Induction furnace - Page 2


Page 2 of 16 FirstFirst 1234512 ... LastLast
Results 21 to 40 of 303

Thread: Induction furnace

  1. #21
    Registered
    Join Date
    Mar 2005
    Location
    Toronto, Canada
    Posts
    1136
    Downloads
    0
    Uploads
    0

    Default

    this subject is interesting - from googling, here's a description of power supply and frequency requirements - what's the frequency of the welder?

    http://www.inductionatmospheres.com/powersupplies.html

    would the basis of the circuit be a timer switching on and off some big power transistors? kind of like a pwm but more heavy duty?



  2. #22
    Banned
    Join Date
    Sep 2005
    Location
    Sweden
    Posts
    229
    Downloads
    0
    Uploads
    0

    Default

    You would want much lower frequencies for meliting metal 50 Hz to 20 kHz or so! Higher is good for hardening and heat treatment as i understand it! I don't know what switching frequency a welder use.. But the principle is:

    AC > Rectifier > H-Bridge(PWM) > Transformer > Work coil

    If you have 10A DC supply... 50% Dutycycle... 10:1 Output transformer and a 15 turn work coil you should get ~700A, including losses, running trough the workpiece



  3. #23
    Banned
    Join Date
    Sep 2005
    Location
    Sweden
    Posts
    229
    Downloads
    0
    Uploads
    0

    Default

    Built a H-Bridge for my induction heater project today It's built for 350VDC and up to 30A (To bad i only got 16A). The heatsink i will mount it to is 42cm long (16,5")...

    Have begun designing the driver board with a microcontroller, with lcd and rotary encoder for settings! Cant wait to try it out

    Attached Thumbnails Attached Thumbnails Induction furnace-h1-jpg   Induction furnace-h2-jpg   Induction furnace-h3-jpg  


  4. #24
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    There are two quite different mechanisms at work in induction heating. The first is hysteresis heating of any magnetic material. The higher the operating frequency, the greater the heat generated. But this only works for ferrous metals below the curie point. For non magnetic metals (or above the curie temperature for magnetic metals), the heating effect is created entirely through circulating eddy currents.

    In effect, the metal being heated becomes a single shorted turn of a high frequency transformer. The difficult part is coupling enough energy into the work to be useful. In order for this to work reasonably well, the induction coil absolutely must be made part of a resonant tuned circuit.

    The idea is that a resonant build up of energy vastly increases the circulating current in the work coil, way beyond what the power source would be able to create all by itself. Many hundreds or even thousands of amps are required to pass through the induction work coil in order to couple enough energy into the work.

    So there is a lot more to this than just coupling up a few turns of copper to a welder and hoping for the best.

    The first requirement is to build a tuned circuit that can carry all this power, possibly ten to twenty times the basic input power of the induction heater. Several hundred thousand VAs (reactive watts) of circulating energy may be involved, and a very special and expensive water cooled tuning capacitor will be required that can carry several hundreds of continuous amps without burning up.

    So the starting point of your home made induction heater will be getting your hands on one of these special induction heater tuning capacitors to tune your work coil to the desired operating frequency.

    The next problem is driving this tuned circuit in such a way that the driver will not self destruct if either the driver or tuned circuit get out of step with each other. This is a lot more difficult to do than many people realise.

    An internet search will turn up many people that have tried to do this and failed. It just ain't that easy. Heating a few pennies to red heat, or the end of your pliers with a couple of hundred watts is dead easy. Feeding several (or many) kilowatts of power reliably into several pounds of metal is going to be a far greater challenge.

    My interest in induction heating has just now been aroused, and I am prepared to have a go at this myself. It will take me a while to track down and obtain some of the rather difficult to find (and expensive) special parts that will be required to try out some ideas of my own.



  5. #25
    Banned
    Join Date
    Sep 2005
    Location
    Sweden
    Posts
    229
    Downloads
    0
    Uploads
    0

    Default

    You make it sound like you could get over 100% efficiency by using a tank circuit, all know thats impossible! What efficiency do you expect (not including losses from heat radiation of the workpiece)?



  6. #26
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    Ah yes indeed, over 100% efficiency is obviously not possible, very true, but perhaps I did not explain this very well.

    Think about how a pendulum or a flywheel stores energy. Keep giving it a little kick at the right spot, and the internal stored energy can rapidly build up to fearsome levels. Resonance is a very powerful means of storing or building up stored energy, but obviously you can never draw more time averaged energy out of it than you put into it.

    The problem with induction heating is to get the electrical current to flow in the work. It relies on a magnetic field around the work coil, and magnetic fields do not travel very well through air.

    So what happens is a certain energy is fed into the tuned circuit, and that energy quickly builds up, and keeps building up, until the power draw from the circuit exactly equals the input power.

    So you might have (say) a thousand watts going in, fifty thousand circulating watts, and a thousand watts going into the work. This means that the coupling into the work from the coil is not very strong, only 2% of the circulating energy actually makes it from the coil into the work.

    Without a resonant tuned tank circuit, your thousand original potential watts could only transfer maybe twenty watts into the work. Your thousand watt induction heater would remain essentially unloaded, because of the very poor output energy transfer.

    But by tuning the circuit to resonance, a massive potential energy reservoir becomes available, and that stored energy becomes sufficiently high to be able to leak away into the work at a sufficiently high rate to be useful.

    Last edited by Warpspeed; 03-21-2007 at 05:58 AM.


  7. #27
    Registered sailandoar's Avatar
    Join Date
    Sep 2004
    Location
    USA
    Posts
    35
    Downloads
    0
    Uploads
    0

    Default Go Warpspeed!

    Seems that you just took this thread to the next level with your analysis and explanation.

    Thanks.....



  8. #28
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    In order to build a workable induction heating tank circuit, you will first need to get your hands on a suitably rated tank tuning capacitor.

    I have been investigating the tank capacitor procurement problem for quite a while, and none of the big capacitor manufacturers would even bother to reply to my e-mails. But finally *bingo* I hit the jackpot. I discovered a small company in Israel called Celem that makes nothing else but induction heating tank tuning capacitors. Because it is their only product, they will never tell a small customer to just piss off, like the sales reps of the big companies do.

    http://www.celem.com/

    Take a look at the ratings of these things, they are truly amazing. An example might be something like 400 volts rms, 900 amps rms, and up to 360,000 maximum circulating watts (360 Kvar) if pushed to the ratings limit. They are designed to operate most efficiently around specific frequencies, so just choose your capacitor, and wind your tank coil to tune it to the desired operating frequency, and that solves the tuned tank problem rather neatly.

    There is far more to building a working induction heater, but it does overcome one very huge problem, and it will get you started.



  9. #29
    Banned
    Join Date
    Sep 2005
    Location
    Sweden
    Posts
    229
    Downloads
    0
    Uploads
    0

    Default

    Nice... Did you get any prices?



  10. #30
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    Yes, I have just now ordered and paid for one of these capacitors, and it should hopefully arrive some time next week. What I ordered was:

    C200T 21uF, 400v rms, 900 amps rms, 250 Kvar, best operating range 12-25 Khz

    Capacitor $121.50 (US dollars)
    Insurence fifty cents
    Packing box $2.00
    Freight UPS, Israel to Australia $44.00

    Total $168.00 US dollars.

    I hope the moderators here don't mind this, I am not touting for this company, I have no connection with them apart from being a one time only customer.

    Just trying to help out some fellow CNC zone induction heating freaks.



  11. #31
    Banned
    Join Date
    Sep 2005
    Location
    Sweden
    Posts
    229
    Downloads
    0
    Uploads
    0

    Default

    Thats not bad! If i would go for a transformer instead it would cost at least that! And get perhaps at best get 60% efficiency (i have seen indications of that number). How big heater will you build?



  12. #32
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    That is a decision I have yet to reach.

    It will depend on what sort of power transformer I can get. Once I have a suitable transformer, whatever voltage and current it is capable of will pretty well fix my final power level. But at least 5Kw to begin with. My three phase power here is limited to 25 Kw, so it can be no higher than that.

    At this stage I really have nothing, finding that tank capacitor is the big breakthrough I was hoping for. Without that capacitor this project would not even have begun. The next major obstacle is sourcing a suitable power transformer.

    When I know how many rectified dc volts and amps will be available, then I can start thinking about the rest of the electronics.



  13. #33
    Banned
    Join Date
    Sep 2005
    Location
    Sweden
    Posts
    229
    Downloads
    0
    Uploads
    0

    Default

    Okay, a bit better than here! We only have 16A on the 3-phase, so max is about 10 kW... 5 kW seems like a resonable start, then you should be able to melt about 5 kg of steel in one hour or so.

    I found these numbers on a forum for a induction heating company:
    Steel: 3 lbs per kWh for melts over 100 lbs.
    2 lbs per kWh for melts under 100 lbs.
    Brass: 7 to 10 lbs per kWh
    Aluminium: 4 to 5 lbs per kWh

    If i get everything working good i might get some BIG 1200V 600A IGBTs and a old 200 kVA military generator



  14. #34
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    Yes, I agree, anything less than 5KW is really not a practical usable induction furnace, and 10Kw would be far better. Very many people have reached the several hundreds of watts level, and then given up because of repeated problems with transistors blowing up. If I can get 5KW working reliably as a very first attempt, I will be more than happy. Nobody else that I can find on the internet seems to have yet achieved anything like this in a completely from scratch home built project. But I am game to try.

    I am convinced that using a well thought out and methodical design approach, and correctly specified components, it should be possible.

    The way I see all this, the first design problem to overcome is deciding on the operating frequency and building a suitably rated water cooled tuned tank circuit. I have finally decided on 20 Khz, but that will vary up and down somewhat in operation, depending on the metal load and type of metal (ferrous or non ferrous).

    The second design problem to overcome is getting suitable dc power at a convenient operating voltage to run some IGBTs. I have decided to use 600 volt rated devices, simply because there is a much wider choice of high performance IGBTs and diodes available at a reasonable cost, at that rated voltage. Later I may go up to a higher operating voltage to increase power, but to begin with for this initial prototype, I plan to start off with two dc supply rails of +220v and -220v with respect to mains neutral.

    I was outbid yesterday on e-bay for an ideal three phase transformer, so will now go back to using something less wonderful that I already have here. This has three 240v primaries connected in star, and nine individual isolated 28 volt secondaries, each rated at 15 amps rms. The transformer rating is about 3.8 kva, and I believe it was originally salvaged out of some sort of battery charger.

    What I plan to do is place the three secondaries on each transformer leg in series giving 28 x 3 = 84 volts and use this as a three phase bucking transformer. I have a 240v phase to neutral incoming supply, subtracting 84 volts from that, will give me 156 volts phase to neutral, at 15 amps per phase.

    A six diode full wave bridge will then produce 156 x 1.414 = 220v peak output voltage with a very low 300Hz ripple. Only a comparatively small filter capacitance is required. That should reduce any inrush problems and give me a nice solid low stress reasonably well regulated dc power supply, 440v total at 20 amps or around 8.8kw continuously rated. Those values of voltage and current should be well within the capability of some fairly low cost high performance individual IGBTs and fast diodes.

    Strictly speaking I do not really need any filter capacitance at all on the output of my three phase rectifier bridge. The ripple voltage is quite low, but that would leave my IGBTs susceptible to mains voltage spikes. Some bulk filter capacitance, and some transient supression across the transformer windings should help eliminate any really destructive voltage spikes or surges originating from the mains supply.

    I can get much more dc output power than rated transformer va's because a bucking transformer does not have to supply the total power, only the proportion of the output power required to reduce the voltage.

    This has yet to be assembled and tested, that is the next step.


    .

    Last edited by Warpspeed; 03-22-2007 at 10:29 PM. Reason: orrible spellin bloopa


  15. #35
    Registered DukerX's Avatar
    Join Date
    Jun 2005
    Location
    Norway
    Posts
    24
    Downloads
    0
    Uploads
    0

    Default

    There are two things that immediately strikes me:
    1: The tank cap might be a bit overkill. Remember the current flowing through the workpiece is the current flowing through the induction coil multiplied by coil windings so a 10 turn coil having 20 amps going through it would induce 200 amps into the workpiece.
    2: Why not go straight for the mains supply, bypassing the transformer entirely? Sure, there's a major safety issue here, but as long as you a) know about it and b) got some proper fuses in place, it would save you the transformer entirely as long as you're looking for a "proof of consept" design and not a "production" unit.



  16. #36
    Gold Member
    Join Date
    Dec 2004
    Location
    Newtown, CT, USA
    Posts
    524
    Downloads
    0
    Uploads
    0

    Default

    Isn't the bucking configuration already hot with respect to ground? If so, there is little point in using the transformer.

    Ken

    Kenneth Lerman
    55 Main Street
    Newtown, CT 06470


  17. #37
    Registered
    Join Date
    Dec 2005
    Location
    USA
    Posts
    117
    Downloads
    0
    Uploads
    0

    Default

    Here is a link with some good info on induction furnaces

    http://www.dansworkshop.com/Induction%20Heating.shtml



  18. #38
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    DukerX,

    What you say would be very true if the work coil and the job were both tightly surrounded within an efficient magnetic core structure. In other words they were were effectively both windings that were part of a properly constructed and efficient high frequency transformer.

    But with no really efficient magnetic coupling, and a very open magnetic air path, only a very small portion of the magnetic flux passes through the work. In fact, it is even worse than you may imagine ! The large eddy currents flowing through the work actually sets up it's own counter magnetic field that opposes and repels the field from the tank coil.

    Technically this is called leakage inductance. Any magnetic lines generated by the primary (tank) that do not pass through the secondary (work) therefore cannot create any useful heating output. It is this weak and inefficient coupling of energy into the work that requires such an enormous energy buildup in the tank to create a useful energy flow into the work.

    Lerman, you are quite right. Here in Australia the neutral wire is at ground potential (almost), and there are three 120 degree phases, each at 240 volts with respect to neutral. Without this bucking transformer to reduce the voltage, I would have ended up with +340 and -340 volts dc. That is 680 volts total, and far higher than I feel comfortable with, at least initially to begin with in this prototype.

    The alternative would have been a bridge rectifier running between one phase and neutral. That has many disadvantages, including having a large ac component with respect to ground on both sides of the rectified dc. That makes looking at waveforms with an oscilloscope rather difficult, and it is somewhat more dangerous. Another limitation is only being able to get 340 volts dc that way, which is rather lower than ideal. And lastly, it would mean pulling all the power from only one phase with a very high ripple current requiring an enormous and potentially dangerous electrolytic capacitor bank.

    I really wish to use all three phases to spread the load, and have a low output ripple requiring minimal filter capacitance. My buck transformer also allows me to adjust the dc output voltage upwards in the future just by shifting some transformer tappings around.

    A bucking or autotransformer is also going to be a lot smaller than a true full isolation transformer. A 10 kw full isolation transformer will weigh about 200 to 250 Lbs and would not be exactly cheap. As neutral here is grounded, not having full mains isolation is not really a great disadvantage.

    Getting reliable reasonably well regulated dc power at a suitable voltage for an induction heater project is not as simple as it may first appear. It requires a bit of thought and ingenuity no matter how you finally go about it. It is just one of several rather interesting problems to overcome in this type of project.

    Photographs and schematic circuits will come later, once I have various sections built and working. At this stage I have plenty of ideas, and a plan of action mapped out, but no actual hardware assembled yet to show.



  19. #39
    Registered
    Join Date
    Mar 2007
    Location
    Australia
    Posts
    85
    Downloads
    0
    Uploads
    0

    Default

    Quote Originally Posted by AustinT View Post
    Here is a link with some good info on induction furnaces
    Dan, here are a couple more that may be of interest:

    http://www.richieburnett.co.uk/indheat.html

    http://webpages.charter.net/dawill/t..._IndHeat1.html



  20. #40
    Registered
    Join Date
    Oct 2006
    Location
    Sweden
    Posts
    13
    Downloads
    0
    Uploads
    0

    Default

    I´m currently working on an IH that actually does about 1kW at full power (what the transformers i use are capable of)

    My biggest problem is the imp matching trafo heating up and the work coil getting hot so water cooling of atleast the work coil is a must, esp is heating stuff red hot.



Page 2 of 16 FirstFirst 1234512 ... LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Induction furnace

Induction furnace