Electronics > Beginners

Induction heaters - the theory

<< < (3/5) > >>

T3sl4co1l:

--- Quote from: coppercone2 on January 14, 2019, 11:30:14 pm ---for a high power system say 5kW what I was looking at for a long time is

1) IGBT phase control bridge, use phase the adjust power from a H-bridge to a coupling transformer to the tank circuit
2) water cooling (a bitch with these power levels and voltages)

--- End quote ---

Not 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.


--- Quote ---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 etc
4) analog circuit that handles the PWM frequency and phase shift
5) 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 shit
--- End quote ---

3. 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 L somewhere...

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).



--- Quote ---What 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.
--- End quote ---

Yes, 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. :D



--- Quote ---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,
--- End quote ---

In 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...


--- Quote ---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.
--- End quote ---

You 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.


--- Quote ---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.
--- End quote ---

I 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 ---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 :(

--- End 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

drogus:
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 pm ---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.

--- End quote ---

Wise 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 pm ---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 languages prohibit such 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.

--- End quote ---

Again, 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.

coppercone2:
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 way



manufacturers tutorial
youtube.com/watch?v=0GowaLg_yBM

I 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=HJT2QOpnYb8

WHAT  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.

coppercone2:

--- Quote from: T3sl4co1l on January 15, 2019, 04:11:54 am ---
--- Quote from: coppercone2 on January 14, 2019, 11:30:14 pm ---for a high power system say 5kW what I was looking at for a long time is

1) IGBT phase control bridge, use phase the adjust power from a H-bridge to a coupling transformer to the tank circuit
2) water cooling (a bitch with these power levels and voltages)

--- End quote ---

Not a bitch, just a formality. Helps greatly with power density.

You need pump control, flow measurement (or at least indication) and resistance measurement to make sure its not getting soiled

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.

You need thermostats on the heat exchangers in the very least


--- Quote ---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 etc
4) analog circuit that handles the PWM frequency and phase shift
5) 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 shit
--- End quote ---

3. DC-DC modules are easier, but line frequency transformers with low capacitance (typically split bobbin type) work, too.

Won't a SMPSU conduct more HF noise then a transformer given the types of magnetics that it uses? My plan was a regular transformer between a Rcore and a torroid and maybe some filtering after.

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?).
I thought to use a optical sensor in a feedback loop rather then calculating power and to use some kind of PID control with a low maximum ramp


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.
good point I am not sure what would happen
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 L somewhere...

Anyway, pipes close together, or better yet laminated bus bar, is the way to go.  This never got hot on the front panel (aluminum),
I thought to use bus bar with a bore hole in it for water cooling


and I ran it at 5kW (Q ~ 15).

Tim

--- End quote ---

i need to look at my notes and some resources about the feedbacks still to discuss further

I 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?)

coppercone2:
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?

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod