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induction heating

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dnsmaster

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Hi every one, since it's my first post, let me introduce myself.

First I'm from Québec/Canada, french speakig so excuse my english.
Computer technician and electronic addict.

Now here's what i need.

I'm currently working at an induction heater for my purpose of casting parts( robotic)
I did quite well with a static circuit but want to achieve a feedback loop so the resonant frequency adjust itself as the metal cruciform come in the work coil.
PLL is not really my force.
Thank's
 

Sceadwian

Banned
Could you post a schematic of your heater? I'd love to see it. I'm not entirely sure how induction heating works, why is it necessary to change the frequency, what benefit does it give you over your static circuit?
 

user_88

Member
What is the approximate frequency that you are working with? What sort of precision or resolution are you looking for?

Just from basic feedback control principles, you need a frequency to voltage converter, some means of adjusting the frequency that is sent to the work coil / crucible, and then a summation or error amplifier.... The output of the error amplifier goes to the frequency adjustment section.
 

dnsmaster

New Member
re schematic

Hi !
I dont have the schematic with me but i"ll post it tomorrow.

I have to adjust the frequency because i need to keep the rersonance in order to get full powwer on the coil without damaging the inverter.
When i introduce something in the coil, it value change according to what and how big is that something.

Here is the wschematic who inspire me.
 

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user_88

Member
Looking at the web page here:
High Frequency Induction Heating

In the section titled: The LCLR work coil
There is a suggestion that the series matching coil, apparently Z-match in your previous diagram, can be changed in order to match specific work pieces. Have you considered doing this? Is it effective?
Or do you want to achieve a circuit that is auto-correcting .... for different work loads?
 

tcmtech

Banned
Most Helpful Member
Far as I know the few induction heaters I have been around did not need to change the running frequencyonce they were set up.
While they are heating something up it is basicaly an air core transformer with the secondary coil shorted. There wont be much you can do because the short should look like a severly damped resonance at all frequencies!

So no matter where you move the frequency referance to its still a dead short.

If you short out the secondary on a normal iron core transformer does it care what frequency its at 50 hz VS 60 Hz Vs 400 hz?
unless you really get far from the base opperating frequency it wont matter as far as the heating effect is concerned.

I think.

If your trying to do remelt withan induction heater you may need to make a gas chamber to keep the oxygen from getting in while your metal is remelting or you will end up witha glob of usless oxidized junk.
 
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smanches

New Member
I think the point is to change the heating temperature, which requires a frequency change. I know little about induction heaters other than they are just a huge LC tank circuit, right?
 

tcmtech

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In its basic form yes thats all they are a high powered air core LC tank circuit.

To change the heat you just change the input voltage or PWM duty cycles. Its basicly the same as changing the heat comming out of any electric heater.

If you change the frequency of your portable heater does the heat amount change?

DO some home work on how much energy it takes to melt metals. Even small stuff takes a good amount of energy. Factor in that induction heating is not 100% efficient and the drive system is far less than 100% efficient too.

Plus you need a shielding gas of some sort and all the other miscilanious but nessisary steps and equipmnet it takes too!

Not so practical on the cost Vs outlay numbers for small scale opperation.

Get an oxy / propane torch your money ahead!
 
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dnsmaster

New Member
Thanks for the site but already look and no real shematic or hint for my needs, but find this and i think that ITS my goal already drawn.
Induction heating
Really worth the visit.

Heres is my own actual design that i will adapt in the next few days.

I'll be back soon to tell the story
 

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user_88

Member
So what is the basic principle of your own circuit?

You have a variable frequency PWM modulating the paired FETs on the primary side of the transformer.
Is the idea to manually adjust the frequency so that you have resonance on the secondary side?

The diode bridge gives you about 200VDC .... the current fed by the transistors into the resonant tank circuit... are those BJTs both bipolar npn type?
 
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tcmtech

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it may work but I would question if it will be able to tranfer enough power to even melt a aluminum pop can.

Heres some pictures of an actual induction heater. This one is 77 kw and runs off a 480 volt three phase source at 77 amps per leg.

It was only used to heat bushings that fit over shafts.
The bushings were about 2.5 inches diameter and 3 inches long. The shaft was about 1.5 inches diameter. heat up time was around 10 to 15 seconds as I was told.

Plus the unit is about 3 ft square and 5 ft tall. There is second cabinet that holds the liquid cooling system also. Total system wheight is around 1200 pounds.
 

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Sceadwian

Banned
tcmtech if the crucible is well insulated power isn't too much of a problem, it'll just take a lot longer to melt it unless it's massively under power. It really depends on how much heat loss you get in the crucible. He said the static circuit he built works so it must be doing something.
 

dnsmaster

New Member
User88

those are IGBT handling 50 amps at 800V theres a switch that double the voltage giving almost 350 V at the bridge .
The 77KW is too big for me, i cast very small parts for robotic projects, i melt about 1 cubic inch of aluminium by heating a car caliper piston as a crucible.
I have to monitor the waveform with a scope to make sur its set at the resonant frequency of the coil+piston+aluminium. if not , the inverter heat a lot and destroy itself sometime if left unattended.

I'm learning from the site of my last post, looks like its more a phase matter than frequency to obtain max power automatically.
Working on a board to adapt my own design, come back later with updates.

PS: It takes a lot more time than 15 seconds ;)
 

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tcmtech

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Glad To know yours works! I Figured if it didnt burn up right away it would do something buthow much or how well I had no way to know.
Stuff that small does not take much energy.
I generaly dont work with much that is hobby sized. Most of the industrial equipment I work with looses more power in the opperating process than most hobbiests will ever use!
I dont have much practical experiance with the small stuff!
 

Sceadwian

Banned
It just suddenly dawned on me I've heard of people using microwave ovens for melting small amount of metal, they're far more efficient than induction heaters I think, but you do need to use a non-metallic crucible.
 
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