Induction Heater 1.0

Introduction

This is an emergency induction heater built for my AP Physics C E&M midyear project. When all else failed (I’d been toying around with various other project ideas for a few weeks, unsuccessfully), I grabbed some Tesla coils parts, etched a new control board, and cobbled together this contraption in one night.

Induction heaters work by inducing massive eddy currents in a metal workpiece, causing it to dissipate power through IIR and hysteresis (magnetic) losses until it glows red or yellow hot, or even until it melts, depending on the power input. The workpiece is basically acting like a very naughty shorted secondary winding in an AC transformer. Like in a Tesla coil, there is a resonant LC circuit that allows the heater to push massive currents through the work coil, often in the range of hundreds to thousands of amps. We keep the switching transistors happy by stepping down a high voltage, low current signal to a low voltage, high current one with a ferrite matching transformer. Additionally, since there is a series LC being reflected back through the matching transformer, the bridge soft switches at resonance. There is a detailed explanation describing induction heater theory on Ritchie Burnett’s site.

I ultimately decided to build a more permanent and more robust induction heater with more features such as liquid cooling and digital power control – see Induction Heater 2.0!

Design

Due to the simplicity of adjusting a variable frequency oscillator for resonance as opposed to setting up a current feedback system, the former was chosen as a driver for the induction heater. Since we’re not running a Tesla coil where streamer loading and coupling to nearby objects can cause serious detuning problems for a fixed oscillator, we can get away with manually tuning for each new object being heated by monitoring average primary current with an ammeter, which is thankfully much more portable than an oscilloscope, and varying the frequency until the maximum current is achieved at resonance.

I used a TL494-based oscillator designed by Bayley Wang and an intermediate H-bridge of IRFZ24N N-channel MOSFETs as a gate driver for my CM200TU-12F IGBT brick (using 2 of its 3 phases in a second H-bridge). It’s complete overkill for the relatively low power levels I’m running, but at least it means that everything stays cool and there’s little chance of transistor failure – quite a shift from my DRSSTC projects where the tiny TO-247 IGBTs seem like they’re always about to explode!

(Mostly) hand-drawn schematics:

(20T:2T is the self-wound ferrite matching transformer)

Eagle schematic and board layout for the VFO:

Construction

As this project was mostly constructed in one wild all-nighter, I didn’t get to take many pictures of my progress. That being said, I did manage to get one or two pictures before and after:

Cutting the brick heatsink. This part was originally (and still is, eventually) intended for a DRSSTC, so I assmebled it all months before I ever knew I’d be making it into an induction heater:

SOT-227 “mini brick” dual diode module mounted on the heatsink (part of the 400V power supply doubler):

The completed system:

(From right to left, bus caps, IEC connector, 8uF snubber, brick, GDT, and gate resistors, control board, maching transformer, primary capacitors, work coil)

Testing

Unloaded GDT waveform – lovely!

Uh oh! When the try to drive the brick, ringing and loading are so significant that the gate voltage drops all the way down to 1 or 2 volts during the on-cycle – disaster! Driving the brick like this would result in a dead brick.

Adding gate resistors helps damp the unwanted oscillations, but the gate voltage still drops below 5V during the on-cycle. This setup might work, but the brick would run super-unhappy =(

Finally, the gate waveform became acceptable after I added more decopling capacitors in my intermediate bridge, another resistor at the primary side of the GDT, and some more turns to the GDT. Ideally, the gate voltage wouldn’t spike more than about 5V over the designed maximum, and the ringing would be overdamped, but this is good enough – especially given that the brick doesn’t have to handle hundreds of amps like in a DRSSTC.

Results

(Running at very low powers to avoid tripping my 15A breakers and to keep the work coil relatively cool)

Heating a 3/8″ diameter steel shaft:

At a demonstration in school:

On a completely different note, it was awfully misty/foggy the night I built this, so I decided to take a break and have some fun with my HeNe laser – the beam is visible for hundreds of feet!

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