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Royer induction heater

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LAN_403

Inducktion

Wed Aug 17 2011, 05:17PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

When I made my induction heater's coil, I filled it up with sand to make sure it didn't bend. Make sure you have it full, with no pockets otherwise you'll end up ruining the pipe. (I have a few kinks in mine because of that, but it still works fine)

and, the dual inductor thing probably wouldn't help with a flyback, because the windings on the core are technically inductors/an inductor as well.

Forty

Wed Aug 17 2011, 07:02PM

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

i wonder if filling it with water and freezing it would work the same as the sand (that's what they do for brass musical instruments, thanks "how it's made")
I didn't mean to improve the driving of a flyback, i just wondered if it would work equally as well as a single inductor. with less turns actually required on the core, I could use thicker wire with some of my smaller transformers.

Marko

Wed Aug 17 2011, 11:58PM

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

Hy everyone,

Gabriel: It's important to use a large tank capacitance, 4uF in my case, if you keep 0.68uF or whatever was used for a mazzili driver the power output will likely be pathetic. Using higher supply voltage as well as a step-up isolation transformer also helps. On the other hand it's completely the same whether you use one DC link inductor or two!

Note that in this circuit the device has to be rated at least pi*supply voltage, hence 30V is pretty much the macimum for a 100V mosfet. Your supply voltage will probably sag under 30V even without workpiece, but I would use a variac anyway to bring the voltage up if it sags too much when you put the workpiece in. You would need a huge transformer with good regulation if you want to go without variac and drive the circuit to full power.

Also if you have only a 2uF tank cap, your power throughput might be somewhat disappointing if you don't get 200V mosfets. But your current ones should work for the first try.

Forty,Inducktion: Yes, FDB2552 looks great if you have some, although a 200V mosfet would be even better as I think.

I filled my coil with table salt before winding it, and blown it out (with some difficulty) with a compressor later. I didn't think of freezing the coil with water in it, it looks like even better idea actually.

Marko

Adam Munich

Thu Aug 18 2011, 12:27AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Marko wrote ...

Funny, I've built that same circuit in the past as an experiment. It worked, but my mosfets kept exploding so I didn't bother to post it.

Maybe I just used crappy mosfets, but good job for getting it working marko!

Dr. ISOTOP

Thu Aug 18 2011, 12:56AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

The giant Eurofarad Snubbers on Ebay are excellent tank caps; I've run mine to 4KVA with only mild heating (cooled through the two large metal terminals).
This has got to be the most beautiful Royer project I've seen

Marko

Thu Aug 18 2011, 02:00AM

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

Hey bwang, those caps you discovered do look superb for induction heaters, so depressing they don't ship them out of US. I see some of them have large flat terminals, are those designed to be conduction cooled? And by 4kVA I assume you mean apparent input power, not the reactive power handled by the cap

WIMA caps do surprisingly well though, I was very surprised to calculate that current in my work coil with no load is over 100A, all of which is handled by just 16 small paralleled caps! It's surprising they are surviving it at all without bulging up or showing decreased capacitance.

Grenadier, your circuit might have been unstable if you used high RDS-on mosfets like IRFP450's. That's the main reason why I enforced low voltages here.

marko

radhoo

Thu Aug 18 2011, 03:48PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Marko wrote ...

The radhoo's schematic which uses the transformer is wrong, the tank cap should go after the transformer and not before it by the way.

I think you got it a little wrong. You can't move the capacitor as the oscillator will not work.
The purpose of the ferrite core is as insulation transformer. It reduces stress on transistors and capacitor.

This approach can be used to power the original winding of flybacks as well, since we know that a concentric primary-secondary topology will put out more power.

Your current design is identical to my second variant, without the insulation transformer, just that you increased the power ratings with the bigger tank capacitor and work coil. Others willing to try this might want to look for mosfets with lower Rds instead, since keeping mosfets cool in this circuit is essential.

Marko

Thu Aug 18 2011, 10:40PM

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

I think you got it a little wrong. You can't move the capacitor as the oscillator will not work.
The purpose of the ferrite core is as insulation transformer. It reduces stress on transistors and capacitor.

Hi radhoo,

the simple answer is, why would you want to have your transformer suffer all the reactive power in the tank circuit when you can have it handle only the real power instead? If I was to place a transformer in my circuit this way, it would need to handle the several tens of kVa present there, requiring a huge ferrite pole pig.
On the other hand having the transformer before the tank circuit it would only need to handle 600 watts of real power that are really going in, which could be done on a flyback core or similar.

I've used these transformers all the time, but on low power levels though (<100W). A 1:1 transformer produces absolutely no observable difference in operation, other than the output being isolated from the input. You can center tap the primary and use a single inductor instead of two and everything stays the same again!

If you use a step-up transformer, it will make the impedance of your tank circuit appear lower to the driver section, divided by square of sec/pri ratio: in other words, the oscillator will feel like you're using more capacitance and less inductance, with everything else staying the same.

I would still take care to minimize the leakage inductance on the transformer as much as possible, which may destabilize the oscillator but simply overloading it could do the same. If I slapped a 1:2 transformer to my circuit I'm pretty certain it would explode, but 1:1 should make no difference at all.

I actually wanted to try it out on this IH, but am unsure how to mount it to the existing configuration of the circuit, would have to isolate the mosfets from the work coil which is some trouble to do now :(

Still if you're going to use a 50Hz transformer like rewound MOT for a power supply it might make no sense to use an additional ferrite transformer - unless you have only very low voltage supply and low voltage mosfets like IRFZ44's and you want to push a lot of power.

Marko

Gabriel35

Fri Aug 19 2011, 01:55PM

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

Hey Marko!
I'm experiencing with the circuit again.
I got some cheap as hell IRF540N's just to try out and but how you said, 36v seems to be so much for them to keep.
I'm waiting for my new IRFP260N's to arrive.
I have 8x 1uF 250 X2 Category Capacitors here.
can I use a capacitance of 8uF or something like that? How does it behave on the circuit?

Can you post a video of your oscillator melting a screw? and some photos of your Power Supply stage?

Can I use 13v Zeners instead of 12v ones?
Thank you!! =D

Forty

Fri Aug 19 2011, 03:46PM

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

the 13v zeners should be just fine. I think his power supply is just a MOT with a rewound secondary and then a big rectifier and some smoothing lytics (unless that's what you meant that you wanted to see)

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