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Induction heater theory question

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LAN_403

uzzors2k

Tue Sept 08 2009, 01:28PM

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

Switching in resonance essentially removes the capacitive and inductive impedances, which would otherwise reduce the current flow, leaving only the resistive portion of the impedance. In addition, when using a parallel resonant circuit the reactive current will only flow between the L (work coil) and C (tank capacitor) part of the circuit, not through the switches.

There is a good amount of theory behind this, which Richie Burnett has covered at his site.

I've made a PLL circuit which can lock onto resonance of an LCLR network as described on Richie's site, and it also regulates both the tank voltage and inverter current.

IamSmooth

Thu Sept 10 2009, 12:03PM

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

Has anyone built an induction heater without the use of resonance? How effective would one be that uses push-pull to feed current into a voltage-step down transforer to a work coil? I guess the devices power requirement will be higher, but will it work well enough for a first try?

Dr. Slack

Thu Sept 10 2009, 06:38PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Hint - consider the coupling with typical IH and workpiece geometries, then think about the design considerations of SG, SS and DRSS TCs

Arcstarter

Thu Sept 10 2009, 09:20PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

If you want the simplest induction heater, that works ok and has little mosfet heat, use the Mazzilli ZVS driver.

Just make a transformer with 10 center tapped turns on the primary and however many you want to try on the secondary, going through the work coil. The ZVS is not the best, but it does work. A bit.

With mine i could heat a pretty long nail that is the equivalent of 8 gauge to red in 11 seconds. The ZVS is not good for commercial use, just tinkering.

IamSmooth

Fri Sept 11 2009, 12:43AM

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

So in order to get those thick bolts to go nuclear

I will need a resonant system.

The designs I have perused through monitor the current and then affect a change on the inverter. Is there any oscillator, like an Armstrong, that will automatically adjust its frequency based on the natural resonant frequency of the LC tank?

This would eliminate the need for monitoring the work coil current. The circuit would self-oscillate at resonance. Is this idea feasible?

Dr. Slack

Sat Sept 12 2009, 07:05AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You've got quite a lot of things to think about on this one.

The armstrong oscillator is single-ended. Review your earlier question (different thread) about replacing 4 FETs with one biggy for an IH, and Steve's answer to it.

Tell us what the gate coil is doing in the Armstrong circuit, if it isn't "monitoring the drain/work coil current"?

You've had several threads on switching vs. linear, and whipping through the linear region as fast as you can to a) stay within SOA and b) keep average switching losses down (not quite the same things). In your windmill dump, switch rate could be fairly low so only a) was a problem. In an IH, b) becomes a serious issue. Consider a classical linear feedback topology like the Armstrong, and a detector + gate drivers, which is going to minimise your switching losses?

Steve Conner

Sat Sept 12 2009, 07:58AM

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

Tonskulus has made a couple of induction heaters powered by big transmitting tubes. They are more or less Armstrong oscillators. Search the forum and archives for more details.

The "ZVS" is more or less your self-oscillating induction heater, however in practice death by parasitic oscillations limits what you can get out of it. If you take the same two MOSFETs out of your ZVS and put them into a halfbridge with proper gate drive, you can get more power without blowing them up. Look up that OMG Induction Heater thread for more information.

Tonskulus

Sat Sept 12 2009, 10:43AM

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

Yes, the simplest way to build powerful inductionheater is indeed tube oscillator.
I have two of them, bigger one uses inductively coupled feedback and smaller one is colpitts-oscillator type. Schematic for bigger one (>1kW output power):
http://www.elisanet.fi/tonskulus/ih/ih_schem2.JPG

Video:
http://www.youtube.com/watch?v=ROoN5QFQSqM&feature=channel_page
http://www.youtube.com/watch?v=6IUTKUwtUuY&feature=channel_page

However, tube oscillator type uses extremely dangerous voltages inside of the cabinet. Tank circuit has capacitive coupling for safety reasons so you dont get shocked by touching work coil (except RF burn injuries).

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