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Resonance frequency of heating coil

  1. Aug 14, 2014 #1
    I am doing R&D and have been hit by a problem. I want to find the resonnce frequency to my heating coil without having to hook it up to the rest of my oven. I fall into the problem because I don't have the stray funds to buy a frequency generator. I heard that with capacitors and an oscilloscope that I could find my frequency.

    I have tried to use an oscilloscope, the coil, and a 9volt. My results are varying from17.44MHZ to 35.71MHZ. The non-consistancy is what is frustrating me.

    If anyone knows the backdoor way to test without a generator, the information would be greatly appreciated.

  2. jcsd
  3. Aug 14, 2014 #2


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    Welcome to PF.
    What frequency will the heating element AC power be ? 60Hz ?
    What is the relevance of heating coil resonance ?
    Are you building an induction heater ?
  4. Aug 15, 2014 #3
    Thanks for the welcome, I'm glad to have found this site.

    An induction heater is exactly what I am making. My power source is 100kW and 30kHZ.

    a major reason I am having problems is because my machine is built for multiple coils (round stock, flat stock etc etc etc) I am in no way a professional at any of this. I have notes from a dead guy, and good work ethic to carry me through all of this.
    The relevance is for my IGBTs. I'm looking for about 30-40kHZ resonance. If my numbers are below or above that they blow out.

    I just want to be able to check my frequency without having to hook up each coil I make to the machine. It's a real butt pain.
  5. Aug 15, 2014 #4


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    Years ago, amateur constructors used all sorts of ruses to measure quantities because test equipment was very expensive. These days you can use computers to drive cheap waveform synthesisers and DMMs to measure components and Frequency.

    But this is not a typical home constructor project.
    If you really are considering constructing a 100kW inductive heating source then I suggest you need to get well up to speed with more than just the basics of EE. 100kW of any sort of power (steam engine or internal combustion, as well as electrical) is a serious amount of Power. (Think of your modern saloon car, and the engineering involved in that) What would be your electrical supply, for a start???

    I may have read this wrong, of course. If you are only wanting to test some existing components and check they are within-spec, then a DMM and signal generator card could show you the resonance peak. But what will you do if the resonance peak is at the right frequency? But, if you want to do this all safely, then you should not even be considering doing your own assembly of such meaty stuff.
  6. Aug 15, 2014 #5


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    The output impedance of the generator needs to be matched to the heating coil. If that match is incorrect, then energy will be reflected back from the load to the generator. That may easily destroy the generator.

    I believe you must adjust the coupling network of the heating coil to the generator output lines at the operating frequency of the generator. There should be a low power mode for use while initially adjusting the match. There should also be some simple way to monitor the reflected power or SWR.

    The self-resonant frequency of the coil will not be important without the coupling between the generator and coil in circuit. It will be very hard to make a coil without a tuning capacitor that will radiate 100 kW at 30 kHz. At 30 MHz it might be possible.

    Does the heater coil have an adjustable tuning network connected?
    Or is the only adjustment on the generator?

    What is the make and model of the generator so I can look up the output specifications?
  7. Aug 15, 2014 #6


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    Welcome to the PF.

    30MHz or 30kHz? Makes a big difference.
  8. Aug 15, 2014 #7
    I watched the installation of an ink drying heater in a printing plant. The paper was printed with water based ink and had to be dried before it was wound up in a roll. The dryer operated at 5kW and 27MHz. The machine used a tuned plate, tuned grid, tube oscillator and the web of paper passed between the plates of the capacitor of the oscillator. Since the capacitor was part of the oscillator, the frequency was always at the resonant frequency of the oscillator.

    Would it be possible for you to have an RF engineer incorporate your induction heating coil as part of oscillator in the generator?
  9. Aug 15, 2014 #8


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    Dielectric heating of plastics would use maybe 1 to 20 kW on the ISM bands at 27 MHz or 40 MHz.
    Induction heating of metals would use 30 kHz. Skin effect, time and thermal conduction set treatment depth.

    IGBTs would be hard pressed to work efficiently above 1 MHz.
    Like switching inverters, a bridge of IGBTs would operate best at about 30 kHz.

    The number of turns and size of the heating coil sets the inductance. But the load would need a tuning capacitor.
    There must be a high voltage capacitor somewhere that brings it close to resonance.
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