Adopt induction cooker for metal melting

[Virusonehalf]

Hello everybody, i have 2kw induction cooker, working principles as image below, working on 50kHz frequency with 100uH working coil, try adopt to 11.4uH coil made from copper tube D=10mm. Also i have 0.5L crucible for metal melting. Using coil32 program to calculate 100uH inductance and i get VERY big coil size. I want to build matching transformer from 2 pieces amidon T200-26 (yellow-white) toroidal core, i have L=11.4uH C4=34*0.1uF C3=0.33uF. Please help me calculate matching transformer.

 

[mtwieg]

It's likely that if you use a transformer to work with a lower inductance coil, the overall power efficiency will be very poor. The transformer may end up heating more than the coil. If the original coil was already capable of producing sufficient field/power to a load, why change to the copper tubing?

It looks like Amidon provides some resources that you should find helpful:
Basic data on core loss for 26 material.
Some guidelines on max power dissipation of core sizes
26 is a good material for chokes/inductors, but for a transformer you may be better off with a basic ferrite core (doesn't have to be toroidal either). You'll require less turns and have better coupling, possibly lower core loss.
For the windings you will likely need good litz wire to avoid excessive copper loss.

 

[Virusonehalf]

Thank you for answer.
1. I think heat from crucible melt cable isolation, i need to melt copper 1200-1300 Celsius degree, i have no idea how to cool litz wire.
2. Will be good to have galvanic isolation from power network.
I have litz wire for now 4+1 meters 0.1mm*2000 near 15.5 mm2, i think more than enough for 5+ kW.
I forget little thing, induction cooker have included flat spiral inductor, this mean i have 1/2 transformer, second winding i try to make in flat spiral form with 6 turns and put on cooker working surface.

 

[mtwieg]

Thank you for answer.
1. I think heat from crucible melt cable isolation, i need to melt copper 1200-1300 Celsius degree, i have no idea how to cool litz wire.

If a significant amount of heat from the work is transferred to the coil, then yes cooling of the coil will likely become necessary, even if the power dissipation in the coil is very low. IMO you should prioritize thermal insulation between the work and coil, so that the delivered power stays in the work. Any heat transferred in this way is simply additional waste.

I suppose your intent is to make the coil part of a water cooling circuit?

2. Will be good to have galvanic isolation from power network.

From a safety perspective yes, isolation is good. Though be aware that at such high frequencies, interwinding capacitance on the transformer may still permit hazardous leakage currents to pass into the coil. Overlapping/interleaving the primary/secondary windings will make this much worse. You will have to compromise leakage inductance with interwinding capacitance. I would cover the coil in some good insulation regardless (fiberglass sleeve, for example).

I forget little thing, induction cooker have included flat spiral inductor, this mean i have 1/2 transformer, second winding i try to make in flat spiral form with 6 turns and put on cooker working surface.

I don't think this is an approach worth pursuing. Aside from the huge leakage inductance for such a transformer, this will also result in the effective inductance seen by the driver dropping. I'm guessing the driver for the cooker operates at a constant frequency, so the drop in inductance will further kill efficiency (unless you also adjust the resonant capacitor). If you're open to changing frequency/capacitance, I would strongly suggest driving the coil directly while also optimizing electrical/thermal insulation of the coil.