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Induction heater (featuring my new PLL driver :-O)

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LAN_403

uzzors2k

Sat Dec 06 2008, 02:53PM

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

Nice. Do you have heating problems with your matching inductor? The wire looks a little charred.

Myke

Sat Dec 06 2008, 10:52PM

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

Looks like you used solid wire for the matching inductor. You probably want to use litz wire seeing that it's good for high frequency use.

Steve Conner

Sun Dec 07 2008, 11:45AM

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

The mini radiator is a nice touch

If you're going to talk about your induction heater more, you should think of starting another thread instead of jacking Uzzors' project thread.

Also, you probably ought to get a scope. Troubleshooting stuff like this without one is like doing a jigsaw puzzle with your eyes shut. And asking for help is like asking us to talk you through doing a jigsaw puzzle with your eyes shut, over the phone. For free. So at least meet us halfway, get a scope and learn what the green squiggles mean.

Proud Mary

Sun Dec 07 2008, 01:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

All wonderful, Uzzors!

Is there some sort of relationship between input power and the temperature of the work?

Does efficiency fall as the work gets hotter, and its resistance rises?

Does the Curie temperature have any effect when the work is ferromagentic ?

I know nothing about it, but it certainly looks very interesting indeed.

Raka

Sun Dec 07 2008, 07:29PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
Location:
Posts: 38

Sorry Uzzors, I didnt meen to take over your thread with my stuff. Im not so familiar with the etics. I did get the circuit to work in the meantime and it was largely due to your help. Big thanks!

Steve. Please delete my posts.

uzzors2k

Mon Dec 08 2008, 11:10PM

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

Hi Harry, if we make the assumption that the work peice is a black body, then it will radiate more energy as the temperature increases, as well as receive energy from the surroundings, like anything else. It will also receive energy from the IH, which increases the temperature above ambient. So the maximum temperature of the work piece will occur when the radiated input and output power are in equilibrium. Now actually setting up an equation for this shouldn't be too hard, as it only requires Stefanâ€“Boltzmann law and some assumptions or constants. I've made a crude equation, and the resulting temperatures aren't far off what I would expect. The hard part is determining such things as how "black" the work peice actually is, and how much power is actually being coupled into the work piece at any given time. Also, the curie temperature comes into effect, reducing the magnetic losses in the work peice, effectively coupling less power into it.

The effect the IH has on the work is somewhat complicated and I can't say I understand all the physics behind it yet. But I've observed that the work peice will load the tank circuit more as it heats up. This continues until the Curie temperature is reached (the exact time can actually be seen on a scope because the tank voltage rises quickly), then the work loads the tank less. The power throughput to the work peice is reduced when the tank is loaded, and increases as it becomes unloaded. This has to do with tank Q and circulating current in the work coil. Hopefully this gives you something to go on! As always, I'll point you to Richie's IH page, which has the physics as well as electronics explained.

Raka, don't worry about. I like having user-projects in the thread. Makes it look popular.

Proud Mary

Mon Dec 08 2008, 11:56PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Thanks very much, Uzzors - there's a lot to think about there - and exciting too!

Raka

Tue Dec 09 2008, 06:01PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
Location:
Posts: 38

It seems the diode across the resistor of your soft start is rather critical. I didnâ€™t have the RPG10A, so I used another of the 1N4148â€™s and that caused the soft start to hold the frequency above resonance. Do you have an explanation as to why you used a fast diode there, it looks as if right about any diode will do the job there, but obviously not. Mine is working fine without a diode until I get can get the proper one.

uzzors2k

Tue Dec 09 2008, 07:14PM

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

The diode is there to allow for quick turn-on/off sessions. If you turn the inverter on and off, then quickly on again the soft-start wouldn't work without, unless the diode is in place. Those little 1N4148's are faster than the diode I have there, but their current handling ability isn't good enough.

Raka

Sun Dec 14 2008, 10:16AM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
Location:
Posts: 38

Somewhere Im still missing something. You mention that you can draw up to 18A from your inverter, you supply rail is 320V. Thatâ€™s a lot more Watts than can be supplied by 4 IRFP460â€™s. Did you lower your supply rail with a variac?

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