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Royer induction heater

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Gabriel35

Wed Aug 24 2011, 07:18PM

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

The work coil used, is from a previous Half bridge IGBT Induction heater, and is a little roasted because of that.
I think this heat sink is pretty large, is larger than my hand, and with 13A it stays very cool.
About the capacitors, I need to get some equally rated caps to make a better arrangement.
This thing is getting complicated... Since I don't have proper caps and proper Supply...
I have that two huge transformers here, and I doubt that a rewound MOT will be much powerful than theese ones.
Does someone knows something about turning two transformers in series to raise thye overall voltage?

I was wondering what happens if i take a big ferrite toroid and wound 5 turns of wire to the driver side and about 15 turns for the tank capacitors side? Its a little hard to find MOTs where I Live... so, using a toroid step up transformer between the driver and the resonant tank could be an option to increase the power? Or I need a flyback ferrite without gaps?

Thanks!

Forty

Wed Aug 24 2011, 09:08PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

for a transformer between the driver and tank, you'd need a ferrite transformer anyway, as an iron one would eventually start to glow.
for the voltage sag on the supply side, maybe you could use a high current relay to switch power to the driver once the voltage on the smoothing caps reaches a certain level, thus avoiding the slow climb problem. MOT's are usually rated upwards of 1kW, so they're a bit more powerful than your transformer, but yours should be sufficient.

Gabriel35

Thu Aug 25 2011, 01:46AM

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

So the idea of the ferrite toroid between the driver and tank side is good? Can it be a BIG toroid or i need a flyback ferrite?
Can i achieve more power using it?
I was thinking.... how to amplify the voltage of the transformer's output (the same one that I already have here) Wouldn't be a nice idea to use a voltage doubler circuit?

Marko

Thu Aug 25 2011, 02:45AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi gabriel,

Once the voltage on your transformer has sagged to half of it's initial value, it's putting out it's maximum power and there's nothing you can do about it.
700VA transformer should do plenty for this circuit, probably over 1.5kW for short durations. I think it sucks for you because you're using a tiny auxiliary winding which can't support all the current you need.

Your probably only hope is to either find a more useful winding on this transformer, dismantle it and rewind it or rewind a MOT.

If you find a more useful, but lower voltage winding on this transformer you could try the matching transformer approach. Smaller mosfets would then be in order though, like IRFZ44 or IRF1010 for up to 20V supply. I'd wind a 1:2 transformer tightly on your toroid and see what happens. Make sure to give it a sane number of volts/turn for the core crossection area of the toroid. You can also center tap the transformer and get rid of one inductor. Heck, you could even try supplying it with a car battery if you had one around.

Also, don't be fooled by your heatsink feeling cold, as those mosfets seem to be mounted onto a thin fin on it's edge instead of the base, also with sil pads and poor mounting pressure.. I would expect the dies to cook very quickly at 20A +. I can't see your sil-pads by the way, are they trimmed to size of mosfet packages?

Marko

Gabriel35

Thu Aug 25 2011, 01:44PM

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

Yes, I'm using sil Pads... I have many other heatsinks here, i'll try to change them...
I was thinking yesterday and I got a possible solution...
I have here 2 300VA 24v transformers...
Can I plug their primaries together on the wall and their secondaries in series, giving me 48v AC or in parallel giving me same 24v but with more current capability?
I'm a little scared to turn the circuit on with 48v AC, what means something around 70vDC (rectified) It will sag i know... but the range for IRFP260N isn't 60v

What do you think?
What are the risks of everything explode using a 1:2 toroidal step up transformer between tank and driver?

EDIT> I think that I'm still missing something here... I made an association of a 6v tap of the huge transformer, with the other 24v transformer, I've measured the voltage and current when the workpiece was inside the coil and it gave me 32vDC and 14,9A, giving a theorical output of 480W... and I'm pretty away form melting the steel bolt with it, It just became red...

Marko

Thu Aug 25 2011, 03:17PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi gabriel,

Your current configuration of the circuit looks pretty much like mine, apart from your work coil being somewhat wider. Note that a jump from 30 to 60V is actually a really big one, if the circuit behaved linearly then my IH would jump in consumption (with same workpiece in) from 600W to 2400W for example.

You'll have to decide whether you want to run at 60V, in which case you'll have to remove a fair bit of caps, I'd go down to 2uF or less.
I think you're safe to plug the circuit straight to rectified and filtered 48V AC, just switch the transformers on and off on primary side (and put a breaker or fuse in series with DC output perhaps).

Or you could try using your transformers in parallel first and see what happens, in which case you can leave the circuit mostly as is. I'd recommend you a bit narrower work coil though, I can't tell exactly from the pics but yours looks like 5-6cm wide while mine is just about 3. Smaller coil will have a better coupling to the workpiece.

Regarding melting a steel bolt, it mostly depends on how big it is, 500W isn't really much power so I'd start with a M4 or M5 bolt. I'll try and see what bolt size can I melt after I finish my new power supply.

I'd recommend you to start some water cooling as soon as possible though, at 500W everything got super hot in seconds in my circuit, and I was afraid that all the emitted heat from the copper tube could cook the caps. You don't really see it at first, you just see decrease in performance as their capacitance drops, after which they bulge up and burst in flames :(

Marko

Gabriel35

Thu Aug 25 2011, 04:00PM

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

My work coil is small as yours, it is 3cm wide too.
Why do i need to take caps out and decrease the capacitance with more power?
I was wondering to use my 8x 1uF bank to see if the power levels become higher but now i don't know...

Thanks!

Nah

Thu Aug 25 2011, 04:15PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Marco,

Would submersing the capacitors in oil help them cool?

Marko

Thu Aug 25 2011, 04:49PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

Gabriel, you need to remove the caps in order to get a better impedance match to the transformers. You want your IH to draw much less current (about 4 times) at 30V in order not to overlaod the supply at 60V.

Nah, I'm actually already building a MMC that is cooled by water flow around the caps. Transformer oil is hard to get here and would require heat exchangers and mess.

By the way, I challenge anyone of you guys to attempt an offline IH of this type with 1200V IGBT's and insulation transformer, as I don't ahve any suitable igbt's around myself. It would be quite challenging, as far as I know there have ben few attempts but their stability is questionable.
I would start by using a 30-40V supply for the gates, and use 470 ohm resistors and zeners much like original mazzili circuit.
Then if this is not stable enough, I have another secret little improvement that I'll only tell about if someone attempts it :)

Cheers,

Marko

Gabriel35

Thu Aug 25 2011, 07:26PM

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

I Have some IGBTs here... The model is...
SKM40GD123D
3 phase brick.

So taking off caps is just to decrease the current consumption?
Impedance match between transformers? are you talking about the supply transformers? because I still do'nt have a step-up transformer between the tank and the driver...

I'm getting quite bored with my circuit at all...
No matter what I try, I can't go any further on it's power...
Ive tryed a lot of transformer combinations, i got the voltage stabilized @ 32v even with work on the coil, using 6v +24V in series, the current was something around 14A, and nothing seems to change.
Now I got the both transformers wired in Paralel, giving 25vAC and plenty of current, but It seems like the circuit don't draw any more current...
I'm still impressed like a young child, with the power that you've achieved with this simple IH Marko...

Should I try the 50vAC (72vDC) that the transformers give me wired in series with the two 24v outputs? do I need to decrease the number os capacitors to do that? Something tells me that it is going to explode...

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