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OMG Induction Heater

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Steve Conner

Mon Mar 26 2012, 06:59AM

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

Iamsmooth: I've levitated a small piece of aluminium tube, but I had to push the setup to the absolute limit. I turned the current limit up too high, and when the piece melted, it got unstable and fell out of the coil, and my IGBTs went boom.

Levitating a solid sphere of metal would take lots more power.

Dago

Mon Mar 26 2012, 10:17AM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181

I wonder why (all?) commercial heaters use the LCLR-topology, cause the poor power factor does make it sound pretty bad? Can you compensate the poor power factor in any way?

Steve Conner

Mon Mar 26 2012, 10:23AM

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

Yes, if you drive it at the correct frequency, you get unity power factor just the same as the series resonant.

The correct frequency is not the resonant frequency of the tank circuit, but slightly above, where the tank circuit appears as a capacitive reactance of the right magnitude to cancel the inductive reactance of the matching inductor.

Ameritherm's small heaters use series resonant. I got the idea from some gut shots that were shared in the 4hv chat years ago.

IamSmooth

Mon Mar 26 2012, 01:25PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

As I mentioned earlier, I used series resonant, got it up to 12kw of power and did all kinds of crazy stuff. At these power levels you need to make sure you stay slightly above as Steve mentioned. If you start get to the true resonance or below the switches will go boom. I put a 60A fast-blow going to the inverter to protect my board from secondary frying if the switches shorted.

I minimized my switching loses with lower frequencies and fast turnon/turnoffs. One issue with faster transitions is the ringing, which has to get snubbed, or else you will have a worse situation. As far as the frequency: while I used lower frequencie for melting bricks of iron, I used frequencies close to 120 kHz, but was still able to get rid of the heat without compromising the mosfets.

You might want to consider using mosfets if you are using frequencies above 20-30khz.

Gabriel35

Mon Mar 26 2012, 01:41PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

The discussion Is getting Interesting!
Unfortunatly I've never succeded with a PLL driven Induction Heater, But I'll not stop trying...

Currently I'm in doubt of what topology to use...
I'll try some of them and see what happens...

Kizmo

Mon Mar 26 2012, 03:40PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

IamSmooth wrote ...

As I mentioned earlier, I used series resonant, got it up to 12kw of power and did all kinds of crazy stuff. At these power levels you need to make sure you stay slightly above as Steve mentioned. If you start get to the true resonance or below the switches will go boom. I put a 60A fast-blow going to the inverter to protect my board from secondary frying if the switches shorted.

DRSSTCs are switched at perfect resonance at current zero crossings and they dont blow up...?

EDIT:

I guess i have to explain my idea of how im going to do this heater.

Series resonant tank circuit. 10ÂµF 1kA celem capacitor, about 3ÂµH work coil + stray inductance. Resonates roughly at 29kHz. Coupling transformer is just large ferrite transformer with 1:10..1:20 ratio. Feedback is taken from tank circuit current with measuring capacitor (150nF cap in parallel with celem and small ferrite CT). Inverter is igbt half bridge powered by rectified 3 phase and im not planning to use any filter capacitance on bridge, just couple ÂµF worth of film caps. Driver is normal DRSSTC driver with interrupter.

Interrupter would be outputting pulses with 0-99% ON time. Idea behind this is to be able to have at least some sort of power control by varying on time. Drsstc has of course zero crossings synchronized over current protection which in this case would be measuring inverter output current. I guess if needed i could add another overcurrent protection circuit that measures tank current via same measuring capacitor. That would serve as tank capacitor over current/voltage protection.

Due to my limited knowledge, i dont see why this kind of contraption wouldnt work. At least it has proven itself in drsstc world. If you see potential problems, please stop me before i waste my time ;)

Mcu controlled PLL driver is *far* too complicated for my knowledge

GeordieBoy

Mon Apr 02 2012, 11:48AM

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

> But Richie says the LCLR is theoretically better. I can't remember why, maybe he can comment?

The work-coil circuit is really just an impedance matching network to convert the unfeasibly low impedance of a lump of solid metal up to a figure that is realistic to present to a solid-state inverter running directly off the mains line. I don't think it really matters how you achieve this impedance transformation as long as it is done efficiently because the currents and voltages at work in IH are large.

I prefer the LCLR network for a number of reasons:

1. It keeps the enormous induction-heating current confined to the work-coil / tank capacitor loop. This loop can be made very small and may be located remotely from the inverter.

2. It does away with the need for a high-power high-frequency transformer requiring careful design to minimise core/copper losses and water cooling to carry away the heat from whatever dissipation remains.

3. It is better to alter the impedance matching by varying the air gap in the matching inductor than to change taps on a transformer. A multi-tap transformer implies poor winding window utilisation for certain tap choices resulting in an overall bigger transformer unless series/parallel winding combinations are used.

4. Stray inductance in the wiring between the inverter and the heat station merely adds to the matching inductance, rather than adding to the work coil inductance and reducing coupling to the workpiece.

5. The LCLR network presents a light inductive load to the inverter in the case of a tank capacitor or work-coil short circuit making the system s/c tolerant to faults at the heat station.

6. If the matching inductor is located close to the inverter it reduces HF switching noise leaving the enclosure improving EMI performance.

7. In high-power systems the tank circuit can be fed from several inverters in parallel through several matching inductors. The "ballasting action" of the matching inductors provides inherent current sharing. It also limits the "shoot-between" currents resulting from imperfect syncronisation of the inverters' switching transitions. (This way solid-state systems in the MW and 100's of kHz range can be realised with a high level of redundancy.)

The downsides of the LCLR matching network are:

1. It is more complex to understand the resonance transformation than impedance transformation with a transformer.

2. Leakage fields from the matching inductor can heat nearby metal enclosure!

3. No galvanic isolation of the work-coil from the mains line.

-Richie,

IamSmooth

Mon Apr 02 2012, 09:30PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

GeordieBoy wrote ...

I prefer the LCLR network for a number of reasons:

-Richie,

Richie, you must have seen my schematic for the work coil. It is a coupling transformer from the switches (10-20:1) around the wire going to a series LC tank. If I were to improve the tank circuit, what would I need to do? A different topology or is this one reasonable? I was able to abuse it without failure, but I'm always interested in other and better ways of doing things.

GeordieBoy

Tue Apr 03 2012, 11:50AM

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

hey, if it aint broke then don't fix it! If it meets your requirements and even survived abuse without catching fire then it doesn't sound like you have anything defficient to improve.

-Richie,

Kizmo

Sun Apr 08 2012, 07:10AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Series resonant version has been great success.

Some youtube clips

Boiling water

Melting car motor parts

Aluminum heating fail :)

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