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induction heating faq? beginners' guide?

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Steve Conner

Wed Nov 12 2008, 01:18PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

jshine, WILL YOU QUIT WITH THE OVERSIZED IMAGES ALREADY!
I resized your first one, but you didn't seem to take the hint. Board rules say maximum width of 400 pixels. We've previously posted details of a quick and nasty way to "thumbnail" externally hosted images, so there's no excuse. Edit your first post to see an example.

Aside from that, nice thread.

jshine

Wed Nov 12 2008, 04:55PM

Registered Member #1784 Joined: Tue Oct 28 2008, 02:30AM
Location: Rochester, MN, USA
Posts: 14

Steve McConner wrote ...

jshine, WILL YOU QUIT WITH THE OVERSIZED IMAGES ALREADY!

Sorry about that -- I wasn't trying to piss in anyone's cheerios -- honest.

jshine

Wed Nov 12 2008, 06:50PM

Registered Member #1784 Joined: Tue Oct 28 2008, 02:30AM
Location: Rochester, MN, USA
Posts: 14

GeordieBoy wrote ...

You may also find these two Induction Heating papers interesting:

Thanks for those links -- I'll read through them tonight. One other question regarding your setup: when you were evaluating your actual, functioning circuit, what range of values did you find appropriate for "R" in the LCLR model? How high was it with no work inserted into the coil, and how low did it go with a heavy load?

The rest of the components have actual, physical values that I can read off or measure prior to turning on the current, but I really don't have any idea how to estimate R, and it makes a big difference in terms of the voltage rise in the tank and the current drawn from the inverter.

Thanks!
-Jon

Steve Conner

Thu Nov 13 2008, 11:34AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You're not supposed to double post inside 48 hours either :P

I thought of answers to a few points that were brought up in this thread:

The inverter output is a square wave, but the tank circuit and matching inductor filter out the harmonics, so only the fundamental does any work. This means that you have to multiply/divide by 4/pi somewhere in your analysis, to get from the amplitude of the square wave to the amplitude of its fundamental.

The ringing on the gate drive chip is probably OK, it should go away when loaded, assuming it exists at all and isn't just stray inductance and capacitance in your scope leads. (There are whole books explaining accurate ways of probing fast pulses with a scope.)

As to the value of R, your device should be designed to work safely without self-destructing for any value of R from an open-circuit downwards.

TO-3 MOSFETs aren't best for high-frequency work because the steel flange adds a lot of inductance to the source lead.

Wall warts are usually floating, so you can tie their outputs to anything you like.

GeordieBoy

Thu Nov 13 2008, 05:32PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

If you model the tank circuit as a circuit in which R, L and C are all connected in parallel. Then the effective value of R will most likely lie somewhere between 2sqrt(L/C) and 20sqrt(L/C). The exact value depends on many things like:

1. How lossy the workpiece material is. (Including whether it is above or below Curie temperature.)

2. How tightly the work coil is magnetically coupled to the workpiece.

3. How lossy the work coil and tank capacitor are at the operating frequency.

As Steve Conner said, the driver should tolerate effective values of R from infinity down to a very low value. In practice an infinite parallel resistance in a parallel resonant circuit implies infinite Q-factor and no damping. So, whilst you can get close to this condition with no workpiece present, a thick copper work coil, and low loss dielectric in your capacitor, there will always be some damping and therefore some measurable value of R. (Since the characteristic impedance of induction heating systems is usually in the region of ohms, then even high Q's lead to modest values of R in the parallel resonant model.)

Note the word *measurable*! All you need to do is to remove the matching inductor from the LCLR arrangement and you can drive the remaining parallel resonant tank from the output of a function generator set for a sinewave. Then you can measure voltage and current drawn to find the network's impedance. If you tune around the resonant frequency you will find a point at which maximum voltage is developed across the tank circuit and it draws minimum current. Dividing V by I gives you the resistance. (At the resonant frequency the reactive components XL and XC cancel out so any impedance which is left must be due to the resistive component alone.)

Doing the above measurement will let you get an idea of how lossy your work-coil tank circuit is, and can help identify any poor electrical connections that might later struggle to carry those hundreds of amps! It will also allow you to see a change in resistance (and frequency) when you insert a workpiece. So it is educational to do the test if nothing else. However, driving it from a dinky little function generator is never going to heat the workpiece above Curie temperature, so you won't get any measurements in that region this way!

-Richie,

jshine

Sat Nov 15 2008, 04:33AM

Registered Member #1784 Joined: Tue Oct 28 2008, 02:30AM
Location: Rochester, MN, USA
Posts: 14

GeordieBoy wrote ...

Note the word *measurable*! All you need to do is to remove the matching inductor from the LCLR arrangement and you can drive the remaining parallel resonant tank from the output of a function generator set for a sinewave. Then you can measure voltage and current drawn to find the network's impedance. If you tune around the resonant frequency you will find a point at which maximum voltage is developed across the tank circuit and it draws minimum current. Dividing V by I gives you the resistance. (At the resonant frequency the reactive components XL and XC cancel out so any impedance which is left must be due to the resistive component alone.)

Excellent idea. I did this already (hook up the LC combo in series with a ballast resistor and tune around to find max V across the LC tank) to find the inductance value of the work coil, but it's just hooked up with little alligator clip-leads, so the connections are probably much more lossy than the "final" version will be. I'll have to find some sturdier wire to hook it up in a functional configuration & then try this again to measure R. Actually, my function generator is on its last-leg and the sine wave output is broken, so I was using square wave. The voltage across LC ends up as a sine anyway though...

Hernan

Thu Nov 20 2008, 10:57AM

Registered Member #1614 Joined: Wed Jul 30 2008, 03:08PM
Location: Argentina
Posts: 52

Jon , this paper could be useful for your design. I'm building an induction heater too ,at the moment I 'm looking the way to regulate the gap of the matching inductor to heat diferents workpieces, I thought to do it with a stepper motor .at the startup would be closed and it will gradually opens to match a desired inverter current. I observed that when the curie temperature is reached, the current tends to increase because the electrical resistivity increases more than two or threefold to the initial value (in a carbon steel). the permeability drops to 1 so the penetration depth increases to to six or more compared initial value.
so when this point is reached the matching inductor will be closed reducing the gap so the current will be reduced to safe values . I would like to ask the forum if it is other method to do this because I not using a controlled rectifier.

this is my setup

I melted steel and slags with a graphite crucible as seen in the pdf file atached. later I will post the schematic

regards

]parallel_tank_ih.pdf[/file]
]prueba_-_escoria_alto_horno.pdf[/file]

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