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  1. #1
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    powerful induction heater advice?

    I'm helping out a friend who is a metal worker, thing is he would need an induction heater but he would need to heat both large pieces as well as smaller individual ones like parts of a single shaft or gear.
    My idea is then this, in order to make this safe both for the worker and for the electronics involved I would get a 3 phase mains transformer make a secondary and rectify that secondary so that I get somewhere around 100 volts DC. then use either an half bridge or rather a H bridge to drive the work coil.
    Normally I see some resonant circuit being used for the work coil in order to minimize current and increase impedance for the switches but in my case it would be ideal if I could simply have a H bridge with a big enough dead time between pulses and capable of powering multiple work coils from small ones to large ones so that he can swap them as he needs when working with different pieces.
    In other words have some brute large current IGBT's being powered from a rectified 3 phase DC source with modest voltage being able to drive various loads both small and very large ones.
    Any advice or possible schematics would be much appreciated as well as critique of my plans, thanks.

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  2. #2
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    Re: powerful induction heater advice?

    You need to clarify exactly what you want, what power it has.
    Depending on the power and size of the coils you want to use depends on the required power and especially the frequency you have to drive the resonant circuit, ie the coil and the capacitors connected to it.
    The coil must be a copper tube which is cooled by flowing water.
    The diameter and number of turns of this coil determine its induction , you add high voltage resonand capacitor any as this
    and this LC must be woken at its resonant frequency.
    Whether you use rectified 230V or 400V mains voltage and a small transformer (a few toroidal cores and a few wire twist) or use DC is already a question of choice and power you need
    several examples
    Click image for larger version. 

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  3. #3
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    Re: powerful induction heater advice?

    the first schematic seems like an astable multivibrator type of self resonant circuit, are those generally reliable in terms of that they might not want to start up some times etc?

    I guess I am looking towards some modest voltage DC (50-100) high current mosfet/igbt switching driven by some IC gate driver/waveform oscillator all in one package. I have built smps with similar schematics.
    the problem might be that IC couldn't drive very large IGBt's so one additional stage would probably be needed or are there IC's now out there that can do this on their own?



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  4. #4
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    Re: powerful induction heater advice?

    H-bridge sends full supply power through series LC. Waveform is sine-shaped. Sense resistor automatically detects LC resonant frequency, by detecting zero crossings.

    Click image for larger version. 

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    Sense resistor is large enough value so it provides sufficient bias voltage to drive the transistors.
    However this also wastes power. There ought to be a more efficient way to produce bias, possibly via other components than the resistor.

    The potentiometers are needed to adjust bias so that the circuit starts to oscillate and continues to do so. The aim is to avoid shoot-through although it may be difficult to eliminate it completely.



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    Re: powerful induction heater advice?

    Ok just an idea here but how about I simply build the thing like a simplified version of a smps power supply which it in some ways is. In other words I get my mains isolated AC voltage rectify it to some 100 vDC then I simple have an Ic with some say 50 khz square wave and modest dead time for safety then I simply take this 50 khz square wave and feed it into my work coil from an H bridge and add a strong enough capacitor in series with the work coil to prevent excessive currents.
    Sure the switches would have to work hard for a low impedance but I could control the frequency so that it doesn't approach the resonant frequency of the series LC and so the impedance would not be too low. Having switches with high enough current rating for some safe operation margin , what do you think is this a sound idea?



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