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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, I noticed hfsstc-freak's comment on the Youtube vid, and I agree with his analysis of the situation. The guy probably has a couple of conduction-cooled capacitors, specially designed for induction heating, between those heatsinks. The capacitor is the most difficult part of an induction heater, and if you can get a real induction heating type, your chances of success go right up!
The crucible probably makes a big difference too. It'll keep the heat in, and the steel will get a lot hotter than if it was just dangling in the work coil.
I managed to get one of those Celem conduction-cooled mica caps off Ebay for $39, but so far I've not been able to drive it at more than a tiny fraction of its 100kVA rated power. I've just got it hooked up to a Mazzilli oscillator running at 400kHz, and if I try and increase the power above a few hundred watts, the MOSFETs go into parasitic oscillations and blow themselves up.
It really needs a proper driver and a water cooling system to do it justice. Also, at 0.4uF, 500V RMS, 200A RMS rating, it wants a fairly large work coil of about 2.5uH and an operating frequency of 160kHz to give its full rated kVA. I used it with a smaller coil, hence why I ended up at 400kHz.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Yes, the biggest thing is actually the thermal insulation around the workpiece (meltpiece) which isn't always possible though.
I managed to get one of those Celem conduction-cooled mica caps off Ebay for $39, but so far I've not been able to drive it at more than a tiny fraction of its 100kVA rated power. I've just got it hooked up to a Mazzilli oscillator running at 400kHz, and if I try and increase the power above a few hundred watts, the MOSFETs go into parasitic oscillations and blow themselves up.
Any more information, pics of your setup steve?
I became a sort of a fan of royer oscillator and am becoming more and more aware of it's problems. I can't think of what would ''parasitic oscillations'' be... are you using a ferrite core?
The diodes tend to be a big trouble. Running at 1Mhz no load (air coil) I cooked the 200ns trr diodes, and as they were heating up, I found that peak gate voltage was rising constantly and threatened to blow the mosfets! :O
Putting a gate protection diode cured that, but even at like 50V supply voltage diodes were grossly hot. Even 25ns trr diodes were getting too hot at higher voltages, and a forward drop seriously close to mosfet threshold voltage Think if you needed diodes that would stand 1000V at that frequency. This, together with too hot resistors and slow turn on times, made me to start developing a proper gate driver.
Although shot through itself is not dangerous in ZVS, this appears to break the oscillation down in some cases.
To cut the story, I'd love to see your work steve!
BTW, doesn't this thread look like it doesn't belong to project board, after all?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I can't think of what would ''parasitic oscillations'' be... are you using a ferrite core?
Resonance between the resonant cap and its inductance to the mosfets. I had huge problems with this when attempting a higher power ZVS oscillator when using high speed gate drivers instead of the crappy low speed "gate drive resistors". I believe that using gate drivers makes the parasitic oscillations WORSE because its easier to excite that resonant mode with the faster drive. In fact it was so bad i gave up on it rather quickly and determined that if i was gonna do a ZVS circuit, i would do it properly instead of the "mozilli" driver :P.
Now that i can get my hands on some giant powdered iron toroids, a ZVS inverter looks attractive again for induction heating. Still need to find one of those badass ceramic caps, though.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Yeah, like those water-cooled heat-sinkable ones. I was fairly sure they were ceramic dielectric, though i suppose i might be thinking of the mica types. Probably less likely to be PP for temperature reasons.
Right now i do have a pair of .01uF 10kV caps that are rated 18ARMS, but that doesnt work too well for a solid state induction heater. Really, just give me something around 1uF that can do a few hundred amps RF and i will make something cool . I was also thinking about just using a DRSSTC (with a miniaturized primary) and heating some stuff with that. It wouldnt be terribly efficient, but im sure it could process a lot of power .
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Here's mine:
The dielectric is mica. To get the full rated power, it needs to be sandwiched between water cooled busbars. Using the driver I've currently got, it barely gets warm even with no cooling.
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OMG Induction Heater
Think if you needed diodes that would stand 1000V at that frequency. This, together with too hot resistors and slow turn on times, made me to start developing a proper gate driver.
