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Induction heater project

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eskoilola:


The radiator (oil cooler) arrived today. It is exactly the size and shape I expected it to be. Very nice unit.




The threads might be BSP but at least they are not tapered. That would have made fabricating the adaptor a manly experience. I can see that the adaptor actually consists of two parts. The nut and the hollow piece that makes a seal with the taper. Must be made out of aluminium (which I hate) to avoid electrolytic corrosion.


Two 9 centimeter blowers side by side pushing air through that one should create cooling enough for the heater. I have also planned to create a water cooled heatsink for the rectifier as the water is already available. The rectifier produces almost 100 Watts of heat when operating at full power. Arranging a separate air cooled heatsink for those would consume a lot of space and would require yet another fan.


The fets have their individual heatsinks at the moment. I assume that those could also be water cooled. If the capacitors can survive with replacement air cooling then this would cancel the two fans on top of the driver.


Basically the water could be tap water as this is a low voltage driver but I will use deionized water in order not to collect calcium and iron (our water has much of that) and other salts that can be found in our tap water. Rain water might also do it. Lots of that during a typical finnish "summer".

eskoilola:


This is my insight of the inputs and outputs of the controller.

The inputs are the usual stuff the coil presence detection being a little bit challenging. Other that that nothing new here.

On the output side the FAN might be controlled by medium frequency PWM which in turn would be controlled by the coolant temperature vs. ambient temperature. It is idiotic to blow air through the radiator if that air does not cool it down.

The Driver power will bw controlled by low frequency PWM - maybe 20Hz. Might be higher if the driver can cope with that one.

eskoilola:
Some progress on this one.
Have now designed the MCU and PSU portion of the controller unit. The used MCU will be Atmel Atmega32L which has just enough support for this design.
Next part of this design will be the FET array driver, relay drivers (for the Mains) and other hardware interfaces.

In case someone is interested, the first page (the only page at the moment) is available as a PDF file here.

eskoilola:
Some more progress on this one.
Have now designed the FET array driver, relay drivers, coolant pump interface and the FAN interface.

I have tried out several ways to drive the gates of a FET array. In this case the array will consist of 10 high current fets connected in parallel. The difficulty of driving these is the quite high capacitance at the gates which are all connected in parallel. In addition to this there will be the miller capacitance (feedback from the connected voltage) which does nothing to help the fets switch rapidly.
Most of the commercially available FET drivers do not come even close to the currents needed here. The gate current will peak at about 10 amperes. Fortunately this peak does not last very long after which the current is neglible. This behaviour of fets is one reason why IGBT components are so common in high current switching applications as they do not have such high capacitances.

The next portion of the design will be the analog interfaces.


In case someone is interested, the schematics are available at:
PDF Page 1

PDF Page 2

eskoilola:
The final page of the control schematic for the induction heater is ready.

The last page contains the interface to AD converter including current, voltage and temperature sensing. In addition to these there are the 4 potentiometers to control attack, sustain and decay times as well as sustain level. Those are terms from some musical synthesizer - I just could not figure out better words for these. Hopefully the device does not make loud sounds - I like when a powerful device is doing it's stuff without any additional complaints.

Talking about machine noise. This one should become really silent. I do not know how much noise the coolant pump will produce but at least the cooler fan is the quietest I could find with enough airflow to keep things cool. The two blowers installed by the wise chinese on the driver will have to go. Those are of inferior quality and will make a very loud noise for sure.

If the capacitors on the driver PCB get hot I just simply replace them with better quality capacitors that have smaller losses and do not get hot. Unfortunately I could not find any references and/or datasheets for those capacitors so I really do not know what they are made of so I have to figure out that empirically.

The next step for this controller is to design the PCB. This PCB is quite easy as this does not contain any RF stuff and generally all electricity on this board will stay on the conductors wich would not be the case with high frequency, voltage or current. The high currents are elsewhere in this apparatus.

There is, however, one thing that I must be aware of when designing this device. There will almost certainly be a lot of RF radiation around. For this reason a double sided PCB is a must and the other plane should be kept as much as possible as a ground plane. There is already some isolation for the ADC ground which must be kept separate from the general ground. Further more, all sensitive cables must be shielded and grounded on the PCB. The high power supply will have it's ground connected to the PCB close from the FET array and only there. The wiring of this heater will be an interesting task as well.

For those who are interested about the schematics, those are available as 3 PDF files via the links below.

PDF Page 1
PDF Page 2
PDF Page 3

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