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  1. #61
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    Quote Originally Posted by awemawson View Post
    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 ?
    This sounds like an Emergency situation. In this situation, I would shutdown all power in the induction system AND close a solenoid on the supply and return side. I would still allow water to circulate in the induction power supply, just not the coil.

    But, I thought that the previous comments were talking about shutting the water off to the coil in a non-emergency shutdown.

    So, how do you monitor for a leak near the hot zone?



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    Quote Originally Posted by Shawn D View Post
    This sounds like an Emergency situation. In this situation, I would shutdown all power in the induction system AND close a solenoid on the supply and return side. I would still allow water to circulate in the induction power supply, just not the coil.

    But, I thought that the previous comments were talking about shutting the water off to the coil in a non-emergency shutdown.

    So, how do you monitor for a leak near the hot zone?

    Electrical leakage to earth. My coolant is at something like 40 psi so a small leak goes a long way !

    I keep my coolant circulating for quite a long time after shutting down the driver electronics as if I stopped the pump too soon the now static water in the coils would be in grave danger of boiling

    Andrew Mawson
    East Sussex, UK


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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. ....
    I concur with this opinion. However, read all my posts; I was faced with a situation were the cooling water was shut off externally...it was not my choice. I wasn't the slightest bit worried about cooling the molten copper I was worried about the water getting up to that temperature.

    This is why I postulated my Panic Mode a few posts up to get water out of the hot zone when things have gone seriously wrong and you don't want them to get wronger.



  4. #64
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    Thanks guys, this is exactly the sort of discussion I wish to have, it is bringing up possibilities that need to be thought through very carefully.

    O/k, the bare tank circuit will be designed to resonate at 20Khz (with no metal loaded into the coil). The tank capacitor value will be 21uF, and that has a reactance of around 0.4 ohms. The design tank amplitude will be 300v rms, so the circulating current should end up in the region of 750 amps. Tank power works out to 225 KVAR. The capacitor maximum ratings are 400v rms, 900 amps and 250 KVAR, so it will be running just nicely within it's ratings. The recommended optimum frequency range for this particular capacitor is 12 to 25 Khz so that is o/k as well. Fully loaded the tank Q will be 225KVA/15Kw = 15, so I would expect it should couple fairly well into the load.

    Both startup and shutdown are very critical times for any high stored energy system, and this has all been thought through very carefully indeed. The fact that the tank circuit and H bridge are current driven means that startup cannot overload anything. The H bridge will work at continuous full power into a dead short or into a very highly reactive load, either leading or lagging without any problem at all. The peak voltage across both tank and H bridge will be clamped to operate between the 440v dc suppy rails.

    The H bridge will have a commutation diode in series with each IGBT, so the IGBTs will automatically block in the reverse direction, just as SCRs do. In fact, with this particular topology, all four IGBTs can be turned on simultaneously, or in any conduction order with respect to the stored energy in the tank, without any danger at all. That is the greatest advantage of the current driven converter topology, it is very immune from electrical mishap if the tank and drive system become out of step with each other.

    I have not yet given much thought to thermal shutdown, but agree that the cooling system should continue to run after tank power is removed. As I have never seen a real induction heater, I am still learning. But those types of details can be sorted out later on once the beast is running. Electrical robustness, and possible electrical failure modes are a far more important design considerations at this stage. Thermal design at this point, is just to ensure that there is enough total cooling capacity.

    While I still know zip about induction heating, my previous power electronics design experience is fairly extensive.

    Inductive coupling both into and from any circuit relies on loop area. That means that any circulating currents will radiate in proportion to the size of the current loop as well as the strength of the current. So the trick is to keep all wiring as short and direct as possible, and keep wires that flow current in opposite directions bunched close together. That also applies to the drive and control system layout to make it less susceptible to inductive pickup.

    The physical layout of the parts is just as important as the circuit diagram. I have had plenty of design experience with similar sorts of thing, so feel confident I can succeed with this. I have been designing and building switching power supplies as a professional design engineer for years, and have had some radio transmitter experience both with high power commercial broadcast transmitters, as well as being a radio amateur. So although induction heating is a completely new field to me, I do have a lot of quite relevant experience.



  5. #65
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    One other thought is the problem of the cooling water being electrically conductive. I have absolutely no experience with this, but my first attempt will be to use automotive engine coolant. This has a corrosion inhibitor and PH buffer, but the electrical characteristics of this brew are completely unknown.

    If the whole thing looks like being a problem, I can always use oil as the cooling medium, and increase the flow with a more powerful pump. But it seems everyone else just uses straight tap water, so the problem cannot be too difficult. We shall see....



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    Quote Originally Posted by Warpspeed View Post
    One other thought is the problem of the cooling water being electrically conductive. I have absolutely no experience with this, but my first attempt will be to use automotive engine coolant. This has a corrosion inhibitor and PH buffer, but the electrical characteristics of this brew are completely unknown.

    If the whole thing looks like being a problem, I can always use oil as the cooling medium, and increase the flow with a more powerful pump. But it seems everyone else just uses straight tap water, so the problem cannot be too difficult. We shall see....
    Even if you use automotive coolant it will still have some water, yes? Using anything other than water will greatly reduce your heat transfer capacity; nothing has the same specific heat as water. Using oil could introduce its own hazards; water is non-combustible oil is.



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    I have a GE capacitor catalog handy, so you need GE part # 19L1049WH* The star position relates to mounting style.
    This is a 1095KVAR 600V 20Khz 1659Amp with 8 studs at 2.5uf each (20uf total).

    Use distilled water if you are concerned about the conductivity of the water. Don't use oil, it always has a flash point!



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    Some really interesting ideas there guys, please keep them coming. My main concern with water is the constant high dc voltage potential between various water cooled parts. This is likely going to cause very rapid erosion through ion migration (as in metal plating). I am hoping that distilled water mixed 50/50 with automotive engine coolant will limit that process.

    Reduced specific heat is not really a problem if the cooling system is made large enough. What I have here at the moment can dissipate 1600 watts with an average measured temperature rise of only 16C above ambient. Even with something only half as good as water, the temperature rise would then be 32C above ambient at that same power level. That could be offset somewhat with increased coolant flow.

    Almost any coolant can be dangerous in some respect. I would certainly prefer water, or at leased an aqueous based solution of something or other. But oil is used to cool all sorts of things and the total volume of coolant in this system is quite small.



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    Quote Originally Posted by Shawn D View Post
    I have a GE capacitor catalog handy, so you need GE part # 19L1049WH* The star position relates to mounting style.
    This is a 1095KVAR 600V 20Khz 1659Amp with 8 studs at 2.5uf each (20uf total).
    That would be an interesting alternative, but probably larger than I really need. This is what I am trying to get my hands on at the moment:
    http://www.celem.com/datasheets/C200T.pdf
    x
    http://www.celem.com/images/photos/C200T.jpg



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    Quote Originally Posted by Warpspeed View Post
    One other thought is the problem of the cooling water being electrically conductive. I have absolutely no experience with this, but my first attempt will be to use automotive engine coolant. This has a corrosion inhibitor and PH buffer, but the electrical characteristics of this brew are completely unknown.

    If the whole thing looks like being a problem, I can always use oil as the cooling medium, and increase the flow with a more powerful pump. But it seems everyone else just uses straight tap water, so the problem cannot be too difficult. We shall see....
    I have been advised NEVER to use normal car antifreeze coolant but to use pure ethylene glycol as antifreeze as a 20% solution in water. Apparently there are silicates in car antifreeze that are in solution at the high temperatures of a car system, but settle out of solution at the much lower (under 35 deg C in my case) temperatures of the furnace coolant. These silicates coat the surfaces of the heat exchangers and reduce their efficiency very markedly

    Andrew Mawson
    East Sussex, UK


  11. #71
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    I am looking at some automotive coolant that I have here in front of me right now, and it says "mono ethylene glycol" at 1080 grams per litre. This is what I intended to try first.

    Looking up the specific heat of various types of oil:

    http://www.engineeringtoolbox.com/sp...ids-d_151.html

    Most types of oil seem to have about roughly half the specific heat of water. I can probably live with that if I use a more powerful circulating pump. At the moment I am pushing only about roughly 2.6 litres per minute through about sixty feet of 3/8 inch pipe with a Holley electric fuel pump. It is really only a trickle, but it does match the mass airflow I have rather well.

    Oil will definitely solve both the corrosion and electrical conductivity problem. If I boost my flow up to around five litres a minute, it should work pretty well. I am not too concerned about the fire risk. If my coolant ever gets hot enough to burn, it will be the very least of my problems.



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    I took my advice from a firm here in the UK that services induction furnaces, and they said it was a regular problem that they have to deal with when cooling systems have been contaminated with car antifreeze. Although the main constituant of vehicle antifreeze is indeed mono ethyl glycol, the additions that are made are the issue.

    See paragraph 4 of this url for confirmation:

    http://www.ethyleneglycol.co.uk/antifreezeguide.html

    Andrew Mawson
    East Sussex, UK


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    Thank you for that information Andrew, very helpful indeed.

    What concerns me most about all this, is what happens if you have one piece of immersed copper tubing at +220 volts dc, and another piece of immersed copper at -220 volts dc, with some electrical leakage current flowing through the water based coolant. I am sure that something really bad is going to happen.

    Oil being a pretty good electrical insulator, as well as being fairly chemically inert, may well be the best solution to all this. I am thinking here of very high voltage oil filled substation transformers, and similar applications.

    I would really like the main liquid cooling system to cool all my IGBT and diode heatsinks, as well as the tuned tank system. There will be some fairly drastic voltage differentials between different parts of the cooling system, and that aspect has me a bit worried.



  14. #74
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    The manual for my CFEI furnace driver states that the coolant (Caracteristiques de l'eau de refroidissement !!!) should have a resistivity of a higher value than 2500 ohm cms. The coolant is pumped not only round the furnace tank coil, but also through the tank capacitor, and the main thyristor copper heatsinks. The busbars (which are massive) have copper pipes brazed to them, again with water cooling. Each item to be cooled is connected via a neoprene rubber 1/2" hose and it would seem that the hose lengths have been selected to ensure a long path to reduce conduction.

    Here is a picture showing the 415v three phase bus bars (vertical and 95 sq mm area) coming down to the thyristor rectifiers to produce the DC bus (horizontal and having water cooling pipes brazed to the rear)

    Attached Thumbnails Attached Thumbnails Induction furnace-imgp0708small-jpg  
    Andrew Mawson
    East Sussex, UK


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    That looks to be pretty much exactly how I planned to cool my system. Solid copper heatsinks with a length of copper pipe silver soldered onto it. I have already tested this, and it worked surprisingly well. I fed a hundred watts from a bolted on metal clad resistor into a 2" by 2" slab of copper 1/4 inch thick, with a quarter inch copper pipe six inches total length silver soldered to it. It would have only just been sufficient. My final design will probably be more like 3" by 3" by quarter inch copper slab with about nine inches of 3/8 pipe. There will be six of these heatsinks. Two for the buck regulator, and four for the bridge.

    Something similar but a bit smaller bolted to each side of the tank capacitor should conduction cool that. The expected power loss from the capacitor (according to the capacitor manufacturer), may be around fifty watts. The tank coil is where all the heat is. I am anticipating around a kilowatt to get rid of, but I really do not know for sure. Difficult to estimate the skin effect in the coil, and how much heat soak there will be from the work. But the tank coil will be the last heat load in the whole series flow system.



  16. #76
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    If I remember correctly, you are planning to run at 20Khz, which seems rather high to me. At the 3 khz that I run at the skin effect means that very little conduction happens below 1.25mm, so at 20khz it will be much shallower. What diameter coil are you aiming for?

    My tank capacitors have integral cooling pipes immersed in the oil. When I was doing my low power experiments I was amazed what a variation in dialectric losses there was from cap to cap even from the same batch - I was using polypropolene motor start caps in parallel series combination to get the votage rating and capacity.

    Andrew Mawson
    East Sussex, UK


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    Here's some information:
    Cooling medium conductivity requirements from the Bone Frontier Company manual = 200uS/cm.
    I think you will find that distilled water will be the best solution. If you mix glycol the conductivity will be raised, therefore worsening the situation.
    Electrolysis is a problem when dealing with the voltage potentials that are used in a typical induction system. Here is a discussion that took place on myinduction concerning electrolysis. Electrolysis discussion.
    Here is also an article that Bone Frontier wrote concerning electrolysis and a simple device that we call a water target. Electrolysis article.

    Here is a link to some formulas when dealing with cooling flow requirements.

    And finally, here is a link to a skin depth chart.

    Sorry for all the links guys. Just thought the information might be useful.
    Like I mentioned before, the myinduction site is a bit clumsy. However, it does contain a lot of useful information.

    Warpspeed, can you put some information up on what you are attempting to heat, or are you just building the induction system for the fun of it and to tinker with various heating applications?



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    Great stuff guys. I originally chose 20 Khz more from long established habit more than anything else. It is a good compromise design frequency for designing larger high powered switching power supplies. I can move the operating frequency easily enough later on if I choose to do so. But for better or worse, I have decided to begin at 20 Khz.

    Way back about a month ago when all this began, I did calculate some suitable tank coil dimensions using Wheelers formula, and standard 3/8 copper pipe. The wall thickness of standard commercial pipe in this size is 0.9mm, and I estimated the straight ohmic dc copper power loss at around roughly 500 watts. More of a stab in the dark than anything else, using skin depth curves from a reference book, I figured only about half the outer pipe wall thickness would be seeing some serious action, probably doubling the actual real power loss to about one Kw. Accuracy was not the objective. I just wanted a ballpark figure with which to very roughly estimate the total system cooling requirement. Silver plating the coil should reduce the losses, but it hardly seems worthwhile.

    So much has happened in this last month, with so many different things to think about, the original tank loss calculations for this have been lost and now forgotten. I came up with a final estimate, but cannot now remember all the exact details of how it was derived. Once I have my tank capacitor I will build a real tank circuit and go through the whole process again with a bit more care.

    One question I have. Is it normal practice to surround the work with the total required tank inductance ? I can see that it may be fairly impractical at lower frequencies to do that, and theoretically it is not required. It should be possible to design a suitable work coil to closely couple into the job, and tune the tank with some additional uncoupled inductance. Is that actually done ? I have no idea being totally unfamiliar with all this.

    Wow, thanks for that skin depth chart Shawn, I have been looking for something like that for YEARS. Skin depth curves for copper can be found in many of the reference books, but nowhere else have I been able to find information on skin depth in other metals.

    Shawn, I did begin to build an iron melting natural gas fired furnace, but then became seriously interested in induction heating. What has really spurred me on, is that the more I search the internet, the more people I discover that have attempted to home design an induction heater, and not been able to get anything working reliably beyond a few hundred watts. A kilowatt seems to be something like breaking the sound barrier to most hobbyists. Looking closely at what others have done, and how they have gone about it, I can see all sorts of reasons for their failures. I may fail too, but I am going to have a very serious try. I will also share my success and failure with you guys, and maybe others can follow in my footsteps. Even if I do fail in this, it may advance the art just a little bit and increase the pool of available public knowledge. Any help and encouragement from you guys will be very gratefully accepted.

    So yes, Shawn, I am just building this for the fun and the challenge of it.



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    Default tuning inductance

    Warpspeed,
    Stray tuning inductance has been used in the industry. However, it does have a major disadvantage. Any additional stray inductance will effect the amount of energy that is put into the heated part. Stray inductance will have a voltage drop. Model your parallel tuned tank, only slip an inductor in series with your heating coil. Now, you will see that the voltage is divided among the two inductors. The larger the stray inductance, the less voltage that will be applied to the heating coil.
    For this reason, most manufacturers minimize the stray inductance in a system. For instance, when making transmission leads (wire or rigid copper buss), the leads are kept adjacent to each other so as to minimize the inductance loop.

    Normally, any tuning required for frequency is achieved through capacitance changes. That is why more tank capacitors have selectable studs.

    Another option is to put taps on your heating coil.



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    Thank you for that great explanation, it is all now starting to become very much clearer.

    I can understand the theory, it is just that never having actually seen any of this equipment, it leaves me wondering about a few things.

    Another question. Every amateur home built induction heating circuit that I have so far discovered on the internet, none of them have attempted to control the tank amplitude in any way. There is always just a large power amplifier driving the tank circuit flat out all the time through a series inductor.

    To my way of thinking, the tank amplitude must be controlled in such a way as to be maintained fairly constant. Sufficient additional drive power should then be fed into the tank to make up for the energy consumed by the heating load.

    From the very little I have been able to discover about commercial heaters, the big low frequency SCR ones like Andrew's CEFI work in a rather similar way to the one I am attempting to build. They use a six SCR phase controlled rectifier bridge to generate a variable dc current, and that goes to four more SCRs in a resonant mode bridge chopper circuit to switch the regulated current at the tank operating frequency. As the SCRs are physically constructed in pairs, it looks like only three SCRs plus two SCRs, but there are actually six plus four..

    Do all the commercial heaters use some sort of tank amplitude control system? as I would think that feature absolutely necessary, yet none of the amateur home built circuits have that. I rather suspect it is one of several rather fundamental reasons why some hobbyists are consistently blowing up parts at higher power levels.



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