Metalworking - Induction Heating.pdf - METAL - lapistolla

Induction Heating - Amps, Volts, and Watts

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

Induction Heating

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A conceptual MOSFET self resonant oscillator

This design is a result of small scale experimentation. I

originally wanted to use SCRs because of the simplicity

of the design. But it turns out that low cost, readily

available SCRs do not handle a frequency high enough to

heat small crucibles full of metal, nothing above 10KHz.

My application induction of heating requires frequencies

between 200kHz and 1Mhz. So I turned to MOSFET

devices and a self resonant circuit made the most sense.

Please note that this circuit is conceptual only. Please do

not email me asking for part numbers and component

values. They will not do you any good! I have

experimented using small devices, and because the

circuit is still under development, you should not take this

information as how-to! This page is here only to detail my

research, experiments, and observations.

Below is my understanding and explanation of the

circuit's operation. For those of you who know more

about induction heating than I do, please send me your

comments!

Waveform in MOSFET self resonant circuit

Here's what I observed in a similar concept circuit on my

Phillips 35MHz oscilloscope.

The circuit was running on 12 volts. The top trace was

taken at the drain lead on the MOSFET (I was using an

IRF510 at this particular moment) which is also the

connection between L1 and C2. Basically it represents

the charge on C2.

The bottom trace was taken at the connection between

C2 and L2. It represents the voltage across L2. Following

is an explanation of how this circuit works.

When the circuit is powered up, a charge gradually builds

in timing capacitor C1. When it exceeds the reference

voltage set by VR1, the op amp's output snaps up and

closes Q1. This rapid change sets up a 'ringing' in the

resonant power stage. The positive half of this 'ring' is

conducted by Q1 itself, and the negative half is

conducted by Q1's intrinsic diode (gotta love them

MOSFET's!). In the oscillogram, note only one complete

cycle occurs.

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Induction Heating - Amps, Volts, and Watts

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

During this resonant state, some other things are

happening. Full power supply voltage is drawn across

inductor L1 while Q1 is closed. This builds up flux in L1,

which then releases a spike of energy into C2 when Q1

opens. This spike is seen on the oscillogram on the top

trace as a sloping signal.

Also, while Q1 is closed, the voltage on C1 is drained,

and when it drops below the reference set by VR1, the op

amp's output snaps back low, opening Q1 and letting L1

yank C2 back up to about 1.5 times the power supply

voltage. This rise is timed by C1, and the cycle repeats.

What we ultimately want to achieve is a high voltage,

high amperage, high frequency alternating current

('ringing') in L2, which is the work coil. This will induce

currents in a mass of metal, heating it and ultimately

melting it. The industry has this principle perfected for

industrial use, and my goal is to develop a circuit

applicable to home shop use.

Revised concept circuit

NEWS FLASH!! For all of you waiting with bated breath

for progress on this page, I have revised my concept

circuit. First I will go over the fundamental differences

between this and the original concept circuit:

First, the R/C timebase gets its signal from a current

sensor rather than a voltage sensor. This is the main

difference. Resistor R1 is a very low resistance, in the

milliohms, like an ammeter shunt. 30-50 amps across

this resistor will only draw a voltage drop around 1 volt.

The negative supply to the op amps will probably be -5

volts, to give them a good operating margin.

This current sensing method to charge and discharge

timing capacitor C1 means that the oscillating frequency

of the circuit is proportional to (a) the inductance of L1

and L2, and (b) the resistive load presented to inductor

L2.

The frequency (read: power draw) can be tuned by

adjusting variable resistor VR. Moving the wiper on VR to

the right will cause the frequency to be higher, and power

draw to be less. Moving to the left will cause Q1's

on-state to be longer, making a lower frequency, and a

higher power draw. R3 and R4 simply set the upper and

lower limits of the "gain" of the Sensing Preamp stage of

the circuit. These values are not established yet.

This circuit presents a "convenience" over the previous:

Because the charging and discharging of RC Timebase

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Induction Heating - Amps, Volts, and Watts

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

capacitor C1 follows the RMS or "area under curve"

value of the amperage through (and resulting voltage

across) shunt resistor R1, the current draw of the circuit

will be constant regardless of the inductance values of L1

and L2. The convenience is the ability to operate the

circuit with a variety of inductances and frequencies, and

the current regulation (and to some degree, power

output) will be automatically regulated by the circuit.

No, I do not have an oscilloscope trace photo yet. Haven't

gotten that far yet. That's how new this stuff is. Yes, I

have tested a low-power version of the circuit with a

scope. I don't know when the next update will be, so

please be patient. This schematic may have to be "new"

for a while.

On a further note, I have also discovered a good source

of "scrounged" inductors. Old TV sets have two good

sized ferrite cores. One is from the flyback transformer,

and the other is the yoke inductor. My preference is

probably going to be the yoke inductor because of its

round shape. Toroid inductors, as they are called, are

notable for keeping their magnetic fields confined. And at

the high power levels and high frequencies present in

these circuits, that's a good thing.

High amperage MOSFET for induction heating

A high amperage, high frequency semiconductor is

needed for induction heating. This picture is a SOT-227

package, which is a common choice for induction heating

power supplies. The particular device I see as the most

promising for my application is the IXYS IXFN36N100 .

This is a 36 amp, 1000 volt MOSFET with fast intrinsic

diode. It has a suitably quick rise and fall time for my

design. It is availabe from Digi-Key for around $90. The

IXFN44N50 , rated 44 amp, 500 volt, is $33 and may

suffice as a lower cost alternative.

High amperage diode module for power

supply

Also needed is a good sized rectifier module to provide

DC voltage to the circuit. The best possibilities come

packaged in an 'ADD-A-Pak' module, pictured.

The one I have in mind to use is the IRKC71/06 , a

70-amp 600 volt standard recovery diode module, made

by International Rectifier and available from Newark

Electronics for about $30. This module contains two

diodes in a common cathode configuration, just perfect

for full wave rectification of a three wire, 240v supply. If

the IRKC71/06 isn't big enough, the IRKC166/08 may

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2/28/05 5:46 PM

Induction Heating - Amps, Volts, and Watts

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

have to do, but it's more expensive.

Link to a great website on induction crucibles

Here is a great website that gives a good how-to on

replacing the crucible in an induction melting furnace.

There are lots of pictures of a small furnace being

restored.

The furnace in these pictures appears to be one that

would melt a hundred or so pounds of iron. My furnace

would be quite a bit smaller than that, I suspect, more

like 25 or 30 pounds. More...

SCR based induction heater

The following is a result of my research on the US Patent

Website on the subject of induction heating. I went to

considerable length studying SCR based designs, but

SCR technology is a bit lacking on high frequency

devices, above 20kHz. But you can definitely see the

similarities between these patent circuits and my design,

which uses MOSFETs.

NOTE: These schematics are actual snapshots of

U.S. Patents. These are NOT my 'intellectual property.' I

simply present them here as an outline of my research of

electronic induction heating. Thanks to the Clinton

administration, the U.S. Patent Image database is open

to the public. You can get there and search for yourself

the complete patents here.

Any use of this information is subject to patent law. It is

up to you to make certain that your use of any patented

design is legal! (In reality, all the patents I reference on

this page are from 1974, with the exception of the last

one, which is 1981. So they are more than likely expired!)

The schematic above is from patent number 3,786,222.

Click the image to see the full page. It is an induction

heater that was designed to heat up food or other

substances wrapped in foil, by heating the foil itself. It is

the actual schematic that we are interested in, note its

simplicity! I have scoured the internet for circuits like this,

and the patents are where I have found the best

information. These are nothing more than induction

cooking range schematics, but I believe the principles

could be applied to heat treating and melting of metals.

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2/28/05 5:46 PM

Induction Heating - Amps, Volts, and Watts

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

High speed SCR module for induction heating

For an SCR based home shop induction heating power

supply, the most likely candidate I could come up with

was International Rectifier's IRKHF200-12HJ . This is a

200 amp, 1200 volt, high speed 'MagnaPak' SCR module

with recovery diode.

This is an amazing unit that can actually handle its rated

amperage at 10kHz! As you can see in the picture, this is

a very rugged unit with a thick heatsink base and large

terminals. The price is also a bit rugged. Arrow

Electronics lists this baby at $180!

Its limitation is the frequency at which its rating peaks out

at: 10kHz. My application (small crucible sizes) requires

much higher frequencies, between 200kHz and 1 MHz.

But the principles remain the same.

Another simple SCR circuit

From patent number 3,786,219. This one is practically

identical to 3,786,222 but shows the power source with

rectification of AC. Also note the 'tank' circuit formed by

capacitor 27 and inductor 31, whose LC characteristics

no doubt sets the frequency of the induction heating

signal.

Waveform in single SCR design

Here's what happens in the circuit. Note how the SCR is

triggered and conducts positive, and then when the

inductive components 'kick' then the parallel diode takes

the negative half.

Twin SCR induction heater, half bridge

From patent number 3,814,888. This circuit is

fundamentally different from the others in that it uses

SCRs in a half bridge that trigger in alternating sequence

to form an ac signal.

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