Induction heater plans

2020-2-24

Control


To maximize the power output of an induction heater, maximizing the current in the tank circuit should be a reasonable approach.
To generate the driving signal for the H brige, a TL494 might be used. It provides PWM control which can be used for output power control.
The frequency can be adjusted using a photoresistor as a timing resistor, coupled to an LED which is controlled by an integrator.
The integrator would get a signal from a microcontroller to increase and decrease the frequency.
A microcontroller (probably an ATMega328) would measure the current of the resonant circuit using a lightly coupled coil (otherwise it requires lots of secondary turns, especially at 100+ A) whose output is proportional to the tank current. It is rectified and lowpass filtered. If the microcontroller measures that the dI/dt is negative, then the direction of dF/dt gets changed, such that the frequency tracks the resonant point.
It also needs to track the voltage on the resonant circuit to prevent the capacitors from breaking down and the power dissipation of the circuit besides the load to prevent overheating (which could be done by measuring the cooling water temperature).

Components


The CM75DU-12F IGBTs could be used to build an induction heater, it is relatively fast (combined delay, rise and fall time of 730 ns) and with zero current switching should not have too high switching losses. It has a rather low current rating of 75 A and the IGBT thermal resistance is approximately 0.5 °C/W so the maximum power is somewhat limited.
It will probably be mounted on watercooled aluminium heatsinks.
I have no plans for the resonant capacitors yet, they need to be able to handle a few hundred amperes at probably a few hundred volts and not smoke up while doing so.
An option could be to get cheap induction stove capacitors, sand off the sides and then solder directly to that to get better heat and current transfer (though that risks damaging the capacitor and is a bunch of work).
The induction coil itself would probably be 6 mm copper pipe since that is easy to work with, though it is somewhat lossy and may get damaged by deionized water.

Magnetic levitation


Using a backwards turn at the top and making the bottom of the coil conical would make it suitable for magnetic levitation.
That would be very useful for melting in vacuum while keeping the sample clean and reaching temperatures which would damage a crucible.
A problem is that on power loss the molten material just drops down, so a suitable catcher (fireproof brick with graphite on top?) is needed.

Control


To maximize the power output of an induction heater, maximizing the current in the tank circuit should be a reasonable approach.
To generate the driving signal for the H brige, a TL494 might be used. It provides PWM control which can be used for output power control.
The frequency can be adjusted using a photoresistor as a timing resistor, coupled to an LED which is controlled by an integrator.
The integrator would get a signal from a microcontroller to increase and decrease the frequency.
A microcontroller (probably an ATMega328) would measure the current of the resonant circuit using a lightly coupled coil (otherwise it requires lots of secondary turns, especially at 100+ A) whose output is proportional to the tank current. It is rectified and lowpass filtered. If the microcontroller measures that the dI/dt is negative, then the direction of dF/dt gets changed, such that the frequency tracks the resonant point.
It also needs to track the voltage on the resonant circuit to prevent the capacitors from breaking down and the power dissipation of the circuit besides the load to prevent overheating (which could be done by measuring the cooling water temperature).

Components


The CM75DU-12F IGBTs could be used to build an induction heater, it is relatively fast (combined delay, rise and fall time of 730 ns) and with zero current switching should not have too high switching losses. It has a rather low current rating of 75 A and the IGBT thermal resistance is approximately 0.5 °C/W so the maximum power is somewhat limited.
It will probably be mounted on watercooled aluminium heatsinks.
I have no plans for the resonant capacitors yet, they need to be able to handle a few hundred amperes at probably a few hundred volts and not smoke up while doing so.
An option could be to get cheap induction stove capacitors, sand off the sides and then solder directly to that to get better heat and current transfer (though that risks damaging the capacitor and is a bunch of work).
The induction coil itself would probably be 6 mm copper pipe since that is easy to work with, though it is somewhat lossy and may get damaged by deionized water.

Magnetic levitation


Using a backwards turn at the top and making the bottom of the coil conical would make it suitable for magnetic levitation.
That would be very useful for melting in vacuum while keeping the sample clean and reaching temperatures which would damage a crucible.
A problem is that on power loss the molten material just drops down, so a suitable catcher (fireproof brick with graphite on top?) is needed.




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