Induction heaters are used to heat conductive materials in a non-contact process. Commercially, they are used for heat treating, brazing, soldering, etc., as well as to melt and forge iron, steel, and aluminum.
This Instructable will walk you through the construction of a high-power (30kVA) heater, suitable for melting aluminum and steel. Note that to take full advantage of this design, you will need a 220V outlet, at least a 50A single-phase one and preferably a 50A or 60A 3-phase outlet.

About the author:

Bayley Wang (me) is a EE student at MIT. I'm responsible for a variety of nefarious power electronics projects which you can find on my blog; perhaps most interestingly is oneTesla, which has since gained a life of its own as a startup creating DRSSTC kits.


  • This project uses mains voltage. While well-behaved, 110/220 mains can seriously injure, maim, and/or kill you if used improperly.
  • The voltage across the tank capacitor can potentially ring up to hundreds of volts. Don't let the 20:1 step-down ratio fool you!
  • When scoping the circuit, beware of ground loops.
  • The work piece, naturally, can get very hot. DO NOT TOUCH! Less obviously, do not rapidly quench the work piece with water, as this can lead to dangerous sputtering.
  • This project uses power electronics. Under fault conditions, semiconductor devices used in this project may rapidly heat, vent, and/or release rapidly moving shrapnel. Shield appropriately.

WIth that said and done, let us move on.

Step 1: Bill of Materials

For this build, you will need:
  • 2 IGBT half-bridge modules. I used Powerex CM400DU-12F 400A 600V Dual IGBTs; anything of similar power handling and switching speed should work. These can be purchased as cheap surplus from Ebay.
  • 4 MOSFETs or IGBTs for the gate drive. I used HGTG30N60B3D's, which are way overkill for the application. They need to be able to dissipate about 30W without burning up.
  • 2 gate drive IC's, of at least 9A peak current capability. I use the UCC37322 from TI.
  • 2 ferrite toroids. These are your gate drive transformers, and should be able to pass a reasonably clean square wave at 50 kHz. Magnetics, Inc. and TSC Ferrite International are good manufacturers, or you can salvage them from old CRTs or switching power supplies. The powered iron cores from ATX supplies rarely work.
  • Large ferrite toroids for the toroidial coupling transfromer.
  • 1 TL494 PWM IC.
  • 1 at least 20 uF, at least 20V film or ceramic capacitor.
  • Assorted resistors, capacitors, and potentiometers for the driver.
  • 10' of 1/4" soft copper refrigeration tubing.
  • A water block capable of accommodating the two IGBTs. A large heatsink may also work, but I haven't tried.
  • 2 aluminum or copper bars, ~3/4"x8"
  • 2 1/4" compression unions
  • A 4-position rotary contactor, good for several tens of amps.
  • A screw-terminal electrolytic capacitor of reasonable quality. I recommend at least a few hundred uF for 3-phase operation.
  • A high-quality, low inductance snubber capacitor for the bridge. Ebay has cute brick-mount 20 uF blocks for $5.
  • One or more high-quality polypropylene capacitors for the tank capacitor. More on this part later.
  • An analog current meter good for several tens of amps.
  • A 3-phase bridge rectifier (or single-phase if you are willing to settle for single-phase operation only).
  • A suitable project case and associated hardware (3-phase breaker, cord, plug, etc).
  • A water pump capable of a couple GPM
  • Tubing appropriate for hooking up the water-cooling.
  • A Variac for testing.

Step 2: Words of Wisdom

The IGBTs: or "bricks", as we like to call them. They should be good for 600V (not a concern, I've never seen a brick rated under that before), at least 200A (I use 400A modules to be, safe), and more importantly, need to be fast. This is where you need to check the datasheet - IGBTs have an inherently long turn-off delay. For 65 kHz operation, rise time + turn-on delay + turn-off delay + fall time should be under 2 uS.
Bricks come in several types: single-transistor, dual transistor, 6-pack, and some rarer types such as chopper modules. Single-transistor modules are prevalent for 1200V and larger IGBTs, and have the highest thermal ratings and are the most difficult to mount. Duals (half-bridge modules) are the much easier to mount and can dissipate less. They are most common for 600V modules. 6-packs are used for 3-phase inverters, require no external power connections, and have the lowest thermal ratings.
Use what you see fit; this tutorial uses half-bridge modules.

The tank capacitor: is very very important. It handles tremendous amounts of reactive power at very high frequencies. It is absolutely essential that this part be selected appropriately. It must be a high-quality polypropylene or mica capacitor. I use giant snubber capacitors made by Eurofarad; alternatively, a series/parallel array of smaller capacitors (such as the Tesla coiler's beloved CDE942 series) should work. The ultimate capacitor, of course, is a water or conduction-cooled unit made by Celem, but such caps will run you about $150 for a 2 uF unit. You want enough capacitance to resonate with your work coil at no more than 70 kHz.

Step 3: Principle of Operation

Induction heaters function by surrounding the work piece with a coil carrying a high-frequency (kHz to low MHz) alternating current. This induces eddy currents in the work piece, which acts as a shorted 1-turn transformer secondary. The currents can be tremendous, on the order of several thousands of amps. This causes high I^2R losses in the work piece, heating it.

Schematic Description
Ignore the transistor model numbers; I just used what Eagle had built in.

IC1 is a TL494 acting as an oscillator with adjustable dead time and frequency. The output is fed into the input of two UCC37322 9A gate drive ICs, which "beef up" the signal into something capable of driving high-capacitance transistor gates. The output signal is passed through C5 to insure only the AC component reaches GDT1, a gate drive transformer. This transformer provides the electrical isolation necessary to drive Q1 through Q4, which form a full-bridge. This intermediate bridge is necessary to provide the high average power necessary to drive Q5 through Q8, a full-bridge of large IGBT modules.
This bridge forms the main inverter. The output of this inverter is stepped down through a 20:1 torodial transformer TR_MATCH, which provides impedance matching as well as isolation for L_WORK, the work coil inductor. The capacitor C_TANK forms a resonant LC circuit with L_WORK; when driven at resonance, this circuit displays zero reactive impedance to the inverter, allowing for higher powers and minimizing switching losses in the inverter.

Step 4: Construction: Controller

Construct the logic circuit as you wish, either by using the attached images to make boards or using perfboard or a breadboard.
The gate drive transformers must be able to pass a high-quality square wave at your operating frequency. To check this, wind 10+10 turns on the toroid, connect one set of windings to a signal generator, and scope across the other. The output should look like a reasonable square wave.
The GDT should be wound with 5 twisted wires to minimize leakage inductance. Many people have had luck with using CAT5 cable, which comes pre-twisted.

Step 5: The Inverter

The inverter should be very well-cooled, either with a large heatsink or a waterblock. I used a waterblock for compactness and robustness; but a big (think 12"x12"x3" with several hundred CFM of forced-air cooling) should work too. The pump should be relatively large to handle the pressure drop through the work coil (mine was rated for 2GPM).
The main filtering capacitors should be placed close to the bridge itself, preferably bolted across the busbars. You should also use a snubber capacitor (the black box in the picture) placed directly across the transistors to reduce voltage spikes caused by excitation of the parasitic inductances in the inverter layout.
Using half-bridge or six-pack modules is the easiest way to buld the inverter; a bridge of single transistors will require access to a machine shop to do right.

Step 6: Work Coil/Tank Circuit

The coupling transformer should be toroidal. Wind ~20t around some large ferrite cores (I was using a stack of 4 ~4"x1" cores).
The tank capacitor willget warm. It should have significant terminal area to conduct both heat and thousands of amps. If you are using a MMC of small capacitors, solder them individually to large copper plates. If you are using a Celem or a giant snubber, bolt large copper plates to the terminals. Then in either case, solder the terminals to the copper tubing that forms the rest of the tank circuit.
Attach the work coil to the tank circuit using compression fittings; this allows you to change work coils to accommodate different loads.
Make the work coil out of at least 1/4" copper tubing. Thicker tubing is less lossy, but harder to handle; trade-off between the two as you see fit. When winding the work coil, it helps to fill it with sand to prevent the tubing from collapsing. As a rule of thumb, the resistance of 1' diameter copper tubing at 65 KHz is 0.8 mΩ/m; that is, to compute the resistance of your secondary, multiply 0.8 mΩ by its length and divide by its diameter in inches.

Step 7: Testing and Usage

Assemble everything according to the schematic. Use a current transformer on the primary side (100t burdened with a couple ohms around a ferrite toroid will do) to monitor the waveforms.
Using a current-limited bench supply (preferably 30V, 10A), slowly ramp up the voltage until enough current is drawn to give a clear reading on the 'scope. Adjust the frequency pot until the waveform is a clean sinewave, and current draw is maximized (you may have to search a little to avoid harmonics). If you don't have a scope, just tune until current is maximized (mine drew something like 40A at 200VDC on the bus, unloaded).
With ~30V on the bus, load the work coil with a bolt. At a few hundred watts in, it should get hot within a couple minutes. If it draws power, but the workpiece doesn't get hot, check the transistors for heating. If they get excessively hot, your bridge is shooting through.
If all is well at low powers, you are ready for a high-power test. Use your favorite DC source (single phase, three-phase, smoothed, unsmoothed, etc - it doesn't really matter) to power the bridge. Preferably, use a Variac, in case it draws too much current (you can predict current draw from the low-power tests by noting that the heater is a fairly linear load). Don't forget water cooling!
At a few kilowatts, without a crucible, you can melt aluminum and copper and make steel orange-hot. 10 KW+ (50A dryer/stove line or 3-phase) is necessary to melt steel in open air. A crucible helps a lot.
You can control power by very slightly detuning the inverter, or by changing the bus voltage, or by tapping the matching transformer. The latter is a recommended feature, and steel and copper have very different effective "resistances".
Good luck, and have fun!

Step 8: Sources for Components

By popular demand, I've added this page.

For the power components, one word - EBAY.
EBAY EBAY EBAY. There is no way this project could have been remotely affordable without it. For the IGBT's, the most reliable source is CTR Surplus, who goes by the usernames ctr_surplus, deals_ctrsurplus, and lisa_ctrsurplus. CTR Surplus also has a constant supply of large electrolytic capacitors, snubbers, and heatsinks used in this project.
The capacitors are also from CTR Surplus - a search for "Eurofarad" works wonders.
Copper tubing is best purchased from Home Depot (assuming you live in the US). They have prices that beat most Internet sources.
The toroids can be from Magnetics, Inc or TSC Ferrite International.
Miscellaneous small components can be purchased from Digi-key.
Arrow has very good prices on transistors, far lower than most other suppliers.

Submitted by MITERS for the Instructables Sponsorship Program

<p>Is the complete schematic available? Thanks.</p>
<p>kindly send all project details on alpesh.dobariya@gmail.com</p>
<p>Could you please email a clearer schematic to me? Thanks</p>
<p>I really would like to view a schematic. The version in this instructable is such low resolution that I cannot see values of the components. Could you either email one to me, or better yet improve the quality of the one in your instructable so that everyone could see it?</p>
<p>This looks like a great project! Could you please send the schematic? Thanks./ds</p>
<p>Hi,</p><p>Nice build!!! Would it be possible to obtain the schematic? Thanks.</p><p>dturnbull316@gmail.com</p>
<p>Hi thanks for the project, I just had a question on the gate drive stage 2, you added a second capacitor on the circuit as C2 but it doesn't have a value, and in the Eagle schematic it isn't even mentioned. So I was a bit confused, thank you!</p>
<p>Hi! I would like the schematics if possible!</p><p>carrillo.joseph.r@gmail.com</p>
Would like the schematics please. <br>jeffmiles1@yahoo.com
<p>Sorry for the stupid question, no reply necessary. Answer; self wound isolation transformer, 10+10. I should read the text more carefully and not just stare at schematics. Thanks</p>
<p>Hi, building a furnace from CM600NFH's, hope to debug the ATmega controller. Till then would like to run it off your design, but have one question; the blocks between the drive stages and power are 1:1:1 couplings? Don't want to assume. When I work out the controller I'll post it, unlike ReactorForge. What a DIY cop-out they are. Thanks</p>
<p>Hi,</p><p>I am interested in building the similar schematic. I am wondering if the schematic is similar with the AC motor controller?</p>
<p>Hi,</p><p>Interested in getting the schematics</p><p>yousif1966.2@gmail.com</p><p>Thank you</p>
<p>Hey there,</p><p>Interested in building this design.</p><p>Was wondering if i could get the schematic</p><p>bensonr08@hotmail.com</p><p>Thanks</p>
<p>Hi,</p><p>Interested in getting the schematics</p><p>andrewholden00@gmail.com</p><p>Thank you</p>
<p>HI,</p><p>Can i get schematic?</p><p>My email is <a href="mailto:daehyunele@gmail.com" rel="nofollow">daehyunele@gmail.com</a></p><p>Thank you..</p>
<p>my email mohammad.daryono@gmail.com</p>
<p>hii </p><p>Can i get all schematic,pliiis</p>
<p>Hello ...... my dear friends extreme need and schematic circut this Please if anyone's got the induction furnace Send me anymore&lt;enginervaziri@gmail.com&gt;</p>
<p>Do you have a ballpark estimate of how much the total bill of materials costs? I don't need an exact number.</p>
<p>I have approximately ten toshiba MG200q2ys40 half bridge igbt modules 1500 volt 200 amp for $75 a piece if anyones interested. email me Andrewboerder@gmail.com</p>
<p>All the schematics please! asraith@gmail.com </p>
Good day <br>I am interested with your estimated induction heater so I hope you send me the schematic circuit <br>Thanks a lot
<p>hi </p><p>can I get circuit schematic?</p><p> jformm3@gmail.com .<br>thank you</p>
<p>HI, can I get circuit schematic? My email is anpaguiar@gmail.com</p><p>thank you</p>
<p>HI, can I get circuit schematic? my email id is prashantgauravbaghel@gmail.com.<br>thank you </p>
Hi can I get the schematics my email is benngalaska@aol.com thanks!
Hi, I want to make this project. You can send me its schematic to this email : shivana.5511@yahoo.com<br>
<p>Hello,</p><p>I was wondering why the ferrite toroids for the coupling transformer are so large?! Wouldn't it be better to have toroids with a smaller diameter to reduce magnetic losses?!</p>
<p>Magnetic losses occur more with smaller toroids because magnetic saturation occurs faster with a smaller amount of iron</p>
Ok, it makes sense...thank you for your reply.
<p>By the way, I would have another question if you don't mind...maybe you have an idea.<br>I saw a comment from lavila540 on this page who ordered the following ferrite cores for his induction heater:</p><p><a href="http://www.mag-inc.com/company/news/new-4-inch--kool-mu-toroid" rel="nofollow">http://www.mag-inc.com/company/news/new-4-inch--ko...</a></p><p>But I would aim for those cores (with the P material):</p><p><a href="http://www.mag-inc.com/home/Advanced-Search-Results?pn=49725" rel="nofollow">http://www.mag-inc.com/home/Advanced-Search-Result...</a></p><p>where the inductance and permeability are much higher. But in the first case they call it the &quot;Kool Mu Permeability&quot;...so is there a difference with a &quot;normal permeability&quot;?! I am a bit confused...<br>What would be the most important characteristics to look at?<br><br>Thanks</p>
<p>do the ferrite cores have to be that large?, I am having difficulty finding any that big here in the UK</p>
<p>I bought mine last week from mag-inc. I`m in the US but, they might be able to ship. Seen some one e-bay too. </p><p>http://www.mag-inc.com/company/news/new-4-inch--kool-mu-toroid</p>
<p>Hello lavila540,<br>Have you finished your induction heater? Does it work well?<br>I was wondering why you chose these ferrite cores? Is it working well with these ones (no heating)? What is the most important characteristics to look at?<br>Do you think these ferrite cores might work as well:</p><p><a href="http://www.mag-inc.com/home/Advanced-Search-Results?pn=49725" rel="nofollow">http://www.mag-inc.com/home/Advanced-Search-Result...</a></p><p>I'm aiming for the P material. But comparing yours and these ones, the permeability and the inductance are much different...I am confused...<br></p>
<p>Greetings,</p><p>Is it possible to get the schematics emailed.</p><p>jeff@fd-intl.com</p><p>Thanks</p>
<p>If possible, I would like receive schematic diagramms by my e-mail.</p><p>FYR, e-mai address is <a href="mailto:franzhwang@hanmail.net" rel="nofollow">franzhwang@hanmail.net</a></p><p>Thanks</p>
<p>i want document. you can send me? please...</p><p>sarawut262535@gmail.com</p>
Where is your scmatic? <br>I want to build a very small one
<p>hello </p><p>need to circuit diagram to progect </p>30 kVA Induction Heater
<p>what is the current going into the tank? The capacitors I have found by Eurofarad have been rated around 150 amps max. After the current is stepped up by the coupling transformer won't the current exceed this?</p>
<p>Not sure if you know, but if not then your work has been stolen:<br><a href="http://ofslides.com/bwang-546040/presentation-1019981">Check here</a><br>Brooklytonia found the website and opened a thread about it in the forum as well.<br>However, if that is something you published there yourself forget about it ;)</p>
<p>I'm wrong-1,800,000 VAR=1,800KVAR.If I'm getting 400 amps to the work head than the tank circuit should have unreal current in it,I can't even calculate it!</p>
<p>My system will be operating at 30khz with 1.92 microhenry work coil with 1/2&quot; tubing I believe I will sweat 56% Silver solder onto .This F0 will not over drive the IGBT's and create great penetration.I Should have many thousands of amps in work coil with Q of 126 icrease in input tank current.</p>
<p>I'm building my system around 4 600 amp Powerex's and BG1A gate drivers.I have a 16 tap 800KVAR workhead.My main tank capacitor is 15uf at 6kv,300amp oil filled capacitor rated at 1,800,000KVAR,weighs about 30 pounds! I am driving the inverter with a 130 vdc at 400 amp military grage generator and a 60 hp Suzuki samari engine.This gives me This gives me 52,000 or 52 kiolwatts of power to work with.This should defiately smelt a little steel?</p>
<p>Okay, bare with me because electronics is far from my field of expertise, and I have a few questions I'm hoping someone can help answer. </p><p>First, why is it necessary to have a rectifier (its not really mentioned) if we are just going to reconvert the signal back to AC? I'm guessing that a DC source is needed for the input to the IC which is the oscillator which then drives the gate drives, the power stage and eventually the tank circuit. That being said it also appears that the rectifier is wired directly into the IGBTs, is it necessary to use DC to drive these? Am I on the right path with any of this? </p><p>Second, it's stated that the the frequency and dead time of the oscillator can be controlled, but how exactly? How is the frequency of the AC current in the tank circuit actually controlled? </p><p>Lastly, what is the function of the input capacitor and why is it necessary? There's no mention of it other than in the picture at the beginning. </p><p>If anyone could take the time to explain these things to me I would really appreciate it. </p>
<p>Going another direction, I want to anneal brass tubing. I'm *trying* to build something that will heat rifle cases to around 750*F. without over annealing them, and do it in a very short period of time so I can maintain production volume.</p><p>This takes a coil that is open enough for the brass to enter on a belt/conveyor and exit the same way, so the coil is longer, oval with the ends bent up to allow for brass to pass into the induction coil, and out of the induction coil as it proceeds down the production line.</p><p>I CAN regulate the belt speed through the coils... And I can get the coils very close to the brass, but what I'm having an issue with is the amount of brass moving on the line (Mass) and building an inductor with enough power to make the brass reach the designated 750*F.</p><p>With rifle brass, It's a pretty good conductor, so it doesn't heat as fast as steel, and I'm not trying to anneal the entire brass, just the top 3/4&quot; or so... But production still puts a lot of brass in the coils at one time, so the heating is VERY slow, which leads to thermal transfer down into the case body/bottom, which I DO NOT want. I would rather the sides/bottom stayed under 450*F.</p><p>I'm considering a water bath for the bottom of the cases, which is going to add more mass and slow the induction heating of the top of the brass...</p><p>Since this is for a high volume competition shooter (100K rounds/year), and not a business, I would be glad to pay a REASONABLE amount of money for someone that can help address these issues.</p><p>I've seen the high volume MANUFACTURERS have induction heaters that would knock out the entire process in just 3 or 4 seconds, I'm not looking for something that draws 440 volts AC and spins the electric meter off the side of the garage, but I would consider 220 VAC single phase, providing the unit works efficiently and doesn't cost a fortune to build or operate...</p><p>Anyone got any ideas how to build something that doesn't bust the bank?</p>
<p>I'm sorry to disappoint you a little bit, but there's one caveat to your water bath plant: all the copper alloys are not only good electrical conductors, but good heat conductors as well (there's a reason why all the high-quality heatsinks are made either from pure copper or aluminium with a copper core just above the CPU). Thus if you'd add water to any parts of the brass casing, you'd make it even harder for it to heat up to your desired temperature of 750 &deg;F, because water is a VERY good coolant (it has one of the highest heat capacities of all the fluids).</p><p>Since there seem to be no 3-phase electric outlets installed in US homes at all for some strange reason, I'd recommend you obtain some REALLY high amperage outlets for this project (a 100A one would get you reasonably close to the 30 kVA mentioned in the project). Moreover I'd also consider lowering the frequency a tad bit (no more than 10-20 kHz tops), get a bit smaller pipes, wind them in very close loops (while making sure that the loops are adequately insulated from each other e.g. by a lacquer) and wind multiple loops on top of each other (this increases inductance a LOT). All of these things (and probably quite a few others folks more knowledgeable about this topic than me perhaps know about) are just tweaks though: the main issue is that you MUST deliver more energy to your brass cases than necessary for heating them. Frankly if I were you I'd just use some heating elements instead.....</p>
<p>Im thinking if i skipped the rectifier in this, could i power the bridge directly with a 3 phase, 400V stick welder with some unknown DC output at 150A. ??</p>

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