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need help building a induction heater

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haxor5354

Sat Mar 05 2011, 03:46AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

so i bought 2 copper tubes from ebay and it finally arrived after 2 weeks. each tube is 12 inch long and 1/8" in diameter.
I thought I was gonna get atleast 10 turns but turns out I was only able to get 6 turns out of 24 inch of tube.
so I just hooked it up to my ZVS driver with a 0.9uf cap and the damn thing is already draws 5 amps w/o any load

when I increased the input voltage to 24v, the wires in my ZVS driver starts to heat up like mad and made the insulation all "gooey"
what should I do now?
use a choke of high inductance value to lower the current?

James

Sat Mar 05 2011, 04:08AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Ebay? Copper tubing can be bought from any hardware store. You will probably want to use a matching transformer between the driver and work coil, that's what I did. A FBT coil with 2-3 turns of heavy wire for the secondary instead of the HV winding. Even then the frequency was too low to really be efficient.

Marko

Sat Mar 05 2011, 06:50AM

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

haxor5354 wrote ...

so i bought 2 copper tubes from ebay and it finally arrived after 2 weeks. each tube is 12 inch long and 1/8" in diameter.
I thought I was gonna get atleast 10 turns but turns out I was only able to get 6 turns out of 24 inch of tube.
so I just hooked it up to my ZVS driver with a 0.9uf cap and the damn thing is already draws 5 amps w/o any load

Well, I hope you can recognize which part of your 'ZVS' is the one that carries heavy current, and make sure you have no thin wires in that path, The common amateur approach of building IH's is winding the coil, and then leaving out 20-30cm of straight pipe from the coil ends, to which you will solder a bunch of small FKP caps, all in parallel and as close as possible to the pipe, and then run water through the pipe for cooling. If you only have a single cap, it is most likely that it will turn gooey before doing anything.

Marko

Inducktion

Sat Mar 05 2011, 05:52PM

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

Did you still add the center tap? It needs a center tap to work properly otherwise it will not oscillate.

haxor5354

Sat Mar 05 2011, 07:15PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

Inducktion wrote ...

Did you still add the center tap? It needs a center tap to work properly otherwise it will not oscillate.

yes the work coil is center tapped.
what frequency does your induction heater work at?

Marko

Sun Mar 06 2011, 12:53AM

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

Inducktion wrote ...

Did you still add the center tap? It needs a center tap to work properly otherwise it will not oscillate.

You don't need a center tap if it's not practical. You can use two inductors instead each going from Ucc to each end of the coil/each mosfet drain.

Marko

Inducktion

Sun Mar 06 2011, 02:48AM

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

haxor5354 wrote ...

Inducktion wrote ...

Did you still add the center tap? It needs a center tap to work properly otherwise it will not oscillate.

yes the work coil is center tapped.
what frequency does your induction heater work at?

My induction heater doesn't run at any frequency, because I have yet to build a working ZVS induction heater.
BUT.
My non-protected induction heater, that blew up your mosfets? (sorry)

That ran at about 300 khz with a .5 uF capacitor. But, the coil was only 3+3 turns each side.

You can figure out what frequency your ZVS will run at by using resonant frequency calculators. Just google one.

A good rule of thumb though, is the higher the capacitance, lower the frequency. More turns on the coil will equal lower frequency as well. Generally, for our induction heaters, meaning hobbyists, and enthusiasts, you want to run your induction heater as high of a frequency as possible. Higher frequency works better for heating smaller objects, unless you're trying to heat up your car or something. :)

200 khz to 400 khz is usually what ZVS drivers achieve with no isolation transformer. With the transformer the frequency drops, if you're using the same .68 or .5 uF cap, to around 60-100 khz. If you do choose to use an isolation transformer, you'll generally want to decrease the capacitor value alot, so that the frequency range is still within 200-400 khz range. Otherwise it will not heat anything very efficiently.

Good luck. :)

IamSmooth

Sun Mar 06 2011, 07:15AM

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

I spent a lot of time and a lot of blown mosfets on this tutorial. If you are serious about building an induction heater I suggest you read this:

For one thing, you will see how to cool the workcoil. I've probably had close to a couple of thousand amps going through without an issue at various times.

"and the damn thing is already draws 5 amps w/o any load"

The coil will draw amps as you increase the voltage, and this can be substantial. When you have a workpiece inside the coil it will quench the resonant tank, dropping the current. This current will increase as you go through the curie point of the metal, and the resistance falls. How are you tuning the circuit for resonance?

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