Help needed for Induction Heater circuit

[ramuna]

Hello friends,
I am attempting to design a 1 kW induction heater operating at 979Hz. Two IRFP250N mosfets are operating in Push-Pull. The parallel resonating capacitor of 33uF is on the primary side. The 2uH induction coil is on the secondary side. The Push-Pull transformer has a split primary and the primary to secondary turns ratio is 20:1. The power supply voltage is 60V DC. I have been simulating the circuit in LTSpice.

My simulation shows a sharp current spike on mosfet switch ON. My question is, how can I eliminate/greatly reduce this spike.

I have uploaded a zip file, which contains the LTspice *.asc file for the simulation, the *.spi file for the IRFP250N and a screendump of the simulation which shows the current spike. In practice I will be using a bank of 2 or 3 IRFP250s per split primary, but for the present I would like to know how to remove/reduce the current spike. Many thanks in advance!

NB: Please amend the spice directive giving the location of the IRFP250N spi file in the Inductheat_1.asc simulation file to match the actual location of the file on your computer.

 

[ramuna]

Hi Tony,
Let me find some values. ( are we assuming pulverized Cu mixed with pulverized Pb or large chunks? or what? and what mix ratio?)
It will be strips of 10 cms wide, 3mm thick lead sheeting, wrapped around a core of 2mm diameter bare copper wire folded lengthwise into a 10cm long stick. Pb/Cu ratio by weight = 70%:30%. So, the lead melts first, then rises up to copper's melting point of 1084 degrees Celsius, and finally both metals melt and alloy together.

Thermal diffusivity, α is a measure of how quickly heat propagates through a material per unit of depth over time, [m²/s]....

Good analysis, but you are assuming that the primary heat transfer mechanism is thermal diffusion. What I'm saying is that in the skin depth zone, heat is being generated instantaneously, and before this heat can diffuse inwards into the rest of the metal, due to lead's poor thermal diffusion coefficient, the heat stays in the skin depth zone of the lead and melts it. The molten lead pools at the bottom of the crucible and gets heated along with the now exposed inner sections of the heterogeneous Pb/Cu composite slug. The skin depth region repeats its magic, more molten lead pools at the bottom of the crucible and so on.

And hence, the key to the entire process is to maximise skin depth.

 

[Tony Stewart]

I assume eddy current induced heat conduction is the primary transfer and thermal diffusion defines the gradient velocity.

I imagine the phase change of materials and oxidation complicates the process with contaminants and lead vapors are extremely neurotoxic so strict protection , cleaning protocols are necessary. I would not stand around watching it melt like the video demo.

 

[Tony Stewart]

Induction heating works better with smaller, irregular pieces (faster heat transfer, better packing).

Looking at the youtube video, the E-field drops rapidly outside the crucible coil volume thus reducing efficiency of long pcs. 1800~1900 Hz square wave audio would be the power spectrum of 2f so 950 Hz excitation approx.

I see 480 Vdc * 200 A = 96 kW source using 3 phase step down transformer designed for induction furnaces with wide BW. and large water cooling hoses for conductors.

I have a physical therapy induction machine at home with 15 cmD coils in a handheld paddle 1kW @ 1MHz 1~2 Tesla with a 3 cm water circulation hose over fine Litz wire to the paddle. It vibrates 1MHz RF with low repetition rates that toggle ever 15 sec cycle with modulation that smoothly changes from rapid muscle flutter to RF heat in way that mimics 15s ocean waves without any skin sensation like e-pads. I noticed the water cooling hoses are similar except mine surrounds the wire inside warm flexible hose rather than inside copper tubing. ( Search Tesla Stym, a similar device to mine if interested in therapy.)

 

[ramuna]

Hi Tony,
I have a physical therapy induction machine at home with 15 cmD coils in a handheld paddle 1kW @ 1MHz 1~2 Tesla with a 3 cm water circulation hose over fine Litz wire to the paddle. It vibrates 1MHz RF with low repetition rates that toggle ever 15 sec cycle with modulation that smoothly changes from rapid muscle flutter to RF heat in way that mimics 15s ocean waves without any skin sensation like e-pads. I noticed the water cooling hoses are similar except mine surrounds the wire inside warm flexible hose rather than inside copper tubing.

Does this work for lower back pain? If it does, how effective is it (how long does it take to get rid of the pain) ? An aunt of mine sometimes gets bad LBP in winter.

 

[Tony Stewart]

Grok 3 suggested this .
Final Suggestions
Shredding Size: 1 cm × 1 cm × 0.3 cm chunks.

Frequency: 25 kHz.

Current: 244 A (coil).

Voltage: 76.6 V (coil).

Power: 18.7 kW.

Duration: 5 minutes.

This setup melts Pb first, then Cu, alloying at 1084°C efficiently.

 

[Tony Stewart]

Acupuncture works cheaper for LBP. I fixed mine after motorcycle accident in my 20's later cured when 50 after 6 treatments. Now 72.

FMS is an expensive machine but found in many EU & Asian physio clinics. The Tesla Stym is $25k . I got a Chinese clone for $2.5k which avoided knee surgery on my dog who walked on 3 legs ($10k) but also used triple dose Nigella Sativa oil, glucosamine, Omega3 for 3 mos. and 0.5 to 1 hr FMS daily while she sat on my lap. Now she she can run on 4 legs. That was 3 y ago.

 

[ramuna]

Thanks for the details of the FMS. FMS is indeed an expensive machine. Did you ever look at its current waveform ? Is it pulsed DC (so unipolar magntic pulses) or is it a more complicated waveform?

I'll pass on the Grok 3 suggestion. I prefer the uncomplicated unshredded sheet on wire core slug. Shredding the metal will make eddy current formation more difficult - it may be suitable for a ferromagnetic load where hysteresis heating is a major contributor, but for non-ferromagnetic metal we need a closed (shorted) current loop in proximity to the primary winding (ie the induction coil). Shredded metal will equivalent to an iron powder or ferrite core, which avoid eddy currents by having discontinuities (ie insulation) between the core particles.

 

[Tony Stewart]

No I haven't scope it.

I suggest you avoid FETs with high output capacitance and use IGBT's


If using a 2 uH induction coil near 1kHz is which is ~10 mOhms but with an open air you get significant bottom leakage unless there is a way to return flux under the crucible

If you want very high currents e.g. 100A you may need or already have an MFDC transformer rated for 1 kHz with an IGBT controller. This reduces the stress on IGBT's.

I wonder what industry needs 70% lead 30% Cu. Cosmic Ray shields?