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Induction heater project

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eskoilola:
Just reading the "Data Sheet" of the coil driver. Definitely a google translation. Here is an extract from the document. This part is warning about the slow rising voltage. Somehow I fin this hilarious:

You must pay attention to when using switch power supply, because the high-power switching power supply usually have soft start function, namely the output voltage is slowly rising, and if the voltage rise to 10V before connected induction heating circuit will be caused by lacking voltage lead to circuit can't afford to vibrate, so that the two MOS tube conduction and burned components at the same time, so be sure to switch the power supply voltage stability and then access the induction heating circuit.

Here is another:
The minimum input voltage of the module is 12V, it doesn't mean all the 12V power supply can be used, some novice buyers may use a smaller power supply. This module has specific voltage and current requirements, the voltage 12V current must be more than 10A power supply can be used, if the power is too low will lead to the power output is greatly reduced, when the voltage is lower than 12V, it will lead to two MOS at the same time conduction, will burn MOS tube;

After reading this it seems like the circuit does not need a kick to start - it just does not oscillate if the supply voltage is below 12 volts. So switching with a FET array should be safe. This is actually a BIG assumption but I am used on exploding FETs - they are actually quite fun.

I am thinking on how to control the output power of this device. Basically there are two options:
1. Control the on-time. Sort of slow-motion PWM.
2. Control the input voltage. Maybe 18V to 48V. I could make a BUCK regulator for that.

From those the first one is more appetizing as it does not alter the operation parameters of the circuit. It is also easier to do. Could do the switching from the low-side with a FET array thus avoiding usage of high-side FET drivers.

And here is another funny extract from the documentation:
When heating the metal objects do not to heat too large metal, otherwise it may be overloaded with power, please add a 35A fuse or ammeter and air switch will be better. (package does not inclues)

So it seems like a current protection should be applied. Apparently sticking in a copper bar of 50mm in diameter will be a no-no-no.
After laughing my ass off while reading the documentation I have decided to do this in a certain way ... will add a block diagram later today.

In case You want to read the complete documentation it is readable in THIS eBay location.

eskoilola:
Here is my interpretation of an induction heater.




This device needs two separate power supplies. First, the high power supply delivering juice to the heater coil through the driver. The other power supply is for all other functionality like control logic, water pump and fans. The high power supply can be switched on/off by the control logic whereas the auxiliary power supply has a real user operated switch.

The high power supply needs a soft start circuit which consists of two relays and a resistor. This is because the transformer I have is a toroid transformer and that devil will blow the fuse every other time if cold connected to mains. The high power supply will be switched on only when needed.

The rectifier and smoothing is just a couple of full bridge rectifiers connected in parallel using resistors (0.1 ohm). I have a box full of 10 amp rectifiers and if I connect 10 of these in parallel that should be more than enough for this application. The smoothing consistsa of 2 capacitors size of a Chevrolet piston each worth of 100.000uF.

Voltage sensor in just a resistor divider. The current sensor I probably make out of a hall sensor glued on top of a wire. If the sensitivity is not enough just cut a gap in a ferrite toroid and stuck the hall sensor in there. The wire then goes through the toroid hole. These sensors are used to cut off the driver in case the voltage is too low or the current is too high.

The FET array switch is a bunch of FETs connected in parallel. I have another box full of 30A/150V fets. Having 10 of these in paralle should do it. With FETs there must be quite a lot of margin as the FET resistance grows when it heats up which again raises the temperature ....

The fans and the pump will be ran only when needed. The temperature sensor is used to switch the heater off if the temperature raises too high. The flow sensor is used to see that the coolant is really moving. The level sensor is used to check that there is enough coolant in the system. These should cover all probable scenarios of overheat, coolant blockage and other situations when cooling is not functioning properly.

The control logic will most likely be a MCU board having an Atmel AtMega64 on it. The user interface is power switch, current (average) and temperature meters, power adjustment, ontime adjustment, heat on/off buttons and indicator lights. Should do it. Will show the desighn of that one later.

awemawson:
The coil needs to be closely associated with it's resonating capacitor bank - the circulating currents are enormous. Mine adjusts resonance frequency dependant on the charge in the crucible, but is nominally 3kHz

The tank capacitors need very careful specifying if they are to survive

Rather than hijack this interesting thread I have started another with some pictures of my 100kW furnace here :

https://madmodder.net/index.php/topic,12596.0.html

eskoilola:
Rather than fiddling with hall sensors and trying to split ferrite toroid it might be better to measure the current before the beefy rectifier with a current transformer. The result might be even more accurate and the experiences I have from hall sensors are not that encouraging.

I made a hall sensor thing for one of my friends to measure the windmill output current and that sensor was far from accurate. The result depended on the orientation with earth magnetic field - this had to be compensated and in addition those ferrites have some sortr of a hysteresis so that after a decent current spike there was remaining magnetism which affected the result as well.

I have also been reading a lot of articles on induction brazing, induction annealing and induction hardening. Those articles are encouraging and it seems like it would be a good idea to equip this with some sort of programmable process control. I do not mean numeric entry but rather a set of knobs to set heat-, sustain and decay times and the sustain power.

I would be really interested  on Your comments on the user interface - with a bad one it is easy to convert a perfect tool into crap.

awemawson:
If you look at one of the photos of mine you will see that power is monitored by current transformer on two phases (no need to measure the third) of the buss bars, and the voltage is monitored there as well so the microprocessor can do a simple calculation and display power rather than current.

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