Induction heaters - the theory - Page 1
Forum section: electronics / beginners

author: drogus
author_rank: Contributor
timestamp: January 14, 2019, 01:08:59 pm
content: Hi,I'm a noob in electronics and I recently got really interested in induction heaters. Eventually I'd like to build one (mains powered, maybe 2-3kW), but for now I'm trying to learn the theory needed to build one.A bit of background: I'm a software programmer and I started learning electronics a few years ago. I know basic principles of passive elements and my most advanced project involving analog electronics was a 10A buck converter without using a dedicated IC for control (it was a lot of fun).So I'm looking for resources that would be good for learning more stuff that is involved in designing an induction heater. Preferably books and preferably something aimed at beginners, but online resources will work as well. Just to clarify - I don't necessarily look only for resources on how to exactly build an induction heater, but also to learn about the concepts that are used, so stuff like AC voltage, transformers, inverters, power electronics - I guess when I understand this stuff better the tutorials that are available online will make much more sense to me. I know that it's quite a broad and not well specified request, but I'll accept broad answersIt can be a good book to learn electronics in general, a good book on power electronics or a book about electroheating or anything in between.

author: rhb
author_rank: Super Contributor
timestamp: January 14, 2019, 02:07:17 pm
content: This is the one I could find in my library. I think I have another, but it's 5000+ volumes and rather disorganized at the moment. But there are a lot of modern books on Amazon. I just bought two more :-(https://www.amazon.com/Frequency-Induction-Heating-Frank-Curtis/dp/0917914716Unfortunately, Lindsay Books closed so all that is available is old stock.However, there are a lot of example builds on line. Try an advanced search with google of the hackaday.com site.

author: drogus
author_rank: Contributor
timestamp: January 14, 2019, 03:07:00 pm
content: Thanks for help! This book looks interesting.Quote from: rhb on January 14, 2019, 02:07:17 pmI found quite a few online writeups about induction heaters, the problem is that I also lack knowledge about how certain elements that are needed to build induction heaters, so resources about the basics will be appreciated too.The stuff that I found so far, maybe it will be helpful to others as well:https://www.instructables.com/id/30-kVA-Induction-Heater/https://www.instructables.com/id/12KW-Induction-Heater/and a website with another version of this tutorial:http://inductionheatertutorial.com/inductionheater/induction5.htmlanother tutorial:https://www.instructables.com/id/DIY-Induction-Heater/here's a good write up on how they work:https://www.rmcybernetics.com/science/diy-devices/diy-induction-heatera good video on a simple design:here's a post about someone's journey in designing a heater, I haven't gone through it yet, though:https://www.dansworkshop.com/2008/03/induction-heating/I also found the "Foundations of Electroheat" book:https://www.amazon.com/Foundations-Electroheat-Approach-C-Metaxas/dp/0471956449,but it's over $120 (+ $20 shipping to EU + duty taxes) and it doesn't look like a good starting point for a beginner, so I'm not going to try it unfortunately.Sorry, I should have started with what I found so farIn any case, I'm aware of a lot of online resources, but a book is often much more thorough and I want to have a much better understanding on all of the components involved.

author: drogus
author_rank: Contributor
timestamp: January 14, 2019, 03:10:47 pm
content: Oh, and there's also a great open source project:https://reactorforge.com/about/,but it's still in the works.And I knew that I forgot about something. I didn't have this link saved on my list, but I had it sitting open in my browser, here's a great article and a great blog in general:https://teslascience.wordpress.com/how-to-construct-simple-powerful-induction-heater/(he also have a very good YouTube channel:)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 14, 2019, 05:25:24 pm
content: I've also got an old series here,https://www.seventransistorlabs.com/tmoranwms/Elec_IndHeat1.htmlwhich has partial schematics, which aren't quite right but show a lot of the concepts needed to build a proper machine, not just a toy. (Example: this is a better oscillator-inverter section,https://www.seventransistorlabs.com/Images/Small_2.png)FYI, in typical Instructables fashion, the Instructables articles are toys, if not jokes.Think I was the only one in the induction community circa mid 2000s that was designing (and building) anything practical, namely, with full controls and protection features included. Don't know what anyone has been up to in the last ~decade.Tim

author: drogus
author_rank: Contributor
timestamp: January 14, 2019, 08:21:48 pm
content: Quote from: T3sl4co1l on January 14, 2019, 05:25:24 pmThanks for the link. I've read a few first parts and it's very interesting. I really like it that you describe the process and results of what you tried, not just the end result.When you say that the instructable articles are a joke you mean that in context of manual tuning and lack of protection features? Or is there anything else wrong with them?

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 14, 2019, 10:50:44 pm
content: Partially off topic: ever seen?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 14, 2019, 11:20:48 pm
content: Quote from: drogus on January 14, 2019, 08:21:48 pmYes, exactly.One must also not underestimate the hazard of working with these machines -- a human can only output a few hundred watts, and subjecting the body to any more than that is very likely to cause bodily harm. Or less, if applied correctly (e.g., a punch to the head is a lot more dangerous than a punch to the shoulder). We're talking levels of multiple kilowatts -- multiplehorsepower -- you wouldn't want to get yourself in the way of a full grown horse, would you?Protection is mandatory in my opinion, both as a lesson in patient and safe design practices, as much as for practical use.It is my philosophy that no system, given the controls provided by the manufacturer, should be able to damage itself. It should preferably be easy to use. Manual frequency control, that's a joke. No current limiting? It's a danger to itself. No insulation or shielding? It's a danger to others, too.Of course you can construct scenarios where things should be able to damage themselves, but these should likewise be infrequent exceptions, and carefully labeled, and interlocked if possible. (A lot of power tools are in this class: it should be obvious where the power cord is, so you aren't cutting into it. Neon orange cable, great.)It's the same as the now-ancient argument over buffer overflows in programming. "Duhuh, only idiot programmers do that. Git gud!" No. Besides being elitist, that only ignores the issue. Some languagesprohibitsuch abuses, and fundamentally cannot have such errors, no matter how good* or bad you are. It's an eminently solvable issue, and it's a lesson that has been illustrated time and time again throughout history.Learn from others' mistakes, and do it right the first time!That's the kind of design I like.(*Mind, given that "nature always makes a better fool". There are bugs in many languages, and there's always some way to abuse the system to go around or outside of it -- it wouldn't be a very useful language if not, i.e., probably not Turing complete. But the reason I asterisked "good" is because you should have to be very ingenious indeed to discover these abuses, not stumble into them during an occasional lapse of awareness.)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 14, 2019, 11:27:31 pm
content: Following on from the safety aspect, just to include it in this thread:Electrical systems are not all that dangerous, by themselves. They don't tend to mangle and dismember, as mechanical or chemical systems do.Shock is the primary bodily hazard: disruption of the heart. But also muscular twitch response, that can make things worse, say jumping away from a live circuit and banging into something, or grasping harder onto a DC conductor. Be mindful, and respect it. Take simple precautions, build habits to avoid getting in trouble.Electrocution is not much of an issue at these voltages, i.e., unless you stick wires into your body, you're unlikely to getdestructiveamounts of power through you. This does pick up in the 400V+ range, where electricity goes fromzappy, to decidedly...angry.Next comes burns, due to sparky, burney and explodey things, and in the case of induction heaters, uncooled coils and hot workpieces. Just because it's black (not glowing), and not electrically energized, doesn't mean it's safe to touch!Lacerations are a risk. Transistors will explode into fragments at these energy levels.Simple, unobtrusive PPE is really all that's needed, for the most part. Safety glasses, maybe cotton or wool clothing (burn resistant). If you have to work on live equipment, use insulated tools and probes, and maybe rubber gloves.You sometimes will have to work on live equipment. Don't make a habit of it -- de-energize when possible. When you do, just make sure that you're doing so mindfully, and taking those extra steps.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: January 14, 2019, 11:30:14 pm
content: for a high power system say 5kW what I was looking at for a long time is1) IGBT phase control bridge, use phase the adjust power from a H-bridge to a coupling transformer to the tank circuit2) water cooling (a bitch with these power levels and voltages)3) use isolated drivers with individual power transformers to control the H-bridge (iso5500 or beefier), using all its capabilities like DESAT protection etc, make sure the transformers give clean power and the circuits dont interrupt eachother with pulses or cause ground bounce etc4) analog circuit that handles the PWM frequency and phase shift5) digital control of analog circuit setpoints (LTC timerblox might be good for this, it is what my control system prototype was going to use) (i.e. weighs and scales the analog part)6) welded steel enclosure with test points so you don't mess around with high energy shitWhat I have not figured out that well is the feedback to control tuning. Apparantly if you put stainless or aluminum into the coil it can decrease inductance and cause excessive power problems. Most of the ones online ignore feedback and over specify everything and require manual tuning of frequency.Getting information back from the tank after the isolation transformer seems to be a bitch. You can't use CT or rogawski coils because of noise on current switching, so you want to use a voltage signal. I don't like the idea of a voltage transformer controlling the frequency. I also don't know enough about the high power transformer to be confident about its behavior or if it will have some kind of inrush problem etc being a doughnut. The idea would be some kind of phased lock loop.In regards to the control system I had something in mind like the F-111 ground follow radar.I just have a feeling the impedance matching transformer and feedback are going to have some kind of funky quirks about them. recently I discovered powdered magnetic transformers age............... wonder what other surprises are in stockfyi a very nice lab oven for r&d is rated at 5kW, melts around 30cc of metal @ 1800C or so in a vacuum.Another problem is I have no idea what I am going to do with it.What kind of metal research can you do with a induction heater? I don't know if I even see the point in using one for brazing because a kiln would work better in most cases and the shop ones are pretty cheap now for losing bolts etc.something like this would be a goalhttps://rdoinduction.com/supercast-pro.htmlkeep in mind theirs weighs 200kg. that keeps the project in mind of how difficult it might end up being. its probobly equivalent to building your own lathe.Also keep in mind how much crucibles can contaminate what you are working with and how destroyed they get. If you are interested in making custom steel alloys have a read on forums about crucibles. you can get something like 7% carbon inclusion from a melt in a graphite crucible.the whole induction project kinda lost most of its utility from me when I looked into the applications side of things as cool as it is.If you never used one, get yourself some time on a oxy-acetylene torch. it kind of made my high temperature chemistry interest more reasonable and less mystic. if i did not play around with torch brazing and actually try to do some destructive testing on stuff I made just by slamming it into walls etc I would not appreciate the strength (very easy to think you need some kind of 110KSI strength on a braze joint till you do a shitty one with no setup with the most common cheap possible alloys and realize you can slam the thing into a brick wall all day and its fine. Then I might have gotten stuck spending 10 years of my life building some crazy contraption. Not saying don't try, I was just kinda misguided about it for a long time.I am still waiting for some awesome post about someone using a induction heater to make some kind of rocket or quad copter or something that is just not possible any other way to help justify this avenue of research........ I have a feeling with my goals I would end up feeling a bit insane after I put this thing together. A kiln at least you can do weird stuff with, like fix irregularly shaped copper blocks to make your own lapping plate out of dissimilar materials or something........... the induction heater just seems like I would need to be a heavy diesel mechanic or need to produce 50000 of something to appreciate.................... someone tell me what small scale stuff I can do with a induction heater that I can't do with a oxy-acetylene torch for 1/15th the price and 1/500th the effort..... some kind of cool metalurgical mixture that needs precise heating and mixing I am sure??/I am even wondering what kind of custom alloys you can actually develop for home use. Seriously I thought about it alot the best I can see happening is like better ice skates. And I have not been skating in around 15 years. And maybe we are at the apex of ice skates. I just can't see the next development in small engine pistons or something coming out of some garage............. and the world is oversaturated with crazy knife makers already.................. I can't see like high performance turbines made at home either..... unless your richy rich.. maybe powder metalurgical alloys but even that you are going to need SERIOUS money to have any sort of decent research effort into given how all the big boys are all over that now.... it just seems fucking dismal.....teslacoil must have some great uses for these things right? i got fired up about induction heaters like once every two years for the last 12 years or so but I always run a bit short on the application

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2019, 04:11:54 am
content: Quote from: coppercone2 on January 14, 2019, 11:30:14 pmNot a bitch, just a formality. Helps greatly with power density.Cooling plates with embedded pipes are off-the-shelf, it's pretty good actually. Pair that with an automotive radiator and fan, a small diaphragm pump probably, and a reservoir, and you're set for life.Quote3. DC-DC modules are easier, but line frequency transformers with low capacitance (typically split bobbin type) work, too.5. Just use the canonical analog-digital control scheme: analog takes care of itself, setpoints set by DAC. Operating point read by ADC. MCU does whatever heavy lifting is needed (e.g., calculating output power?).6. Aluminum can be better, depending on how well you control the fields inside. Even that can be bad, if you put too much field into it. The tabletop industrial unit I designed, was forced into such a path by the management... turns out putting a work coil half the size of the enclosure, inside the enclosure, turns it into a pizza oven. Had to line the enclosure with ferrite plates, in the end.This is considerably easier to deal with, in a series or parallel resonant network (of which, series resonant is easiest with conventional semiconductors). An LLC ("series tuned parallel resonant") network only has that problem because you need to put that first Lsomewhere...Anyway, pipes close together, or better yet laminated bus bar, is the way to go. This never got hot on the front panel (aluminum),and I ran it at 5kW (Q ~ 15).QuoteYes, a close-fitting load can halve (or worse) the coil inductance, while dropping the Q only modestly (for the case of copper or aluminum; for stainless, the Q drop is appreciable of course). A steel load can double it (or more), until it passes Curie temp, at which point it flips to the other side (dropping).So, frequency control is mandatory for any real work.A typical use case is forging, where work is constantly being placed into, and removed from, the coil. The supply can be simmered or stopped inbetween, but sooner or later you'll leave it on too long (or turn it on too soon), and be operating at full power into a high Q load. With no limiting, poof, goodbye inverter.With frequency control and voltage and current limiting, you can set the supply for full output into a steel load, and let it limit automatically when no load is present. Typically you might have a Q of 5-10 with work inserted, and 30-50 without; at the same coil voltage or current, that's a ~5x difference. If it's set for 10kW into the work, it'll simmer at only 2kW into the coil -- it throttle down automatically, tracking the load.You could even add a feature to monitor the ratio of voltage and current to inverter output, i.e., the load resistance, and throttle it down even further when no work is detected.This is how induction cooktops work: the coil is pinged from time to time, and if a load is detected, it can move into operation.Or for geeky purposes, you can measure the V, I and F at the inverter output, which is therefore equivalent to the L, C and R of the tank. It's a network analyzer, solving for a simple RLC load, operating at full power.QuoteIn a series resonant circuit, current sense is exactly what you want. CTs work very nicely. The current ripple does not have sharp transients, at least for any sane build.Here's an example from my archives,As measured on a Tek 475, so not that you'd necessarily see tiny squiggles, but they're really just not very important.You can still see a little bit, the current waveform is broadened at time divs 3 and 8. That's the actual ringing here. It's not much because the inverter voltage waveform is heavily snubbed (hence the trapezoidal waveform), and it's easily filtered in any case.A shielded CT goes a long way, too. Basically, anything but a Triad CST206-1A, which is absolutely the worst CT I've ever used...QuoteYou still want to sense voltage, but not because of tank parameters so much as to limit voltage safely, lest you burn through the capacitors. Which you've more or less spent $100-300 on at this point, whether because you've spent that time soldering together an array, or bought a proper Celem or other name brand.With voltage, you can of course compute R, L and C from V, I, phase and F.For synchronization purposes, I've been tempted to use a depletion mode MOSFET current limiter, to sense the high voltage directly with minimal phase shift. Should be very effective at lower frequencies, but probably not as good in the MHz.In any case, it's just signal analysis, with varying levels and impedances of signal, depending on how you sense it. Dynamic range limits apply, so don't expect to track a super weak resonance. It's a pain trying to make an output transformer suitable for a huge range of impedances anyway. A modest operating range is fine (say 30:1 coil resistance?), which keeps the signals from being gross.QuoteI was going to make a high frequency ("high" is relative; it would be 1-2MHz) model suitable for soldering pipe, brazing stuff, etc.; it would be an all-electronic torch substitute.With an intuitive control showing tuning, and inverter capacity enough for a generous tuning range, it should be pretty easy to use. The main trouble is changing out coils, and making coils in the first place.The other thing I was going to do was.....Quote...My original motivation, back in the days I did home foundry. I would've done iron and steel with induction. Pretty easy metallurgy, no worries about contamination and such (clay graphite crucible, BTW), nothing picky.I still have a number of my foundry supplies around, but nowhere to use them, so also no motivation to work on a medium frequency (10-50kHz) heater (the one pictured above).Tim

author: drogus
author_rank: Contributor
timestamp: January 15, 2019, 10:19:33 am
content: Thank you all for the input! I'll admit that I don't understand some of the discussion, but it will surely come in handy when I learn more.Quote from: T3sl4co1l on January 14, 2019, 11:20:48 pmWise words. I'm usually paranoid about safety, so I agree completely. I also do some hobby machining and I cringe when I see youtube videos where people are doing things that could possibly get them killed. That stuff is powerful.I remember when I was a teen and I was forging some stuff with my brother (we were reenacting history). When we were working on small stuff we often used metal rods that we held by the "cold" end. My brother put one such rod aside in a way it was possible to grab the hot end. As it happens when you allow for a problem, a problem occurred and he grabbed the hot end. It was in a day light, so even though it would be probably glowing in the dark, it seemed cold on a quick glance. That was a very painful and lasting lesson.Quote from: T3sl4co1l on January 14, 2019, 11:20:48 pmAgain, I agree. A bit of an offtopic, but that's why I recently started learning Rust, which is an awesome language. Speed close to C without buffer overflows, data races, null pointers with the expressiveness of high level languages. I love it so far and once I'm more experienced I think I'll try to make it my go-to language for most of the stuff.

author: coppercone2
author_rank: Super Contributor
timestamp: January 15, 2019, 01:35:22 pm
content: To go further on applications:If you look at stuff like harris brazing catalog, with all the alloys they have, for doing pipe you actually get a stronger bond using their low temperature solder then the brazing because you don't have heat making the pipe weird. At least for copper the easy-simple harris special solder ends up having a higher burst strength then the damn braze. Foundary work is kinda iffy for me because you don't get a great surface finish and you really need a 3d printer to make the parts that will release nicely from molds unless you need alot of mold layers.I am just saying this about brazing because when I got fired up about induction heaters many times I thought that I could get super high pipe braze joint quality but its not really that good, and you need inert gas inside of the pipe ANYWAY if you braze, and you are not benefiting from higher strength ANYWAY because the copper will weaken from the heat. And experimenting with copper, you can get a nicer polish then the store sold for like 30$ of sanding and grinding equipment that will last you a long time so long you have some basic tools. All the cool hydraulics components that don't swage are designed for TIG welding too (like look through a swagelok catalog). You just need to do that inert gas fill when doing nice pipes ANYWAY so it cancles out the induction heater benefit. It will also polish up super easy if you just gas braze them (not sure if its even OK because I think its a face joint between the pipes so you need a damn filer material, the joint might actually be considered weak even with 56% silver because the surface contact area is so small).With parts joining you can only join narrow things with the coil unless you make magnetic directors for it. It's just so god damn non-versitile, something like a Henrob Cobra 2000 torch kit will just end up doing a much better job (and its only going to be like 600$ with all the good accessories if you already have oxy-acetylene equipment), and you can also weld steel if you have that.1:15 shows how they weld together the high end stainless pipe systems. I think you would have a better shot trying to weld that sucker shut with a gas torch with the cobra head (tig like) and stainless steel filler rod then trying to braze the face seal on those pipes with any technology.All those pipe systems want something referred to a 0 dead space, meaning they put face seals whenever they can so contaminates don't hide in the overlap joints and so you can polish the interior perfectly if you so desire with special tools. They even sell little inserts into the valve bodies and stuff to reduce their volume as much as humanely possible to get laminar flow inside of them so you don't have areas were corrosion stuff can accumulate etc (i.e. in a high purity water system with a scrubber you don't want a bunch of ions accumulating in some gap despite the flow, then it gets turbulant and they emerge and ruin your process)You can weld stainless with oxy/gas but I don't know how good it is, hopefully your not trying to build a nuclear reactor cooling system this waymanufacturers tutorialyoutube.com/watch?v=0GowaLg_yBMI think you could get something usable if you practice alot, even on tubing, even if swagelok says not to (because they need to cover their ass in case they are responsible for a 3-mile island 2.And again you said graphite but look at how contaminated the iron will get from dissolving the crucible! You need special crucibles and they all get destroyed to if you have hot molten steel/iron in them, it acts as a reductant I think.And the best usable projects I found for steel casting = making your own vises. There is one neat vise,that is sold commercially, that actually requires casting because it has a little ball bearing hemisphere in it that does force vectoring and actually clamps down 50% of your clamp force, and its cast with basically unmillable geometry, so when you machine the part cannot be lifted up *unlike a regular vise that has no real anti-upward downforce*, but you still need a at least a powerful milling machine to get it usable with a flycutter and you need a surface grinder to make it non shameful.. It's called a precision milling vise, but its not actually that precise (that would require one of those vices with the universal joint notches every 1/2 inch or so that is tightened with a wrench), but its good for milling machine tolerances. For the grinder or more precise milling *not even sure how* you need the universal joint vise or preferably a magnetic table. But it was a breath of fresh air for me to find that vise, because I thought wow you actually need to cast this thing unless you have some really weird tools.Unless you already have a machine shop you will be relegated to casting hammers! I made a bunch of hammers already and it gets kinda old. I say this because for wood working you already have advanced hammers (i.e. I own a overpriced overengineered titanium strike hammer that is one of the weirdest thing you have seen for building a house), and all the machinists hammers etc have advanced features anyway that need a machine shop to make like removable heads, knurling on the knobs (nice oil resistant grip finish), etc. Copper, lead, bronze hammers are very good, especially considering how much a lead one wears down, over the long term you might be able to save some money making your own lead hammers if you use them alot (especially since you want them real soft). And you can use sanding/etc to make a nice looking hammer with acceptable tolerances. But for steel hammers you can get parts that need very little work to turn into a nice hammer... almost no point in casting them.AND I DONT THINK YOU WANT TO WORK IN A STEEL MILL ANYWAY!!!!!!!! *not the best project to put on a resume*I need time to think about the electrical things you said though, its still interesting, but I lose interest because the project does not have clear goals or objectives... with the electrical sensor stuff at least you can make it so it operates near theoretical measurement limits.. this thing not so much.. its just not saving me money or giving me unique capabilities or anything!!!!how much does the induction machine you made weigh? with my induction heater I got as far as making a wooden mockup of a possible chassis and mounting the transformers and IGBT to it etc to see how it would look so I can think clearly in the future, but the control system and feedback network and coupling transformer are still unsolved problems that I keep giving up the research effort on.......What kind of crucible do you use for high purity metal work if you don't want the alloy completely changed by dissolved carbon? I thought if I picked a crucible at least and got that bag of snakes out of the specification it would be easier to focus on. There are some options but I don't know which would have the longest life and be the most robust (AND THESE ARE NOT CHEAP!). If I had a crucible at least I could have a coil inductance and nominal frequency spec LOL. RIght now I don't even know what that would be. Right now my spec is in the flubber stage, I don;t know what it really is and it bounces all over the place and I suspect it might end up benefiting the world by building better basketball hoops. (great movie). youtube.com/watch?v=HJT2QOpnYb8WHAT ARE MY ODDS AT MAKING VIBRANIUM AND BECOMING THE REAL CAPTAIN AMERICA??/ALSO there are many different graphite crucible grades/types out there, they are not equal.Also if you are building one, they have special IGBT series that are advertised with switching frequency/induction heating use.

author: coppercone2
author_rank: Super Contributor
timestamp: January 15, 2019, 05:14:45 pm
content: Quote from: T3sl4co1l on January 15, 2019, 04:11:54 ami need to look at my notes and some resources about the feedbacks still to discuss furtherI did think though that the control board should have isolation on all the feedback signals. Would you just let the secondary sensors into the processor PCB directly? I don't know if you can turn them into digitally transmitted PWM signals through isolators (the galvanoresistive ones might make sense here because they have less coupling them the RF ones) for use in a analog loop or if you would digitlize them. I was weary when I thought about plugging a CT into the control PCB directly. If they are fast enough you can use V-F converters and discriminate them into digital signals. Or for things like RMS just measure it with a timerblox and use the PWM output as a metric for amplitude. Conversely F-V to set timerblox PWM as a frequency indicator. I found there are problems with this kind of thing on startup sometimes but what I did is just put a time delay on the control circuit so everything can get to oscillating before it regulates itself (put some kind of weak minimum bounds on the startup condition)I like the idea because then you can use digital isolators for all your signals. And then you can isolate all the subsections with their own transformers since its already a giant box that weighs 200kg commercially. Who gives a shit at that point.For how everything effects the scaling parameters for the frequency and power (phase shift ratio) it seems like a spider jumping on a trampoline. May be complicated to make all the summers/scalers. Then just make a temporary panel with a bunch of pots on it to adjust the circuit to find the values that kinda make it run and then replace those with digital signals from a MCU once you honed in on it.Maybe turning it all into PWM would make it too drifty for case hardening turbines but I think it would be fine for a vacuum smelter.Can you use RMS converters to get the amplitude information from the currents and the voltages? Or do you need hump-by-hump monitoring? How fast will the situation break down? What kind of response times do you need from the protection systems? (i.e. what filter cap on the rms converter is OK or is it just not gonna work?)

author: coppercone2
author_rank: Super Contributor
timestamp: January 15, 2019, 05:49:14 pm
content: otherwise how do you condition the sensor signals (other then the fairly obvious IR sensor output)?I guess it will be fairly predictable in a shielded box, but if you are just using a exposed coil to heat a bolt near a running car's spark plugs might it cause a interference problem ??Or if its in a laboratory/shop and someone does something high energy/RF while its running, arent their risks of the sensors getting HF pickup?What signal conditioning did you use?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2019, 06:59:46 pm
content: Quote from: coppercone2 on January 15, 2019, 05:14:45 pmIn the industrial design, we used a flow meter, I believe the free turbine type with Hall effect sensor. No fouling problem (not beyond what anything else will get, anyway), very reliable, fail safe (no pulses = must be no flow, shut it down).A pressure switch is often used, too, since a pressure drop is expected across the plumbing. We didn't do that (I think the pressure drop of the internal piping wasn't very high -- it had pretty good flow). Obviously that's not telling you if pipes get blocked, so it can only be used with a flow sensor of some sort.In a product, I'd be happy with a flow switch or meter.In my project, I didn't get that far, but the trickling sound of water falling into the milk jug reservoir is pretty obvious, and failing that, the rapid bumping of water boiling in the work coil is a further reminder. No, the semiconductors don't heat up nearly as fast as the coil -- this is fine for testing purposes.QuoteYou can. A thermistor measuring outlet temp is not a bad idea (which we did on the industrial design). Thermostat, no. If the flow is >= enough, you don't care.QuoteAnd the inverter isn't completely fucking beating the shit out of the SMPSU..?We used RECOM Medical type (8kV reinforced, low capacitance) DC-DC's, which handled 650V, 50ns edges just fine. They made no detectable noise.What did create detectable, in fact annoying amounts of noise, was the 100V 10A Chinese shitbox PS we got for testing. Its output had 10s of volts of common mode noise (spikes). The 100V inverter waveform was noticeably fuzzy. I had installed extra filtering inside the bastard to help. Later, it cooked itself off; seems that, when operated at full current into a <10V load, it's a class A amplifier, rather than class D... its IGBTs didn't take kindly to that. Which were overrated and under-driven anyway, in typical Chinese fashion.QuoteYou can do that, especially now with thermal sensors/arrays/cameras being common (they were still very boutique and expensive circa 2010 when we looked into this).Thermal is slow, though, and you're likely to overshoot the setpoint. Typical industrial application is a square pulse of power. Open loop, just a programmed sequence. Lock parts in jig, hit the go button, apply e.g. 8.2kW for 1.23s, off, let cool.The tech designing the profile, adjusts P and t until the part has the desired heat treat pattern, or filler is distributed, or whatever. Dwell or simmering is rarely needed. The characteristics of steel mean that it's not much point to adjust power during a case hardening step (the heating zone follows the region below Curie temp, which is coincidentally very close to the austenizing temperature; the region above Curie doesn't overheat very much, because of the huge shift in properties). And that covers 95% of applications, so there's very little need for more control.QuotePPECO does this, apparently.Gun drilling is a huge waste of machining time. I guess it's cheap enough in Taiwan.More than adequate is brazing a pipe onto the bar. Until you get to very high frequencies and powers (100s kW, >200kHz?), you don't have to worry very much about the combined thermal and electrical conductivity of copper.Stacked and soldered/brazed construction is also excellent. Or an embossed shape against a plate. Good way to make big pockets connected to many fine channels. Unless you find one off-the-shelf, it's a lot of custom design and building though.QuoteMaybe. The coil can be grounded, one side or the other, or CT grounded or whatever. It's usually kept isolated, so you don't get screwdriver-melting fault currents when something accidentally touches it. But that doesn't preclude a modest impedance (say a couple 100k resistors in a voltage divider and differential sense amp?) from being useful.High frequencies don't feel the same as mains frequency. Better to just insulate or guard the coil in the first place. You can get electrocuted (cooked) by that kind of power, without feeling anything at all. Again, not something a few 100kohms will aggravate, but a big risk if it were grounded, or if someone grabs both terminals at once.In any case, everything is transformer isolated, so it's also no problem to transform it right back. A hundred turns on a pot core makes a damn high impedance (effectively no load on the work coil circuit), and say a single turn secondary makes a quite acceptable signal level for the control circuit to work with (say 100:1 ratio, so 500V on the tank --> 5V at the circuit).The direct-connected current limiter/clamp idea I mentioned, could be sent into a digital isolator, yes. That would be a fine way to get phase information out (of course, the limiters destroy the amplitude information).QuoteDigital isolators are good for digital interfaces. For example, you might have a core control board, that's mains referenced (to the inverters specifically, and maybe those use bootstrap gate drivers, hence the mains reference?), which does all the analog control itself. It has an isolated digital input, and DACs onboard, to communicate enables, status, faults and setpoints. The digital control board then is ground referenced, and has user input controls (pots or encoders on the front panel, say), or external inputs wired from a PLC, or comm ports, whatever's good.QuoteWell, no, those are just design parameters. Min/max V range, I range, F range, and control loop time constants (and zeroes if applicable). Or PID parameters, as the case may be. The loop parameters are the hardest to know from basics, but the others are just AC steady state solutions.QuoteOne thing we played with was a direct drive inverter, 1MVA capacity at up to 50kHz, expected output ~150kW (depends on load, obviously). The point was to direct drive so the square wave PWM% and frequency are adjustable, controlling the heating depth with the programmed cycle (which was linked to a CNC axis which moved the work through the coil).The cross section from the process is pretty cool. It looks like this,https://www.industrialheating.com/ext/resources/Issues/Issues2/2018/Dec/ih1218-opportunities-lead-900.jpgbut imagine the depth can vary by section. So, maybe those inside corners could be made deeper for better strength, or something.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: January 15, 2019, 07:44:43 pm
content: I don't understand what you mean by the hammering the shit out of the power supplies?Maybe I am not being clear, I just meant that all the isolated control electronics would be powered by small signal transformers with extra filtering to power rectifiers to power linear regulators to power the electronics. I thought it would be more robust then using switching converters.My first mockup of this actually used RECOM brand dc/dc converters (for the gate driver and MCU circuits), which were cheap on ebay as NOS, but I thought if one of them breaks it could be a hassle since they were not cheap on digikey. I thought if I built a serious shop tool it would be unobtanium to obtain the same footprint DCDC converter in 15 years. Then it would turn into a horrific bodge to fix it. I did not recognize them as a standard footprint. But I think the best recoms I found on ebay still needed a 48V stepdown transformer or stepdown converter to work. I just see it going south in the deep future if I make a nice fit chassis for it with all the ebay deals.Of course I want to also power the power circuit through a big multiphase isolation transformer (but not used for isolation) but I also expect to run into design difficulties here, with inrush and humming. I don't know enough about those power levels to say anything ATM. But for the initial testing I would like to get series high power regulated lab supplies to run it on before letting it go on mains.I thought they might do this given that it weighs 200kg.And I am pretty sure flow switches can get stuck open. If you don't have a second backup then stick to the flow paddle wheel or whatever, but keep in mind if it leaks you still get flow (particularly if you test it from a faucet or plant water), hence the desire for thermostats. If its only going to run with a closed loop tank then I guess it will drain fast enough before it explodes unless you decide to use some giant drum. Unless you put the flow sensor on the outlet then it will not be stopped by leaks. But the thermostat is going to break a circuit and not rely on some kind of interrogator that can malfunction. And of course you might hook it up backwards which will fool a single flow sensor in a leak condition if it does not have some kind of direction control. I also don't know what steam can do to a mechanical flow sensor. I suspect it would be likely to but not necessarily give a nonsense reading that would flag a fault.I suspect it would be advantageous to the room to hook up multi kW waste heat to a radiator situated outside of the building and not include a pump inside of the unit. It would be unpleasant to work around the pump noise and the heat. Even an exhaust fan into a duct would be kinda loud and irritating, otherwise your air conditioner and AC bill would suffer. And no filters to clean for a interior vent with high air flow, no annoying duct work to run, no clogs, you just need to hose down some shit outside once in a while and give it a scrub with a brush.A very interesting IH design I saw on youtube is that the guy totally sealed his chassis (to eliminate shop dust) and put a heat exchanger inside of the unit with a recirculating fan to maintain low chassis temperatures and cleanliness.

author: coppercone2
author_rank: Super Contributor
timestamp: January 15, 2019, 09:16:37 pm
content: I am kinda thinking about some kind of high capacity outdoors located pump/tank/radiator combo that would do some kW of cooling at a decent flow to cool various power electronics things for a power electronics lab (i.e. heat loads). It would have 2x flow meter and 2x pressure meter and differential temperature in addition to a pump controller and a tank. I wonder if you can mount it to a overhang or something. Something that does not look like a industrial site. Maybe built to look like a wall extension.If you had that capability you can add cooling jackets to vacuum pumps and other things too. And put little heat exchangers behind equipment that generates lots of heat to keep the room cool. Like a rack of commercial equipment with a suction fan. Then you can run stock linear high power supplies.It would beat the shit out of a extraction vent (omfg don't you hate that corrugated tubing shit?) if you need to move something around, especially if you put a grid of metal plates on the ceiling to which you can attach magnetic hooks if necessary to suspend heat extraction piping.I guess you can cool it to with a comperssor if you want.

author: coppercone2
author_rank: Super Contributor
timestamp: January 19, 2019, 01:50:49 am
content: identified a use for large scale home iron casting (other then precision machine shop work and making your own fence):capability to quickly make a custom weight for making grilled cheese sandwiches. Low tolerances and delicious result. Imagine stackable cast iron weights of various sizes that mate with each other so you can find the right amount to put the correct amount of pressure on bread.You no longer have to be at the mercy of either bread selection, using your hands to press odd sandwiches etc (imagine like a hoggie or large cuban on weird shaped bread).Should do nicely with my planned experiments in bread manufacture.

author: drogus
author_rank: Contributor
timestamp: February 03, 2019, 10:28:19 am
content: Thanks for the discussion here, it's a lot of interesting info.That said, in the topic I was asking for thorough theory sources, ideally books, because I think that reading a good book on the topic is usually really valuable for me. I've been digging more in the topic and I found something that looks very promising: "Elements of Induction Heating. Design, Control and Application". It's on google books and first 37 pages are available to read and it looks very promising:https://books.google.com/books?id=zXmTLYwO3McC&pg=PA97&hl=en&source=gbs_selected_pages&redir_esc=y#v=onepage&q&f=falseThe book doesn't seem to cover a lot of the electronics involved, ie. it rather covers high level design considerations (parameters tuning, coil design etc), but it still seems like a very good knowledge source.This also seems like an interesting book:History of Induction Heating and Melting. Again, it doesn't really go into electronics theory too much, but it seems that there's a bit of info about design.Next book that I found isHandbook of Inducion Heating. Most of the book is about induction heating application, but there's also a chapter on power supplies for induction heating, which among the other things describes different topologies of various inverters.A book that seems to go a bit more into electronics side of things isResonant power converters. It has a chapter on inverters. It's very theoretical, though, I don't think that there is a lot of info about practical considerations (most of the book is not available for free reading, though, so it's hard to say).All that said, I figured out that I need to get a thorough understanding of the basics because while I have some understanding of analog electronics, it definitely has holes and I'll start by going through "The Art of Electronics" (and maybe some other books about fundamentals) to have a really good understanding of all of the components involved. And then I'll try one of the books outlined above and then I'll get back to tutorials and articles online.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 03, 2019, 01:03:48 pm
content: Check out P. G. Simpson's "Induction Heating Coil and System Design", which is out of copyright and freely available on archive.orghttps://archive.org/details/in.ernet.dli.2015.147941. Compared to "Elements of Induction Heating" and "Handbook of Induction Heating", it covers the fundamentals in much more detail. I haven't come across a copy of "History of Induction Heating and Melting" yet, but it looks like an excellent resource as well.

author: Nominal Animal
author_rank: Super Contributor
timestamp: February 03, 2019, 08:06:24 pm
content: Quote from: coppercone2 on January 19, 2019, 01:50:49 amBread-facing one with an inverted U -shaped handle, with additional weights having a slot in the middle? Oh yes; you could share the extra weights among a number of bread-facing weights. A damn good idea, IMO.Quote from: coppercone2 on January 19, 2019, 01:50:49 amCiabatta-type sourdoughs with a pinch of rye (or barley) to taste may just blow your mind like it did for me. Playing with the starter and its exact composition in itself is really fun for an engineering-oriented mind.

author: coppercone2
author_rank: Super Contributor
timestamp: February 04, 2019, 02:46:36 am
content: yea something like that. I always wonder about a fusion of copper and cast iron some how (blast welding?) to make 'cooking press stones' with high thermal mass.