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LCLR Induction Heater

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Josh Campbell

Mon Jan 28 2013, 03:32PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Ha, that's funny. Looks like yours has been through a lot! My father was an electrical engineer for a dairy processing facility and he had an even older one that he used most of the time. I think this one was his "nice" one. It's still in the original yellow fabric case it came with otherwise it would probably look similar.
Anyway, back to work! :)

teravolt

Tue Jan 29 2013, 02:50AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Exalent work Zum Beispiel. Hi 2bytes do you think that a low iron ferrite toroid or a rogowski coil might work?

Josh Campbell

Tue Jan 29 2013, 07:06AM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

It's possible, a less magnetic material may lower the heat from magnetic hysteresis. It's worth a try, what type of material are you thinking?

I've tried a typical Rogowski coil wound on a 1/4" pvc tube. The problem with using it on an induction heater is that rog inherently has a very low full scale voltage, a few milivolts if your lucky. So it requires an integrator circuit, the circuit isn't complex but dealing with the noise when such low signals are involved is. I was not able to get a clean current reading using a rog when running at working power levels. If you were able to find a material that basically brings the rog closer to performing more like a typical CT yet doesn't heat up significantly that would be great.

More Q!

Zum Beispiel

Tue Feb 05 2013, 11:04PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Hello again. Got some more work done on this. The new inverter has been built and tested. Have some video evidence below.

Everything seems to run nice and smooth. The driver is in dire need of an upgrade, though. Trying to keep the thing in resonance manually is a pain.

I did a stress test for a couple of minutes at around 300W, but I didn't notice any heating in the current transformer. Or if there was any, it was too slight to really notice. Of course this is with only a kVA or two in the tank, so the effect might kick in at a higher power level. Measured tank current was around, maybe a little over, 100A peak during the test run. For now it seems to be just fine, though.

The work coil got nice and hot though, but that's to be expected. Water cooling will fix that. The caps stayed nice and cool with only natural convection cooling at this power level.

Anyway, it's getting late. I'm off to bed. If anyone has any questions I'll be glad to answer, and if there's any comments I'll be glad to hear them.

Tetris

Wed Feb 06 2013, 01:17AM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Why does it glow purple?

Josh Campbell

Wed Feb 06 2013, 06:07AM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

HighVoltageChick wrote ...

Why does it glow purple?

ZB's camera has no IR filter.

Good job ZB! Signals look decent and your board looked very clean. Next step, bigger power supply?

Something to keep in mind when/if you go to higher power levels.
In voltage-fed series resonant inverters, when switching above resonance the antiparallel diode (body diode in the MOSFET) is turned-off softly due to the voltage drop of the MOSFET that starts conducting. This low reverse voltage, that is proportional to the loadâ€™s current, causes a slow reverse recovery of the antiparallel diode. Afterwards, when the MOSFET turns-off, a high reverse voltage is applied to the diode. Due to the stored charge, this high reverse voltage can generate a reverse recovery current that can result in the turning-on of the parasitic BJT. Because of this special care has to be taken regarding the reverse recovery time and current of the antiparallel diode when using MOSFETs. It is generally recommend to use MOSFETs with fast diodes, ideally fast-recovery epitaxial diode field-effect transistors (FREDFET).

To have a more complex but more reliable inverter, I would recommend canceling the antiparallel diode and adding an external free-wheeling diode like this:

1360132586 5258 FT1630 Canceling Mosfets Antiparallel Diode

Some possible fast diodes would be MUR1560 (35nS), RHRG75120 (85nS), or DSEI120-12A (40nS) this one is a FRED rated at 109A & 1200V. The antiparallel diode in your IRFP460's has a Trr of 570 to 860 nS a continuous source-drain current of 20A and a pulsed forward current of 80A. If you find yourself blowing FET's at high power levels this could be a possible fix.

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