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OMG Induction Heater

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Steve Conner

Sun Mar 25 2012, 07:25AM

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

Kizmo, from your waveforms you look a bit out of tune.

The inverter current is supposed to be in phase with the voltage so you can have ZCS. That's more important than ZVS, especially when pushing the limits of slow IGBTs.

Kizmo

Sun Mar 25 2012, 07:37AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

How can it be out of tune with self oscillating driver? :)

This is just drsstc driver and im using tank current to generate feedback signal which is in phase with tank current.

EDIT:

I was told earlier that poor inverter power factor is known "feature" of simple LCLR heaters and there is not much one can do about it.

Steve Conner

Sun Mar 25 2012, 08:38AM

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

Well, surely you should be using the inverter output current as the feedback source, not the tank current.

With the series resonant heater, the two currents are in phase because they're the primary and secondary currents of the transformer. But in the LCLR they're out of phase, because a phase shift is necessary to transfer real power through an inductor.

Kizmo

Sun Mar 25 2012, 08:42AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I tried taking feedback from inverter current and guess what happened..

I found out that the resonant frequency of my matching inductor and DC block capacitor is about 6kHz. Couldnt get it working at all.

Steve Conner

Sun Mar 25 2012, 11:58AM

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

Oh, I forgot you don't have a proper driver with a PLL.

A simple feedback circuit will oscillate at whichever resonant frequency has the highest loop gain. So, you could include a highpass filter in the feedback circuit to lower the loop gain at 6kHz, and make it prefer the frequency you want.

If you can't easily design a filter that blocks 6kHz without unwanted phase shift at the operating frequency, then design the best filter you can and make the DC block cap bigger to lower the frequency of the unwanted resonance. Although, the phase shift at the operating frequency might even be beneficial as it would work like a "Prediktor".

Kizmo

Sun Mar 25 2012, 12:06PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I think im done with this topology. Already got 2 kilos of power ferrite and some copper tubing for series resonant transformer fed tank circuit :)

Steve Conner

Sun Mar 25 2012, 01:30PM

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

I suppose I have to agree with you, since I've only ever built series resonant heaters!

I chose series resonant because:

The inverter output current is in a fixed ratio to the work coil current, so a single overcurrent detector can measure and limit both. If you put a limit on the PLL tracking range, it can also protect the tank cap against overvoltage. (Tank cap voltage is proportional to current and 1/frequency.)

The feed transformer isn't that much bigger/harder to build than the matching inductor of a LCLR. The only problem is the massive water-cooled secondary winding, but we 4hvers enjoy building things like that

The feed transformer isolates the work circuit from the inverter, which is usually not isolated from the mains.

But Richie says the LCLR is theoretically better. I can't remember why, maybe he can comment?

Kizmo

Sun Mar 25 2012, 01:51PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

The transformer gets rather large when its made from ferrite, drive voltage is high (320V p-p) and highest possible frequency is 32kHz (limited by tank capacitor)

Kizmo

Sun Mar 25 2012, 04:47PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Almost done. One question: With transformer fed series resonant topology i have to include transformer high current side winding inductance with work coil and wiring inductance when figuring out the resonant frequency?

Wonder how low it will drop :)

Steve Conner

Sun Mar 25 2012, 05:47PM

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

Yes, you do.

To avoid excessive inductance, I built my transformer right into the heat station as an extension of the capacitor's water cooling plates. That also got the secondary water cooled as a bonus. And I wound the primary inside the secondary, on the same core limb, again to minimise leakage inductance. (Since the inverter is a voltage source, it's the leakage inductance that's relevant.)

But a little extra inductance may not be a problem, it will reduce the change in operating frequency with workpiece type.

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