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  1. #41
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    What are you actually running there Tekko ? Come on, a full and complete confession is in order <grin>.

    It appears that all the larger commercial induction heaters use water cooled tank coils as normal practice. If you work out the heating effect of all that circulating current passing through a thin walled copper tank coil, the resistive power loss will be fairly high. Making the pipe wall thicker is not a solution, because electrical skin effect forces all that current to flow right near the surface anyway.

    Once I get my dc power supply up and running, the next job to tackle will be the closed circuit water cooling system. After doing some initial speculative calculations, I may need to wind my feed choke out of small bore copper pipe as well, but am not yet certain if it will actually need to have water circulating through it. With all that cooling water being pumped around, I may as well cool my transistors with some home made water cooled heat sinks as well. That should simplify the whole thermal design considerably, and solve many potential problems.



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  3. #43
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    Default Actual induction furnace up and running

    OK a bit of background. Total amateur who has had a propane furnace for years. A couple of years back built a prototype induction furnace good for a few hundred watts, and intended scaling up to 15KW as that was the max electrical power I have in my home foundry. Found a commercial unit on ebay which I am now running so stopped the development.

    System so far: I have a 100KVA 3 phase 415v diesel generator powering a 120KW CFEI (French) induction furnace driver for which I have two furnace 'bodies' both made by Radyne. One is a 12Kg capacity (of stainless steel) inverting body intended for lost wax casting, the other is a 27kg tilting body intended for conventional pouring. The copper coils in the bodies are tubes with water pumped round at a great rate of knots with a 1HP Grundfos CR8 pump though a heat exchanger which I cool using a 35KW commercial water to air chiller unit driven off the mains.

    The CFEI driver takes in 3 phase 415v and rectifies it to a nice lethal DC rail, which is then chopped by an 'H bridge' of thyristors at about 3Khz. The system is microprocessor driven. A second set of thyristors 'ping' the tuned circuit of tank capacitors and furnace coil to determin the initial frequency (which varies dependant on how much metal is in the pot and also how hot it is) and then the micro tracks the charge keeping it on resonance.

    Connection from the electronics unit to the furnce body is by four (two out and two back) 35mm sq mm cross section welding cables threaded through 3/4" 'Brewers Hose' (no carbon content) and chilled water is pumped down the hoses. There are several tens of thousands of amps circulating in the tuned circuit.

    It's been a long journey but I'm getting there ! Both bodies have had to be re-crucibled - they are perminent fixtures, I end up having to have bespoke ones made in the Cech republic, and the hydraulics and pneumatics have had to be totally rebuilt.

    Attached Thumbnails Attached Thumbnails Induction furnace-imgp0692_small-jpg   Induction furnace-imgp0330_small-jpg   Induction furnace-imgp0355_small-jpg  
    Andrew Mawson
    East Sussex, UK


  4. #44
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    Mr Awsome Mawson, GOLLY !

    And you class yourself as a total amateur, hehehe.



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    Thumbs up Induction heating

    Hello everyone,
    I am an avid hobbyest in several different fields. However, I also work. And my job is at an Induction Heating manufacturing facility. (I do field service work on the heaters).
    I may be able to answer some of the "myths" as related to Induction Heating.
    While I didn't read the entire 4 pages of this thread yet, I have enjoyed seeing the Ideas and realisms that you have come to.

    By the way, as some of you have already pointed out, ANY conductive material can be heated with induction. However, the non-ferrous materials do not heat as efficiently as do magnetic materials.
    Most of the time when Al is being melted with induction, a crucible made of graphite, or other material that can be heated fairly efficiently is used. Then the Al is heated both by eddie currents and by thermal transfer from the crucible. Of course, the crucible also has to be made of a material that can withstand the melting temperature of the Al. And another nice side effect of using induction is that the Al is stirred by the magnetic fields.

    There is a fairly new community at myinduction.com it appears to have both hobbyests and professionals. While the forum is not as elaborate as most, it is a great place to get some questions answered. The website also has valuable information in the "Tools" section. I am pretty new to that community also, but it looks to be a good start.



  6. #46
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    Hello Shawn, welcome to the CNC Zone Forum. I am pretty new here myself. I am just about to register in at the myinduction.com Forum, so expect to see you over there as well.

    My home built induction heater project is coming along very slowly, but it is certainly progressing. This is going to take more like several months to complete, not just a few days or weeks, so for those interested in this, please be very patient.

    I now have all the important large parts to build a 15Kw three phase dc power supply of plus and minus 220v (440v total @ 35A), and after a lot of scrounging and testing now also have a suitable water pump, fan, and water to air heat exchanger for the cooling system. There have been some rather big dramas trying to obtain a proper commercial tank tuning capacitor, and after spending several hundred dollars and wasting a month, I still have nothing. A company messed me around, accepted my money, then sent me the wrong part. I sent it back to be told it never arrived. I will wait another week (it may be still lost in the mail) before forking out even more money and trying all over again.

    If I cannot obtain a suitably rated tank tuning capacitor for this project, then the whole project is killed stone dead before it even starts. But at least all this has given me plenty of time to think in more detail about the electronics. Here is an overall block diagram of the general idea:


    The tank circuit forms part of a classical "Royer Oscillator", or more modernly known as a "current fed converter". This is a very efficient and low stress means of turning square waves into pure sine waves.

    There will be two control loops. The first measures the amplitude of the built up resonant tank voltage, and uses that to control the constant current dc supply which feeds the H bridge current steering section. The constant current supply being a simple buck regulator run in current mode.

    The second control loop senses the tank zero voltage crossings to operate a phase locked loop. The PLL then drives four bridged IGBTs, and steers the constant current through the tank circuit to maintain the circulating tank energy.

    The tank energy should rapidly build up to a set voltage under no load, and it should require fairly little power to do that. As soon as some metal object starts absorbing significant power from the tank, the tank amplitude will fall, and more drive current will be fed into the tank to return the tank voltage to it's set level.

    Being able to adjust the target tank voltage means I can control the power level over a very wide range, right down to zero.

    This is a lot different to how others have attempted to build a home induction heater, but it is a far lower stress way of switching large amounts of power through a resonant tank, than the usual hard voltage switching with a series inductor.

    Last edited by Warpspeed; 04-09-2007 at 09:02 PM.


  7. #47
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    I think Awesome Mawson is correct.

    I have peeked at this thread of and on for a while. Not planning on building anything but interested because nearly forty years ago I ran a 500KW, 20KHz induction furnace at a research facility near Montreal doing experiments in continuous casting of copper alloys. Yes you need enormous capacitor banks to tune the system and the furnace coils are copper pipe for cooling water. I remember the day I had the furnace up to heat full with 500 lbs of molten copper and the idiot plant maintenance engineer shut off the water supply to the building without any warning. My system was busy flashing red lights and making loud siren noises while I ran down five flights of stairs to tell the idiot if I didn't get water soon he was going to see one almighty steam explosion.

    I also discovered that stray high frequency magnetic fields and wire reinforced hydraulic lines don't get on well together. Hot hydraulic oil spraying onto molten copper makes a hell of a stink. I should point out I was not involved in the design so I can't take credit for this discovery.

    The whole installation was built on a steel mezzanine floor in the pilot plant of the research center...yes steel, that magnetic stuff that is also conductive. The floor used to get too hot to walk on around the furnace.

    I lasted six months in that job before I decided my continued health and sanity required a change.



  8. #48
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    Thanks Geof, that confirms something that has been worrying me for quite a while. The magnetic field around the tank coil is going to radiate significantly beyond the actual work being heated. Any nearby electrical equipment or instrumentation could suffer rather badly from induced voltages and currents.

    I would not like to eventually fire this thing up in my home electronics lab, and then discover I have destroyed several thousand dollars worth of nearby electronic test equipment.



  9. #49
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    Quote Originally Posted by Warpspeed View Post
    ...I would not like to eventually fire this thing up in my home electronics lab, and then discover I have destroyed several thousand dollars worth of nearby electronic test equipment.
    A valid concern. I have wondered if you simply surrounded the induction furnace with a steel mesh screen whether that would shield it. A kind of inside-out Faraday cage?

    EDIT

    Its not a Faraday cage is it? That is for electric and magnetic fields? Or maybe it is...electromagnetic theory is not my strong point. (Are not my strong points????...obviously I have problems with grammar also.)



  10. #50
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    I had planned to sit the whole prototype on a trolley and wheel it well away from everything else before running it at anything like full power. But that is all probably still a very long way off into the future. At this stage I just wish to test independent parts of the system in isolation. Never having even seen a commercial induction heater before, this is all rather an adventure.

    My calculations of electrical losses suggest I might need about roughly 300 watts of cooling capacity for the electronic heat sinks, and around 1,500 watts cooling capacity for the tank system. Can some of you old hands tell me if that sounds realistic ?

    My most recent cooling system configuration should do that with a measured 10C water temperature rise. (water into cooler 21C, water out of cooler 11C (above ambient) with a 1600 watt immersion heating element as a test load). The water/air heat exchanger is reasonably large with about 800 CFM of measured airflow going through it.

    Would that sort of cooling capacity be reasonably comparable to a commercial unit in the 10KW to 15Kw class ? I have absolutely no idea about this.



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    Quote Originally Posted by Warpspeed View Post
    I.....Would that sort of cooling capacity be reasonably comparable to a commercial unit in the 10KW to 15Kw class ? I have absolutely no idea about this.
    I have no idea either. The system I ran simply had tap water coming in and then running to drain. I don't even know what the volume flow was.

    When I think back on my experience it was really Keystone Cops. After I gave notice of quitting they hired a couple of other guys to replace me. On their first run on the furnace they did something wrong, panicked and dumped 500 lbs of molten copper onto the mezzanine floor. In the process encasing a couple of rubber propane gas lines in molten copper. I had to walk across the red hot copper to turn off the gas valves meanwhile screaming for fire extinguishers to put out my shoes that had caught fire. I guess I was lucky to survive that job.



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    That is exactly why I prefer to run a fully closed water cooling system. If the incoming power fails, the source of heat goes away as well.

    It sounds like you had a really "interesting" job there Geof. Smoking shoes just add to the excitement. I am very surprised that system did not have suitable automatic safety interlocks and shutdown features. If something can go wrong, it usually does, sooner or later.



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    Quote Originally Posted by Warpspeed View Post
    Thanks Geof, that confirms something that has been worrying me for quite a while. The magnetic field around the tank coil is going to radiate significantly beyond the actual work being heated. Any nearby electrical equipment or instrumentation could suffer rather badly from induced voltages and currents.

    I would not like to eventually fire this thing up in my home electronics lab, and then discover I have destroyed several thousand dollars worth of nearby electronic test equipment.
    To confirm the stray magnetic field issue. When I rebuilt one of my furnace bodies, I marginally re-routed the copper feed pipes to make things more convenient. ERROR! I ended up with a jubilee clip on one of the reinforced nylon water pipes effectively in the centre of a 'half turn' of the current carrying pipes. It took me a while to realise why this clip kept overheating and melting the nylon. Ended up having to source some nylon clips to substitute for the jubilee clip !

    Here is a picture before I changed the clips

    Attached Thumbnails Attached Thumbnails Induction furnace-imgp0336small-jpg  
    Andrew Mawson
    East Sussex, UK


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    Default lookin good

    Warpspeed, Your design is looking good.
    Have you considered how you will deal with a shutdown?
    There is a lot of stored energy in a system like this and the energy will find a way to dissipate.

    As far as stray magnetic fields, you should keep all steel components and electronics a couple of feet away from the coil. Any fasteners on or near the coil should be made of non-ferrous materials. Brass or a high grade Stainless work well.
    There is a discussion on myinduction.com concerning distance magnetic fields will travel. http://myinduction.com/forums/view_p...um-32/topic-40

    What size cap are you looking for? Around 440V @ 20Khz I see, but I need one more datapoint. Either amps, uf, VA - of capacitor. Or if you know the approximate uhy and Q of your coil/load.



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    Quote Originally Posted by Warpspeed View Post
    That is exactly why I prefer to run a fully closed water cooling system. If the incoming power fails, the source of heat goes away as well.

    It sounds like you had a really "interesting" job there Geof. Smoking shoes just add to the excitement. I am very surprised that system did not have suitable automatic safety interlocks and shutdown features. If something can go wrong, it usually does, sooner or later.
    As Shawn D points out you have stored energy so just turning something off may not be the way to go.

    My excited state when my system shutdown was motivated by the fact that I had molten copper separated from a coil of water filled copper pipe by about 1-1/2 inches of refractory material. The auto shutdown closed the incoming water valve and I had no idea if the outlet was now also closed. Water expands when heated and the picture that went through my mind was a crack occuring in the coil squirting water into the bottom of the crucible. Whee!!!! My own mini-Krakatoa.

    I never looked into it further because I moved off that furnace but I have often thought there should be a Panic Mode option which would blow compressed air into the coil to displace all the water if cooling circulation failed with a melt in place. You might have to rebuild the furnace but at least you would not need to rebuild the where it was situated.



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    Quote Originally Posted by Geof View Post
    As Shawn D points out you have stored energy so just turning something off may not be the way to go.

    My excited state when my system shutdown was motivated by the fact that I had molten copper separated from a coil of water filled copper pipe by about 1-1/2 inches of refractory material. The auto shutdown closed the incoming water valve and I had no idea if the outlet was now also closed. Water expands when heated and the picture that went through my mind was a crack occuring in the coil squirting water into the bottom of the crucible. Whee!!!! My own mini-Krakatoa.

    I never looked into it further because I moved off that furnace but I have often thought there should be a Panic Mode option which would blow compressed air into the coil to displace all the water if cooling circulation failed with a melt in place. You might have to rebuild the furnace but at least you would not need to rebuild the where it was situated.
    Many professional furnace bodies have conductive wires embedded in the 'drypack' that the crucibles are supported by, and have electronics to detect the first signs of electrical leakage between these and earth and shut down. Mine doesn't have that system, but the water pressure and temperature are monitored and the system shuts down fast if they go beyond safe limits. The hypothisis is that a coil failure will start small and grow rather than imediately pour forth all its fluid. The conductivity of the water is also an issue that you must consider.

    Andrew Mawson
    East Sussex, UK


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    Quote Originally Posted by awemawson View Post
    ...but the water pressure and temperature are monitored and the system shuts down fast if they go beyond safe limits. The hypothisis is that a coil failure will start small and grow rather than imediately pour forth all its fluid. The conductivity of the water is also an issue that you must consider.
    This does not address what concerned me. You can shut down the energy input if something starts to go wrong but shutting down the cooling when you have a hot system may not be a good idea. Especially if shutting the cooling down means closing valves both on the in and out lines. That is what would be undesirable; a liquid system closed at both ends and getting hotter.



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    Quote Originally Posted by awemawson View Post
    Many professional furnace bodies have conductive wires embedded in the 'drypack' that the crucibles are supported by, and have electronics to detect the first signs of electrical leakage between these and earth and shut down.
    Not that I am trying to sell the kit, but my company has a Ground/Short kit, that connects to the heating coil and monitors the resistance path and voltage potential from coil to earth. The kit also has two contact outputs, one basically to be used as a warning, and the other to cause an emergency shutdown. I think other manufacturers have equivalent detection circuits.

    When I stated that Warpspeed should consider a shutdown mode, I was actually speaking of the energy in the power supply, not necessarily in the coil. I was speaking of energy in the choke that would be in the Buck converter and any capacitors in the system. Many parallel tuned power supplies create a short (crowbar) across the tank by firing all of the inverter switching components at once. But, before you create the crowbar, you would need to disconnect from the utility. This can be done with the Buck transistor or using SCR's for the bridge rectifier instead of diodes.



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    Quote Originally Posted by Geof View Post
    This does not address what concerned me. You can shut down the energy input if something starts to go wrong but shutting down the cooling when you have a hot system may not be a good idea. Especially if shutting the cooling down means closing valves both on the in and out lines. That is what would be undesirable; a liquid system closed at both ends and getting hotter.
    I don't know why anyone would want to shut down the cooling medium (water) in the system. Unless, the cooling is close enough to the heated load, in your case molten copper, and there is a concern that the cooling would solidify the copper.



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    Quote Originally Posted by Shawn D View Post
    I don't know why anyone would want to shut down the cooling medium (water) in the system. Unless, the cooling is close enough to the heated load, in your case molten copper, and there is a concern that the cooling would solidify the copper.

    How about explosive release of steam as a good reason to shut down the cooling water fast in the case of a leak getting near the hot zone ?

    Andrew Mawson
    East Sussex, UK


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