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Hi all. Induction heater question

  1. Apr 27, 2012 #1
    Hi, new to the forum.
    Great place but much of the info far past my interlectual understanding. :frown:

    I have been looking at the idea of an induction heater for use in my job.
    I'm a mechanic by trade, nicknamed proffessor screwtop by my workmates :)

    Snap recently introcuced the mini-ductor II by induction innovations to their line up of tools.

    Lots, (read most) of the induction heater projects/DIY plans I have looked at use quite heftly coils for the work coil, obviously to cope with the amount of current that is being driven through it.

    The mini-ductor II however uses what looks like (in the demo)http://youtu.be/hcVYYVGCt5M rather small coils in comparison to any I have seen. Only 4 windings too whatever size work coil they use. Quite thin too with a heat proof protective braid over the coil to protect it from the heat and shorting.

    My conclusion from what I have seen in the online demo is that the the work coil is working at a very high voltage/low current.
    It would be great just to buy one but that would be no fun and at £650 that Snap On want plus VAT. :bugeye:

    Any thoughts on the subject peeps.

    Sorry about the feet first first post.
  2. jcsd
  3. Apr 28, 2012 #2

    jim hardy

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    i think it's more likely high frequency than high voltage.

    induction works by rate of change of current
    so a large current at low frequency has same rate of change as a smaller current at high frequency.

    Just as piston speed on a long stroke engine at low RPM is same as short stroke engine at higher RPM.

    old jim
  4. Apr 28, 2012 #3
    Thanks for your answer Jim.
    I read 300GHz is pretty much the best frequency for steel.
    One project I looked at used a car ignition coil but they do not give any circuit specifics.
  5. Apr 28, 2012 #4

    jim hardy

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    that sounds way high to me. Heating must be by eddy currents and their depth decreases as frequency goes up.
    Where i worked they discarded an industrial induction heater to the scrap metal pile. I pillaged the driver transistors out of it, they were huge high voltage high frequency bipolars, rather specialized as i recall. I think it operated in tens of kilohertz , 40 khz i think.

    So i'd expect 30KHZ or 300KHZ not GHZ.

    Switchmode power supply could be adapted to the purpose.
    They operate in the tens to low hundreds of kilohertz.
  6. Apr 28, 2012 #5
    Sorry yes KHz my mistake! DOH
  7. Apr 28, 2012 #6
    Here is a dissection of the Mini-ductor.


    The Mini-ductor II is, to the best of my knowledge, the same as the original but it has a better case than the original's PVC plumbing pipe.

    It is a Royer oscillator type heater that uses modest current (about 65 amps or so) at low voltage (about 50 volts, depending on the work coil) to deliver its modest output. It runs at about 60kHz, again, depending on the load. A Royer power oscillator is inherently self-tuning to the work piece. As you can tell from the review, I don't like the Mini-ductor much (see disclaimer below).

    If you don't mind a little work, you can save a lot of money by building my open source Royer induction heater


    This is an improved version of the basic Royer circuitry with digital logic and appropriate gate drive for the IGBTs. One note on the article. The oscillator needs freewheel diodes. They're not shown on the schematic but should be (I need to update that article). You can either buy IGBTs with co-packaged diodes or you can use external ones. In Roy, my commercial product, I use separate diodes to spread out the heat dissipation area over 2 TO-247 packages. The diode that I use is a DSEI60-12A.

    Starting here:


    You can read several articles on using this heater. Note particularly the one where I used the heater to straighten the fork of my Palmer electric scooter.

    One last thing, since you quoted the MD's price in Pounds, I assume you're in the UK. This open source heater can be operated on 230 volt/ 50 hz with a small change in the transformer. Contact me directly if you're interested.

    John DeArmond

    Disclaimer: I'm the chief engineer for http://www.fluxeon.com, a company that makes induction heaters that compete with Induction Innovations.
  8. Apr 28, 2012 #7
    Two rules of thumb:

    1) penetration depth in steel in mm = 400/sqrt(frequency).
    3) use at least 3 penetration depths to avoid flux cancellation in the center of the workpiece and avoid the problem of temperature rise limited to the Curie point.

    These are general rules of thumb that deviate from reality with very thin workpieces. Also, with enough power, one can power through the Curie point limit and continue heating past the Curie temperature.

    This doesn't work out very well in practice. In the early days of Fluxeon, I tried that with several different architectures of switchmode power supplies. I have also looked at adapting a few SMPS architectures such as the single transistor quas-resonant one to induction heating use with little to show for the effort. The usual problem is that the design doesn't scale up to induction heating power very well.

    The Royer architecture is good to at least 5 kW for low frequency work, the catch being that appropriate-for-the-power-level ferrite core material is required. For high frequency work and higher power, the conventional H-bridge transformer coupled series resonant architecture is hard to beat.

    An example is the 10kW unit that my friend Jonathan built:


    Jonathan has posted a large number of videos to youtube using the nym "imsmoother". He does such nifty things as levitation melting iron, copper and other metals.

    John DeArmond
  9. Apr 28, 2012 #8

    jim hardy

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    Thanks Neon John !!!

    I was contemplating trying to adapt one of those "inverter" type microwave oven SMPS boards, they're probably beefier than what's in a PC supply.

    but since you offer a kit - i'll be by your site.

    Thanks !

    old jim
  10. Apr 29, 2012 #9
    Thanks for your reply NeonJohn, I had found the disection of the original mini-ductor.
    Great strip down!
    I have a lot of reading to do!
    Thanks for the links.
    Yes, you were right I am in the UK (the country were we get ripped of for everything!, they're even talking about taxing our hot pasties!!!)
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