How to Design an Induction Heater - With Calculations and WorkingFormulas

The article explains a step by step tutorial regarding designing your own homemade basic induction heater circuit, which can be also used as an induction cooktop.

Basic Induction Heater Concept

You might have come across many DIY induction heater circuits online but nobody seem to have addressed the crucial secret behind implementing a perfect and a successful induction heater design.Before knowing this secret it would be important to know the basic working concept of an induction heater.

An induction heater is actually an extremely "inefficient" form of electrical transformer, and this inefficiency becomes its main advantageous feature.

We know that in an electrical transformer the core needs to be compatible with the induced frequency, and when there's an incompatibility between frequency and the core material in a transformer, it results in the generation of heat.

Fundamentally an iron cored transformer will require a lower range of frequency around 50 to 100Hz, and as this frequency is increased the core may shown a tendency of getting hotter proportionately. That implies, if the frequency is increased to a much higher level may be over 100kHz would result in the generation of extreme heat within the core.

Yes, this is exactly what happens with an induction heater system where the cooktop acts like the core and therefore is made up of iron material. And the induction coil is subjected to a high frequency, together this results in the generation of a proportionately intense amount of heat on the vessel. Since the frequency is optimized at significantly high level ensures a maximum possible heat on the metal.

Now let's proceed and learn the important aspects that may be required for designing a successful and technically correct Induction heater circuit. The following details will explain this:

What you will Need


The two bare basic things required for building any induction cookware are:

1) A bifilar coil.

2) An adjustable frequency generator circuit

I have already discussed a few induction heater circuits in this website, you can read them below:

Solar Induction Heater Circuit

Induction Heater Circuit Using IGBT

Simple Induction Heater Circuit - Hot Plate Cooker Circuit

Small Induction Heater Circuit for School Project

All the above links have the above two things in common, that is they have a work coil and a driver oscillator stage.

Designing the Work Coil


For designing an induction cookware, the work coil is supposed to be flat in nature, therefore it must be bifilar type with its configuration, as shown below:




The bifilar coil type design shown above can be effectively implemented for making your homemade induction cookware.

For optimum response and low heat generation within the coil make sure the wire of the bifilar coil is made using many thin strands of copper instead of a single solid wire.

Thus, this becomes the work coil of the cookware, now the ends of this coil simply needs to be integrated with a matching capacitor and a compatible frequency driver network, as shown in the following figure:



Designing the H-Bridge Resonant Driver Circuit


So far the information should have enlightened you regarding how to configure a simple induction cookware or an induction cooktop design, however the most critical part of the design is how to resonate the coil capacitor network (the tank circuit) into the most optimal range so that the circuit works at the most efficient level.

Enabling the coil/capacitor tank circuit (LC circuit) to operate at their resonance level requires the inductance of the coil and the capacitance of the capacitor to be matched perfectly.

This can happen only when the reactance of both the counterparts are identical, that is the reactance of the coil (inductor) as well as the capacitor are approximately the same.

Once this is fixed you can expect the tank circuit to operate at its natural frequency and the LC network reaching the resonance point. This is called a perfectly tuned LC circuit.

This concludes the basic induction heater circuit designing procedures

You may be wondering regarding what is resonance of an LC circuit.?? And how this may be calculated quickly for completing a specific induction heater design? We will comprehensively discuss this in the following sections.

The above paragraphs explained the fundamental secrets behind developing a low cost yet effective induction cooktop at home, in the following descriptions we will see how this can be implemented by specifically calculating its crucial parameters such the resonance of its tuned LC circuit and the correct dimension of the coil wire for ensuring an optimal current handling capacity.

What is Resonance in Induction Heater LC Circuit


When the capacitor within a tuned LC circuit is momentarily charged, the capacitor tries to discharge and dump the accumulated charge over the coil, the coil accepts the charge and stores the charge in the form of magnetic field. But as soon as the capacitor has discharged in the process, the coil develops an almost equivalent amount of charge in the form of magnetic field and it now tries to force this back inside the capacitor, although with an opposite polarity.




Image courtesy: 

Wikipedia

The capacitor is again forced to charge but this time in the opposite direction, and as soon as it's fully charged, it yet again tries to empty itself across the coil, and this results in a back and forth sharing of charge in the form of an oscillating current across the LC network.

The frequency of this oscillating current becomes the resonance frequency of the tuned LC circuit.

However due to inherent losses the above oscillations eventually die out in the course of time, and the frequency, the charge all come to an end after sometime.

But if the frequency is allowed to sustain through an external frequency input, tuned at the same resonance level, then that could ensure a permanent resonance effect being induced across the LC circuit.

At resonance frequency we can expect the amplitude of the voltage oscillating across the LC circuit to be at the maximum level, resulting in the most efficient induction.

Therefore we can imply that, to implement a perfect resonance within an LC network for an induction heater design we need to ensure the following crucial parameters:

1) A tuned LC circuit

2) And a matching frequency to sustain the LC circuit resonance.

This can be calculated using the following simple formula:

F = 1 ÷ x √LC 

where L is in Henry and C is in Farad

If you don't want to go through the hassles of calculating the resonance of the coil LC tank through formula, a much simpler option could be to use the following software:

LC Resonant Frequency Calculator

Once the resonance frequency is identified, it's time to set the full-bridge IC with this resonance frequency by suitably selecting the Rt, and Ct timing components. This may be done by some trial and error through practical measurements, or through the following formula:

The following formula can be used for calculating the values of Rt/Ct:

f = 1/1.453 x Rt x Ct where Rt is in Ohms and Ct in Farads.

Calculating Wire Size for Induction Heater Work Coil


Once you have calculated the optimized values of L and C for the tank circuit of the induction heater and evaluated the exact compatible frequency for the driver circuit, it's time to calculate and fix the current handling capacity of the work coil and the capacitor.

Since the current involved within an induction heater design could be substantially large, this parameter cannot be ignored and must be correctly assigned to the LC circuit.

Using formulas for calculating wire sizes for an Induction wire size may be a little difficult especially for the newcomers, and that's exactly why a special software for the same has been enabled in this site, which any interested hobbyist can use to dimension the right size wire for your induction cooktop circuit.

Need Help? Please send your queries through Comments for quick replies! And please Bookmark my site :)




Comments

Paul Cárdenas said…
Hi Iam from Ecuador my name is Paul Cardenas Gordon.
I came to your website because I am looking for a magnetic induction circuit. I must thank you for having found several options. You can tell me which design I can adapt to get a power of 1kw maximum output.
And how could I do to vary the output power.
I do not study electronics, I study mechanics but I am struggling to understand the circuit.
Swagatam said…
Thank you for visiting my site Paul!

You can try the following design

https://homemade-circuits.com/2017/01/induction-heater-circuit-for-labs-and.html

in this circuit you can limit or change the max power output by appropriately calculating Rx value.

But please remember that this concept is suitable only for experts in the field of electronics, and is not recommended for any newcomer, therefore make sure that you have an expert electronic engineer with you while constructing and testing this circuit.
rasika said…
how to get value of Rt and Ct thank you sir !!!
Swagatam said…
It is explained in this post

https://homemade-circuits.com/2014/01/simplest-full-bridge-inverter-circuit.html
Saeed Mahdavi Asl said…
Dear Mr. Swagatam:
I am not a new comer , as i have been working on electronics over 30 years but mostly practical and rarely theorycal so I need your help with making this circuit. as a matter of fact i have made it and works, however with a 300 watt lamp in series and the lamp illuminates so brightly and I have just little heat in the metal inside the coil. as i have burnt out a few expensive IGBTs , I fear to use more powerful lamp. the 4.7 volume has been regulated so precisely. however, the coil i have used is 9 centimeters diameter(for i need this size of the coil) and 4 turns with 4.5 millimeters thickness of the wire. the caps are almost 2.3 uf. Do you think i have to make some changes in the coil or it could be OK?
meanwhile, as the wider a coil , the more useful the circuit could be, i want persistently to use it. please tell me:
1- is the heat OK with the 300 watt lamp?
2_ by what i mentioned can I use more powerful lamps? or i have make changes in the coil?
thank a lot \

Saeed Mahdavi Asl
Swag said…
Dear Saeed, If your 300 watt bulb is illuminating brightly, then it is not good. Your circuit might not be working correctly

At resonance your lamp must have minimum illumination and the load must have maximum heat.

Were your able to achieve the correct resonance for the design??

It has to be done by attaching a magnetic plate with the coil, and then tweaking the frequency until the heat maximizes and the lamp illumination minimizes.

By the way did you connect the SD pin of the IC to ground? Otherwise the system will not initiate.
Saeed Mahdavi Asl said…
Dear Swagatam:
I have just LITTLE heat although the 4.7 k volume has been adjusted as precisely as possible but still the 300 w lamp is illuminating so brightly . on the other hand ,
i cant find the Sd pin of the IC although checked with the datasheet.
the circuits has been built exactly in conformity with the plan introduced by you
please tell me:
1- what pin you mean by SD?
2- if the coil is OK. it is 4 turns of 4.5 mm wire with 9 mm diameter.

by the way the only thing to adjust is the volume, so i dont have any more idea of how to regulate the circuit. i have used the shortest wires and four ferittes at the tip of the wires connected to the bases of IGBts.
the last connection points of the caps have been connected to the coil and with thick wires.to me everything is ok but unfortunately the IGBTs will blow out if i use more wattage lamp. for example i lost three of them and one IC today when i used a 800 watt lamp.
please guide me by telling
if the coil is OK and what pin you meant by SD.
your Sincerely Saeed
Saeed Mahdavi Asl said…
Dear swagtam:
here i should add that my problem is concerning the induction heater with 4 IGBTs and the coils with air core.
thanks a lot
Swag said…
Saeed, The SD pin is pin#5 of the IC, just under the Rt pin.... it is clearly visible in the diagram, I am wondering how you could not find it?

By the way where is 4.7K, are you referring to the Rt resistor selected by you as 4.7k??

Please try using the following formula for selecting the coil, capacitor and frequency parameters

F = 1 ÷ 2π x √LC

F can be determined by adjusting Rt/Ct, C can be randomly selected, once F and C are determined L can be calculated by solving the above formula.
Frequency will need to be confirmed using a frequency METER across the output pins of the IC.

As long as resonance is not achieved the circuit will not work optimally, and you will keep losing mosfets and IGBTs

Alternatively you can go with the existing set up, connect an arbitrarily selected Capacitor for "C" and Ct, and then tweak Rt until you find the heat to be maximum and illumination on the 300 watt bulb minimum.

Remember you must do the above by attaching an iron plate with the coil, if you keep the coil air-cored then the circuit will never work, I have already cautioned about this in my previous comment.

try a 7 turns coil, this will allow a lower frequency and stable operations
Swag said…
Dear Saeed, in that case you should have posted the comment under that particular article, for so long I thought you are referring to the above design
Swag said…
here's the link of the original design

danyk.cz/induk3_en.html
Mohammed said…
Hi
Your tutorial was so helpful. please what is the key deference between using low voltage and high voltage for induction heater, and which is preferable.
Swag said…
I am glad you liked it...there's no difference in the working principle of the design for different voltages.. If your requirement is for generating extreme heats over large objects then you might need very high wattage for the process, which will require the use of higher operating voltages.

so it basically depends on the power output, for reasonable values low voltage can be used, but for bigger jobs higher voltage may be required.
Arjun said…
Hi Swagatam, thanks for laying this out clearly in a post. Haven't found as clear a resource as this to understand basic induction heater design. Just one question - the link you have posted to help beginners select wire dimensions seems to be private (this one - https://circuitcalculators.blogspot.in/2017/10/wire-thickness-and-current-calculator.html).

Can you help provide an alternate public link to this?

Thanks once again for posting this here and sharing your knowledge so freely!
Swag said…
Thank you Arjun, please check it now, I have changed the access to public.

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