Series Resonant Induction Heater

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Red hot bolt, run time of about 1 minute, about 120 kHz

Induction heating is heating conductive metals with no physical contact, just high frequency electricity. To induce the high frequency electricity in the workpiece a coil is used, much like a transformer, except here the workpiece acts as both the core and the secondary. Since the core has no real turns, it can be considered shorted. Logically this will induce massive currents in the workpiece. Since the induced current is high frequency, the skin effect becomes noticeable and forces the current to flow in the outer layer of the conductor. This increases the resistance for the induced high current, causing the workpiece to heat. If the workpiece is ferrous hysteresis losses contribute to heating as well.

Some interesting drive techniques are needed to push enough current through the workpiece for noticeable heating, and all of them use resonance. Most commonly used are series and parallel resonance, but the parallel resonance variation LCLR rox0rs their b0xors. Richie Burnett has a very good page on it. High Frequency Induction Heating. Unfortunately I didn't have any success with LCLR, so I went with good ole' fashion series resonance.

Obviously the worst topology of the three, but it worked the best first, so I stuck with it. A series resonant circuit consists of a capacitor and an inductor in series. At their resonant frequency their combined impedance is minimal, which allows us to push current through the work coil at high frequencies with relative ease. In case you didn't know, that would be difficult otherwise. (Inductive reactance increases with frequency) The lossy workpiece makes the coil lossy, which dampens the resonance action. If the workpiece is removed during operation however, your inverter would explode. Trust me, I've had it happen. Never let your brother take care of important tasks like power control. The reason for this is the workpiece kept the resonance damped but without it the resistance presented to the inverter approaches zero, meaning magic smoke in your lab.

Series resonant setup, with matching transformer.

Since the the resistances in the workpiece are fairly small, high voltage is not needed. A typical inverter can only supply high voltage, low current, which is the last thing we want with our series resonance setup. Therefore an impedance matching transformer is used. Basically it steps up the current by a large magnitude, and the voltage down by an equal amount. I've found that the power can be controlled by the turns ratio of the matching transformer, a smaller ratio means less power. I stuck with about 20:1, but if I had a better capacitor bank and matching transformer I wouldn't hesitate to take it up to 10:1. 

The currents flowing in the secondary side will be enormous, so use litz wire, or at least tetrafilar (that's four strands) of 18 AWG wire on the matching transformer secondary. Thinner wires, and more of them would be ideal, since the skin effect limits the usable size of wire. The capacitor bank should consist of at least four capacitors for increased current handling. The two I used became hot after one minute, and the entire setup was untouchable after two. Since they are configured in a series resonance setup the voltage across the components will be much higher than what the matching transformer actually supplies. In my case the voltage across the capacitor was almost 200V. If you plan to use a larger ratio, like 10:1, remember to buy high voltage capacitors.

To drive the setup I would have used my “Multipurpose Inverter” (not published yet), if the frequency control was installed at the time. This would save the inverter from failing with no load, simple frequency control and easy current monitoring. I was forced to lash up a driver, which was almost identical to my “Offline flyback driver”.

Lash-up, notice the cooling fans required to keep everything alive.

Basically you want good frequency control since this controls power. Frequencies on either side of the resonant frequency will result in increased reactance, and less power. Tuning for resonance it done by either watching the output on a scope, or watching the current draw. Max current draw at resonance. Scope the output of the matching transformer while changing the frequency. The off resonance waveform will be square, and as you approach resonance the waveform will become more sine/triangle like. At resonance the waveform will have the largest amplitude.

On some of the pictures the bolts will appear to be pink/violet. This is caused by my digital camera misinterpreting the IR.


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