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Induction Heater testing

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IamSmooth

Mon Nov 09 2009, 01:53AM

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

Well, the good news is I have gotten a 1/4" nut to glow bright orange for several minutes without overheating my mosfets. I seem, though, to have hit a wall in getting it hotter.

My unit is a half-bridge using 170vdc as the power input. The design is similar to

except the inverter output goes to a 20 turn step-down transformer which encircles one of the legs of my LC series tank. The work coil is 2" diameter 3/8" copper pipe with 5 turns. The capacitor is a bank for a total of 1.0uf.

I tune the resonance manually with a pot. I find that right at resonance it flips back and forth, and I have to adjust the pot to stay there, especially if I am near 170v input. My work coil is attached to one side of the capacitor bank, so the capacitors don't share the current equally, but they don't get too hot. I don't know if this is a big deal.

Would heating the nut in a crucible instead of open air help? where can I get one?
Would adding a PLL help, or should manual tuning work well enough?
Would decreasing the turns on the step-down transformer allow me to get more current into the tank?

Thanks.

Myke

Mon Nov 09 2009, 02:00AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I think you can't get it hotter because you reached the curie point making the nut no longer magnetic.
I don't know how much a crucible would help you once you reached the curie point. I'd think it wouldn't do much because it will have trouble getting above the point where it's no longer magnetic.

IamSmooth

Mon Nov 09 2009, 02:51AM

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

I've seen videos of the metal glowing white and sparking, so there is room to get it hotter.

Myke

Mon Nov 09 2009, 04:07AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I should have said "It gets a lot harder to heat it hotter than curie point because you don't have hysteresis losses anymore". I think you would have to heat it up hotter with resistive losses after the curie point. I'm not completely sure though.

Gabriel35

Thu Nov 12 2009, 06:58PM

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

Hi Iam, i was trying to build a IH too, and then i finally got mine to work, with danyk's schematic

works pretty nice
i like to tune it manually, is a way of getting entertainment while watching the screw becoming white...if you like this too, you'll probably like to try this circuit....the IGBT's used are not with anti-paralel diodes inside, so if you can get acess to some diode protected IGBT, you can get rid of the Turbo diode drawed..

Tonskulus

Sat Nov 21 2009, 05:52PM

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

I would say you will need at least 1kW of power to melt some small steel/iron pieces. My induction heater is limited to 3kW wall outlet so far and it can melt some 10mm thick steelbars @ 10A/230V input.

However, we need actually lots of circulating reactive power.

paolss

Wed Dec 02 2009, 07:26PM

Registered Member #2494 Joined: Wed Dec 02 2009, 07:16PM
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Posts: 2

Hello all.

Tonskulus i want to ask You a question.
I want to modify my circuit to work with 220VAC so this will be 310 to 350VDC
I use IRFP460 mosfet transistors.
And now all is working perfect with 154VDC so i have almost 400VAC on "work coil"
And i think if i will use 310VDC i can have max from 974VAC to 1099VAC on work coil.

So do i need to have mosfets minimum 550V / 600V ? or more ? IGBT 1200V ?

Can You tell me what i need also to change - because i want to know more before i "blow" some mosfets :)

Thanks.

my current circuit:

Experimentonomen

Sat Mar 19 2011, 09:27PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Im back again and i have a new induction heater project.

This time im using a PLL based on Imsmooth's circuit except i use the passive RC network rather than the opamp integrator.

Heres a video:

IamSmooth

Sat Mar 19 2011, 10:32PM

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

Very nice.

What was that sound at the end? Water boiling in the coils? Are you using a coiling fan for the chips?

If you get the workpiece any hotter you are going to have molten metal (2400F melts, 5400F boils) falling on your platform. I would have some type of crucible with a water bath below the coil to catch the molten metal.

Experimentonomen

Sun Mar 20 2011, 01:24AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

The sound at the end was steam returning to the coolant tank, pressurizing it and blowing out the vents.

I'm yet to find a cheap crucible that will withstand 1500deg C.

Heres a new video after a few changes:

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