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Induction heater, what frequency to use for heating copper to brazing temperatures?

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Henry

Sat Jan 23 2010, 06:13AM

Registered Member #2440 Joined: Sat Oct 17 2009, 05:49PM
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Posts: 9

We want to see some hundreds of volts there
--
not just some tens of volts as mazzilli ZVS

I dont really know if it is the input voltage to the schematic that is meant or induced voltage in the coil but i have a voltage around 160Vp at the coil. Its a little strange looking sinewave i get its like it has been rectified but i havent done that. But i remember once with a smaller coil i tried once so would it be a perfect sinewave bvut that coil more or less went up in smoke right away. But the waves are 160V top and the schematic draws about 20A+ if heating up a bigger nut for example. Although i havent calculated the coil current yet mainly since i dont remember how right now but i guess it could be a little higher then i might think. The coil is around 7 - 8ÂµH

The input voltage is around 50V but if i go any higher the mosfets max 200V will be exceeded since the coil is at 160V+ and blow them up which have happened 2 times already. Based on this so should the voltage in the coil be quite all right even if ofcourse higher is better. Then its only the current left which i doubt are enough due to, as i guess, the small wire in the coil i use right now so enopugh power cant be transferred and the maybe small capacitance in the bank or in worst case, which i dont hope, the schematic it self.

Yes steel is really easy to get hot no doubt about that but maybe a little challenge though if you want to melt it.

About running directly on mains; i would like to avoid this as far as possible and instead as far as it could go try with an non dangerous voltage first since we use 230V where i live. And higher the voltage bigger the bang if something would break and i would be almost afraid on just thinking about a mosfet that had been shorted and at the same feeded with 230V directly connected to mains.

Tonskulus

Sat Jan 23 2010, 09:30AM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

I think you all know that:

200Amps through 3 turn work coil makes almost no difference compared to 6 turn work coil running at 100Amps. Flux density keeps the same IF both coils are about the same size. If single turn (low inductance/impedance) work coil is used, very high current is required. OR very high frequency.

Maybe i should make ZVS mazzilli to see what it really can do as induction heater. I think it is possible to run it >100volts using separate low voltage bias supply? Anyone tested this method?

Steve Conner

Sat Jan 23 2010, 02:24PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Tonskulus, I tried to build the biggest ZVS I could (see the old OMG Induction Heater thread for more details) and it suffered from parasitic oscillations above a few hundred watts.

Maybe the parasitic oscillations can be fixed somehow, but I couldn't be bothered trying. I changed to an IGBT halfbridge driven by one of my old PLL drivers, and it would happily run straight off the mains at over 1kW and melt steel bolts. The ZVS with the same tank capacitor could only get the bolt red hot, before the parasitics got bad enough to blow the MOSFETs.

Henry

Sat Jan 23 2010, 05:39PM

Registered Member #2440 Joined: Sat Oct 17 2009, 05:49PM
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About the ZVS mazzilli topology i would say if you want an inductive heater for steel to heat it up to its curiepoint and you want a quite easy build without much electronics then go with this topology. PLL; much more work with and a lot of electronics but you can melt steel and pretty much everything else with it.

As much power this schematic has drawn for me is 24A at 50V (althought maybe down to 40V or so at this current) when putting in a big fat bolt just to se. But at this level so was there only a small very short distortion after the sinwaves of about 3 volt, thats all distorsion there was but it doesent disturb anything as far as i have seen on the scope.

Henry

Sun Jan 24 2010, 03:06PM

Registered Member #2440 Joined: Sat Oct 17 2009, 05:49PM
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Posts: 9

The distortion in the end is the only one and the other picture is a zoom in. The top is 2V/div the other is i think 0.1V/div probe at x10 on both.

As soon as i put a load in the coil it disappear the dip is still there though what that now is. This distortion absolutely seem to be resonance somewhere and its in the MHz range but it doesnt cause any problems as i can see.

The signal is taken from one side of the coil and it seems that the sine had been rectified, there should be a full sinewave if im not wrong no matter what buttons is pushed on the scope but there isnt. But once when i tested a smaller coil then there was a full perfect sinewave but distorted with a heavy load, but i cant understand why there isnt a full sinewave now with the little bigger coil i have? I strongly guess that i also only have half the power or so due to this.

BTW i put a little more capacitance in it and steel started to warm up a little bit quicker now and went a little hotter as it seemed but copper no real change. I guess a more or less quite big capacitor bank could do something at least to iron maybe but copper i dont know but one could hope but the caps i use are a little expensive.

IamSmooth

Sun Jan 24 2010, 06:20PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

As promised:
boiling copper

Frequency 62.5khz
Input power 2.5-2.8kw

Result: boiling crucible of copper. Very hot.

Now, what can I do with this?

I have found that the furnace cement is a very cheap way to make crucibles and stirring sticks. I have also found that by coating the inside of my levitating coil with it I have eliminated any shorting of the coils before I get it airborne.

Henry

Tue Jan 26 2010, 05:30PM

Registered Member #2440 Joined: Sat Oct 17 2009, 05:49PM
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Posts: 9

Nice one! That is exactly what i want and probably need to build, altough that high power level is to much for what i neeed i guess and maybe a little hard to control.

I have now read a little more about this and your tutorial as well, great one!

Am i right about this: to heat iron beyond its cure point and to generally heat non ferrous materials like copper so do the resonance frequency need to be changed so neither of current and voltage lags but follow each other pretty much exactly so maximum power would be transferred. Only then can non-ferrous material be heated as much as you want and also beyond the cure point for iron? The Mazzilli topology doesnt do this only resonance at the coil and capacitor frequency which isnt enough since it should resonate at the heated materials point and therefore cant heat non ferrous materials but ferrous material but only to its curie point?

I havent really understand before why the Mazzilli topology couldnt heat copper and so on but i think my theory above might be the reason. If this is what its all about so doesnt it matter how much time i put into the Mazzilli topology it will never been able to heat copper or any non ferrous material more than maybe a little if you lucky by some reason. But i have seen one here that had built a Mazzilli with pretty much exactly the same components then i and he could heat aluminum quite much as it seemed, but i cant understand how that could be done.

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