the crucible & how it reacts to the magnetic field lines.

[Chris Smith]

Aluminum brazing is much cheaper, one can of alu-flux, rods, and a small tourch? I used to fix my aluminum parts this way, in the 70s.

[Dimbulb]

When I looked into the practicality of very small scale foundry applications using inductance heating I do remember that most books described equipment for very large scale. <p>There where a few small applications such as warming ball bearing cases and this equipment was unfortuneatley not readily available to look inside. I don't know much about microwave ovens other than putting aluminum foil was a colorful event. I concluded that cost effect would be to buy an electric kiln.

[Joseph]

Chris, I am working on an induction water heater composed an iron pipe split lenthwise that is clamped onto the existing copper pipe and heated with a waveform driven coil.