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induction heater resonance

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Linas

Wed May 07 2008, 11:20AM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Hi all
does need in induction heater resonance, i will use step down transformer to 80V and then will be work coil, so is resonance make heat faster, and power will be bigger ??
because i want to use fixed frequency

work coil

cap:

uzzors2k

Wed May 07 2008, 07:55PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Fixed frequency and resonance works fine. I have had good results and so have many others.

Resonance is used to allow much higher currents to pass through the work coil, which are necessary for induction heating. Take my heater for example. A half-bridge run from 320V gives +/-160V on the transformer primary. With a step-down ratio of say 20, a work coil inductance of around 3ÂµH and frequency of 150kHz. That's 8V over 2.8 ohms of inductive reactance, so 2.8A of current. Nowhere near enough for induction heating. Add series resonance and the work coil reactance is suddenly zero, viola! more than enough current.

Linas

Sun Jun 29 2008, 03:27PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

finally, i made induction heater, i use 1uF wima FKP1 650Vdc cap, power transformer 20:1, resonance frequency 160KHz, driving half-bridge by FDH44N50 mosfet's, fixed frequency

Marko

Sun Jun 29 2008, 04:01PM

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

Linas wrote ...

finally, i made induction heater, i use 1uF wima FKP1 650Vdc cap, power transformer 20:1, resonance frequency 160KHz, driving half-bridge by FDH44N50 mosfet's, fixed frequency

Hey, nice work sir! You definitely need a water cooled coil now, take care not to destroy your inverter.

What core are you using for the impedance matching transformer, it looks fairly large?

Some thoughts about the circuit:

- You don't really need the additional external freewheeling diodes, but it's OK to keep them. You just need to tune slightly higher than resonance, and take care the circuit remains so, in which case diodes are completely useless and just waste power on series Schottky's.

For driving inductive loads, the diodes just aren't needed - internal diodes in the MOSFET's are completely fine into many hundreds of kHz as they suffer no forced recovery with inductive loads.

In case you are using feedback, you will need to provide some commutating current by slightly gapping your matching transformer.

- your capacitor looks fairly robust, but you might want to space it out and introduce forced air cooling.

Linas

Sun Jun 29 2008, 04:25PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

capacitor are very good, he don't even warm, transformer are 3F3 material, 6*3Cm, he is using in my 8KW inverter (20V 400A), i don't know how to make feedback, because with toroid, 1om resistor burns just in 1secund...
Now i am using bigger work coil, in work, he just go hot, not red

And one more thing, i can't make metal white :(

Experimentonomen

Sun Jun 29 2008, 06:16PM

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

To reach that temperature, you need feedback to track the resonant frequency, as it changes as the work piece heat.

Linas

Wed Jul 02 2008, 06:41AM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

which feedback is best in induction heater Voltage~current zero crossing detector ...?
can i connect CT direct to my driver ?? (CT>>1K resistor>>0,1uF capacitor>>dumping diodes(1N4148)>>74HC14) ?? (t will generate up to 1KV but i believe resistor and diodes make logic level signal

Hernan

Tue Aug 05 2008, 12:32PM

Registered Member #1614 Joined: Wed Jul 30 2008, 03:08PM
Location: Argentina
Posts: 52

Hi, I'm new in the forum. I'm making an induction heater .I want to reach 4 to 5 KW using a full bridge voltage sourse inverter to drive ferrite isolating 1:1 transformer with LCLR resonant tank.for the moment I'm using half bridge inverter with Mosfets IRFP460 for testing but I would like to use large rating switches ..
when I reached 10A In mains input current the mosfet driver Ir 2110 failed and the power mosfets too.I will use a small ferrite to drive mosfets to isolate the control electronics.I also would like to use a PLL to track the resonant freq.

here is a picture

Myke

Tue Aug 05 2008, 05:58PM

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

It seems like the caps should be connected directly to the work coil... Also try to minimize wire length as that introduces stray inductance.

...

Tue Aug 05 2008, 06:20PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would assume that us is using some conduction cooled caps (the black blocks) between the 2 copper plates. The bank of electrolytics is just energy storage for the DC bus.

If your mosfet driver failed then it is probably time to get a better one

The ir2110 wasn't designed for this kind of abuse, you would be better of with some ixdd414 or the likes.

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