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Re: First attempt at induction heating

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  • tmoranwms
    The usual computer supply uses a half bridge circuit, which allows it to recirculate energy through the transformer. When you have a coil tightly coupled to
    Message 1 of 16 , Jan 31, 2012
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      The usual computer supply uses a "half bridge" circuit, which allows it to recirculate energy through the transformer.

      When you have a coil tightly coupled to the workpiece, like a coil wrapped around a soup can, relatively little energy has to be recirculated for the amount of heating you'll get. As a bonus, the soup can is thin steel, so it heats up very quickly.

      What you won't be able to do is heating from any kind of distance -- with even a small gap between can and coil, the physical space inbetween is filled with extra magnetic field, which stores energy. And each cycle, that energy has to go back and forth from the power supply. All the extra cycling puts a lot of stress on the circuitry, and pretty easily, you'll burn out the transistors, or trip the protection circuit, all for just a few watts of heating. (I would rate a server-class PSU at maybe 100 or 200W useful induction, which is hardly enough to power a large soldering iron.)

      Very few industrial machines are designed to "direct drive" the coil like this. The reason this is used at all is to achieve unparalleled control over both temperature and depth of heating. These are used in combined case hardening and tempering processes, customized hardening profiles (shafts, gears, bearings, etc.) and specialized brazing applications. The price you pay is by spending literally more than five times the cost in electronics -- recirculating that energy at will doesn't come cheap.

      Almost all power supplies use a capacitor to gobble up the excess energy. This makes driving easier, but now it has to be tuned, because the right balance of currents occurs only at one frequency. And in general, you don't know exactly what frequency that is, and it varies by coil, work and material!

      For this reason, a conventional PC power supply will not work anywhere near its nameplate rating, because it's designed for one frequency only, or adjustable only over a small range.

      Almost all induction heaters have automatic frequency control, so they keep themselves locked to the resonant frequency, or a little off as required. The basic circuit which does this is called a phase locked loop (PLL). There are single chips which do this (CD4046 is often used by experimenters), or you can build your own (the circuit on my website uses a TL494 oscillator, whose frequency is controlled by a phase detector and feedback loop).

      Unfortunately, you can't just toss a circuit board together. A successful induction heater requires a compensated feedback loop. Improper compensation results in the operating point "chasing its tail" and, at the very least, making an awful whine or growl instead of smooth power, and at worst, explodes as soon as power is applied without any apparent sign of just why it exploded.

      Even with a well built induction power supply, tuning can be cryptic. I have enough experience designing and operating machines that I can run a few quick numbers and figure out what most types of supplies will require for operation. Without a strong theoretical grounding, even experienced operators can be left at little better than guessing -- okay, it didn't work with this combination of capacitors and tap settings, maybe this... Most machines have two adjustments, usually output voltage and capacitance, and with perhaps a hundred combinations between the two, trial and error can take a very long time to achieve the desired result. Suffice it to say, once you've gone to all the trouble of designing and building a reliable controller and switching supply, you still have the continuing challenge of simply operating it. On the upside, after all the experience of designing it, operation will be fairly simple.

      Tim

      --- In hobbicast@yahoogroups.com, "ferrman1001" <ferrman1001@...> wrote:
      >
      > I had a go at modifying a computer power supply so it will heat up a soup can. It was amazing to watch it get to red hot in about 15 seconds. All I have to do now is to get another nine computer power supplies and hook up the outlets in parallel so I can have a serious output for a small induction furnace. Click on the link for the video.
      > http://youtu.be/X4zIO4zYSnk Ernie
      >
    • David Knaack
      Fantastic information, thanks! You mentioned your website. I m not familiar with it, can you post a link? DK ... [...] [Non-text portions of this message have
      Message 2 of 16 , Jan 31, 2012
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        Fantastic information, thanks!

        You mentioned your website. I'm not familiar with it, can you post a link?

        DK

        On Tue, Jan 31, 2012 at 9:28 PM, tmoranwms <tmoranwms@...> wrote:

        > **
        >
        >
        > The usual computer supply uses a "half bridge" circuit, which allows it to
        > recirculate energy through the transformer.
        >
        > When you have a coil tightly coupled to the workpiece, like a coil wrapped
        > around a soup can, relatively little energy has to be recirculated for the
        > amount of heating you'll get. As a bonus, the soup can is thin steel, so it
        > heats up very quickly.
        >
        > What you won't be able to do is heating from any kind of distance -- with
        > even a small gap between can and coil, the physical space inbetween is
        > filled with extra magnetic field, which
        >
        [...]


        [Non-text portions of this message have been removed]
      • Les
        Hi, How do you plan to synchronize the oscillators in a number of power supplies so the outputs are as the same frequency and in phase ? Les.
        Message 3 of 16 , Feb 1, 2012
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          Hi,
          How do you plan to synchronize the oscillators in a number of power supplies so the outputs are as the same frequency and in phase ?

          Les.

          --- In hobbicast@yahoogroups.com, "ferrman1001" <ferrman1001@...> wrote:
          >
          > I had a go at modifying a computer power supply so it will heat up a soup can. It was amazing to watch it get to red hot in about 15 seconds. All I have to do now is to get another nine computer power supplies and hook up the outlets in parallel so I can have a serious output for a small induction furnace. Click on the link for the video.
          > http://youtu.be/X4zIO4zYSnk Ernie
          >
        • tmoranwms
          Sure, my website is: http://webpages.charter.net/dawill/tmoranwms/ Specifically you ll find the induction heater project under Projects/Electronics. Tim
          Message 4 of 16 , Feb 1, 2012
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            Sure, my website is:
            http://webpages.charter.net/dawill/tmoranwms/
            Specifically you'll find the induction heater project under Projects/Electronics.

            Tim

            --- In hobbicast@yahoogroups.com, David Knaack <davidknaack@...> wrote:
            >
            > Fantastic information, thanks!
            >
            > You mentioned your website. I'm not familiar with it, can you post a link?
            >
            > DK
          • Glenn N
            Tim, Great webpage. :) I found a pump like this one
            Message 5 of 16 , Feb 2, 2012
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              Tim,
              Great webpage. :)
              I found a pump like this one
            • Glenn N
              That was odd. It stripped the url even along with the rest of the text. Anyway I have a piston pump from a small electric steam carpet cleaner I bought at
              Message 6 of 16 , Feb 2, 2012
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                That was odd. It stripped the url even along with the rest of the text.
                Anyway I have a piston pump from a small electric steam carpet cleaner I
                bought at Surplus center. It runs my TIG cooler nicely so it should work
                for your coil.
                About 30 years ago I repaired a tube type induction heating device that was
                used to temper chamber reamers. It was very much like the low frequency
                beacons I maintained. (Homer, Compass Locator) and ran a lot of the same
                tubes. I only changed a few tubes and tuned it to the chart on the side and
                they were happy so I really didn't get to investigate it a bunch. Point
                being that you may be able to still find some surplus transmitters cheap
                that could be turned into a heater with the right tanks.

                Glenn
                ----- Original Message -----
                From: "Glenn N" <sleykin@...>
                To: <hobbicast@yahoogroups.com>
                Sent: Thursday, February 02, 2012 12:06 PM
                Subject: Re: [hobbicast] Re: First attempt at induction heating


                Tim,
                Great webpage. :)
                I found a pump like this one


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              • ferrman1001
                Last time I looked the seat of my pants was not burning but all jokes aside I learned alot from this project about electricity that I did not know before so I
                Message 7 of 16 , Feb 2, 2012
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                  Last time I looked the seat of my pants was not burning but all jokes aside I learned alot from this project about electricity that I did not know before so I think that has to be a good thing for me.

                  Ernie.
                • Jeshua Lacock
                  ... Thats all good and fine, but playing around with a arc welder for a power supply is a whole other ball park. Best, Jeshua Lacock Founder/Engineer 3DTOPO
                  Message 8 of 16 , Feb 2, 2012
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                    On Feb 2, 2012, at 6:46 PM, ferrman1001 wrote:

                    > Last time I looked the seat of my pants was not burning but all jokes aside I learned alot from this project about electricity that I did not know before so I think that has to be a good thing for me.

                    Thats all good and fine, but playing around with a arc welder for a power supply is a whole other ball park.


                    Best,

                    Jeshua Lacock
                    Founder/Engineer
                    3DTOPO Incorporated
                    <http://3DTOPO.com>
                    Phone: 208.462.4171
                  • oldstudentmsgt
                    Why? 50 milliamps is enough to kill you. The extra amperage just flash-cooks the remains. You just want to be sure you know enough not to get that first 50mA.
                    Message 9 of 16 , Feb 3, 2012
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                      Why? 50 milliamps is enough to kill you. The extra amperage just flash-cooks the remains.

                      You just want to be sure you know enough not to get that first 50mA.

                      Bill in OKC


                      --- In hobbicast@yahoogroups.com, Jeshua Lacock <jeshua@...> wrote:
                      >
                      >
                      > On Feb 2, 2012, at 6:46 PM, ferrman1001 wrote:
                      >
                      > > Last time I looked the seat of my pants was not burning but all jokes aside I learned alot from this project about electricity that I did not know before so I think that has to be a good thing for me.
                      >
                      > Thats all good and fine, but playing around with a arc welder for a power supply is a whole other ball park.
                      >
                      >
                      > Best,
                      >
                      > Jeshua Lacock
                      > Founder/Engineer
                      > 3DTOPO Incorporated
                      > <http://3DTOPO.com>
                      > Phone: 208.462.4171
                      >
                    • Jeshua Lacock
                      ... Because 50 amps is way more likely to kill you then 50 milliamps. For 50 milliamps all of the conditions have to be exactly right. 50 amps @ 240 volts -
                      Message 10 of 16 , Feb 3, 2012
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                        On Feb 3, 2012, at 4:11 AM, oldstudentmsgt wrote:

                        > Why? 50 milliamps is enough to kill you. The extra amperage just flash-cooks the remains.
                        >
                        > You just want to be sure you know enough not to get that first 50mA.

                        Because 50 amps is way more likely to kill you then 50 milliamps.

                        For 50 milliamps all of the conditions have to be exactly right. 50 amps @ 240 volts - there is way less margin for error.

                        Its just extremely dangerous. There is no comparison of playing with a 12 volt versus a 240 volt power supply.

                        In additional to increased electrocution you also have increased danger from starting a major fire and god knows what else.

                        I just wouldn't play with a 240v 50 amp power supply unless I really knew what I was doing. I don't think it is something to tinker around with. "Gee what does it do when I connect this to this?"


                        Best,

                        Jeshua Lacock
                        Founder/Engineer
                        3DTOPO Incorporated
                        <http://3DTOPO.com>
                        Phone: 208.462.4171
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