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Understanding relationships in Induction Heating circuits

  1. Jul 10, 2016 #1
    Hi Everyone,

    I will try to keep this short as I can and to the point, there is a bit of a back story, so bear with me.

    For the last 18 months (on and off as life allows), I have been trying to develop an induction heating assisted machine for 2 reasons.
    1 - To learn as much as I can by doing so (I want to actually make the circuits etc myself)
    and 2- To actually achieve what I want the machine to do, as I cannot get what I want anywhere else for a decent price and functions the way I want.

    I have been researching induction heating circuits for quite a while now. But there is something I cannot work out. And I cannot find any specific info on the internet for exactly what I am asking.

    Just a quick review of the machine I need to make:
    The machine will be called a Rifle Case Annealing Machine.
    It will incorporate an induction circuit to heat rifle case mouths to an annealed state (399C), so that the cases can be reloaded and fired again. This annealing relieves the work stress created by firing the cases.
    This is specifically useful for competition shooters, that require all their brass to be of the exact same hardness rifle round, to rifle round, and to get decent life out of the brass, as sometimes, specific calibres can get VERY expensive.

    Now, I initially purchased this circuit from ebay -

    I had a play with it and a 36v 13.6a power supply.

    Now just using this, I was able to anneal my small rifle (.204 Ruger) rifle cases manually, in about 25 seconds.
    This actually cooked the capacitors and blew them up, they got way to hot. Needless to say I learned something LOL.
    I then connected water cooling to the coil.
    I then replaced with the capacitors with some slightly lower value units, to try to increase the frequency with which the unit operated at. I did some very rough maths, and hypothesised that lower caps, would raise the resonance frequency. Apparently, to heat non-ferrous metals, you need a higher resonance frequency (not sure if this is true). I was hoping to increase the speed that the cases annealed at, as 25 seconds was far to slow.
    I did this, with not much noticeable difference, maybe slightly faster.

    I then saw that someone gapped a ferrite core, and wrapped the work coil around it a few times.
    This made the ferrite core concentrate the switching magnetic field.

    I tried this and I annealed the 204 brass within seconds!! Success!!
    Except incorporating a ferrite core (one I had to gap myself with a dremel and great difficulty) into the machine, and trying to water cool the whole coil was going to prove to be very difficult.
    There must be a better way.

    Then I came across this video, where someone had actually made a very similar machine to the one I want.

    Now, my question is this:

    In that last video, as far as I can tell, the guy that made it, has used water cooling (water bottle in the box), and has obviously used a normal copper coil (not a ferrite core concentrator).

    How the hell did he get that brass to heat up so fast!?!?!
    Realistically, that is the speed I am after, with that size case.
    Smaller cases will anneal faster, which is no problem.

    Is he just using more power? maybe 2kw power supply, running at 40 amps?
    Obviously he has designed the whole circuit himself, and kudos, as that is an awesome job.
    I have actually tried contacting him directly. I got a "read receipt" through the email, and no response :(

    Has he used very low capacitors, to get the resonance frequency right for the non-ferros brass to heat fast?
    Has he changed the inductors, and some how got the heating speed from that?
    I have a feeling he is just using more power?!? I don't know.

    I just cannot find any information, on how to speed up the induction heat rate, without putting more power into that system, and having to up the capacity all the components, to handle more power.....

    Any ideas electro wizards?
    Last edited: Jul 10, 2016
  2. jcsd
  3. Jul 10, 2016 #2


    User Avatar
    Science Advisor

    Welcome to PF.
    Energy circulates between the inductance and capacitance. Circulating energy is not output power, but it will have some real losses. The output power is the heating due to induced eddy currents. The place to heat things quickly is within the inductor where the eddy currents induced in the brass will be greatest.

    Changing frequency will not have a significant effect. Capacitors are selected based on cost and ripple current ratings. Frequency is then decided by the inductor design. The use of a ferrite core will make it more difficult to access the strongest changing magnetic field. You should change the inductor turns count and size to bring it into resonance with the capacitance at the design frequency of the driver circuit.

    Wind your own coil from flat copper strip, or from thin copper tube if you need water or oil cooling. Make the coil length similar to the heating zone you need. Make the internal diameter sufficient to pass the cartridge case. Work out the number of turns needed to make a similar inductance to the coil originally provided with the driver.
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