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Registered Member #610
Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110
Ahh I see he updated his page.
I'm not going to make an LCLR heater. Just an 'L' heater. I don't really want to spend money on high-performance capacitors, and instead am just going to put the load directly on the bridge output and smart-switch it. I'd rather have a little bit of firmware and 10 different work coils then a water-cooled cap bank.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Good capacitors are what make your induction heater actually work! The non-resonant kind really don't work very well, for the reasons that HFsstcfreak hinted at. You have to spend all the money on high-performance semiconductors instead, and even then you don't get as much bang for your buck.
I've seen Tonskulus get good results with a bank of metallized polypropylene capacitors that he got goodness knows where. They didn't look expensive. I think they died eventually, but it looked as if he had a lot of fun melting bolts and coins first.
Registered Member #610
Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110
Should I dare say that I don't want maximum power? Though saying something like that is considered profanity on this site!
Seriously though, maybe I need help understanding. I don't see the difference between 20 amps flowing through a 5 turn coil and 1 amp flowing through a 100 turn coil. I also don't see the difference between a 600V 2A mosfet with 0.8 ohms on resistance and a 200V 40A mosfet with 0.01 on resistance. Yeah, if your supplying reactive current you end up wasting a little bit of power while you wait for diode recovery, but if you don't need high voltages then is it really a big deal? Its not optimum, so can't you just make a coil thats wicked coupled to the work piece?
The way I see it is as long as there is power loss, the piece is getting heated.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
nrhoades wrote ...
Yeah, if your supplying reactive current you end up wasting a little bit of power while you wait for diode recovery, but if you don't need high voltages then is it really a big deal? Its not optimum, so can't you just make a coil thats wicked coupled to the work piece?
Well, you want to supply HUGE amounts of reactive power. This is where parallel LC comes into play as it saves the FETs from most of the reactive current. I have never examined an Induction heater but I can imagine the reactive current going well into 10s of KVA.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
nrhoades wrote ...
I don't see the difference between 20 amps flowing through a 5 turn coil and 1 amp flowing through a 100 turn coil.
The difference is that the 100 turn coil will need a high voltage to drive the 1 amp through it at induction heating frequencies, maybe several thousand volts. So you end up needing capacitors anyway to make a series resonant circuit. (The capacitors are saving the FETs from the high voltage this time, not the current, so you don't need to make a 5,000 volt H-bridge.)
This is the method used in induction stove tops and rice cookers. The lesson to take away is that capacitors are a cheaper and more reliable source of reactive power than silicon.
Registered Member #610
Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110
Oh. I see what your saying. Wouldn't what I'm talking about though be a lagging power factor (since the load is purely inductive?)
So, with LCLR, the FETs switch states as current crosses zero, and this makes the body diode happy because it never has to recover from an extreme state?
And when using a load that ISN'T resonant, there will be a power factor less than 1 (because its not matched) with a phase thats leading or lagging. Kind of like trying to swing a kid on a swingset... if you try to force him to swing at an off-resonance your going to end up smacking him around, and if your name is MOSFET and you have weak hands, your going to get hurt as well. Is this right? haha.
Question: And from the power point presentation I attached earlier, even if I used an L-only load and switched to allow breaking (like a motor) I'd be supplying reactive current? Or is this what yo meant by "not working as well".
Registered Member #941
Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497
You think capacitors for induction heating are too expensive ?
Have a look at this: <-- there you can see i use cheap off the shelf polyprop caps, this is a quasi-resonant inverter using a single igbt and a high impedance work coil around 50µH inductance, and even with thwse cheapo caps (less than a buck eack) i managed to generate enough current to melt the coil.
The caps are four 3.3µF 200VAC rated caps in series for 0.8µF, resonant freq of the system measures 22.4-22.6KHz. And these caps i use are about the same stuff as those inside your average induction cooktop. With a high impedance work coil you dont need to worry about finding very high current capacitors, just ones capable of voltage up to 1200V, which can also be obtained by seriesing, just look at tesla coil mmc arrays.
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Feedback system for induction heater
