Many of you who are reading this may be asking "What is a ZVS driver"? Well, it is an extremely efficient oscillator circuit that is able to create an extremely powerful electromagnetic field that heats up the metal. It is the backbone of the induction heater that this instructable is showing you how to make.
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To understand how this power supply works, I will explain the different sections of it. The first section is the 24 volt power supply. The power supply needs to produce 24 volts at a current of 10 amps. For my power supply, I will be using two sealed lead acid batteries wired in series. The power is then fed into the ZVS driver board. The ZVS oscillator pushes and pulls current though a coil around the object that is being heated. This constant changing of the current's direction creates a fluctuating magnetic field. This induces many small eddy currents in the metal(refer to the diagram above). All of these currents are relatively high, and because of the low resistance of the target metal, heat is generated. According to ohms law, power converted to heat in a resistive circuit is P=I^2*R.
Now, the metal type of the object that is being heated is very important. Ferrous metals have a higher magnetic permeability, so they are able to harness more energy from the magnetic field. This allows them to be heated quicker than other materials. Metals, like aluminum, have a lower magnetic permeability, so it takes longer for them to heat up. Things that have a high resistance and low magnetic permeability, like a human finger, will not be heated at all by an induction heater. The resistance of the material is also very important. If you have a higher resistance in the target metal, then less current will flow, so the power converted to heat gets exponentially smaller. If you have a metal with a lower resistance, then the current will be higher, but power loss will be lower due to ohms law. It is a little bit complicated, but because of the relationship between resistance and power output, the highest power output is achieved when the resistance of the object approaches 0.
The ZVS oscillator is the most complex part of this circuit, so I am going to explain how it works. First of all, when the current is switched on, it flows though 2 inductive chokes into each side of the coil. The choke is to make sure the circuit does not draw to much amperage on start up. The current also flows thought the two 470 ohm resistors into the gates of the two Mosfets. Now, because no component is perfect, one Mosfet is going to turn on first. When this happens, it hogs all the gate current from the other Mosfet. It will also draw the drain of that Mosfet that is on to ground. This will not only let current flow though the coil to ground, but it will also let current flow though one of the fast diodes form the other gate of the other Mosfet, locking it off. Because there is a capacitor in parallel with the coil, it creates a resonant tank circuit that starts oscillating. Because of this resonant action, the drain of the other Mosfet will swing back and forth in its voltage, eventually reaching 0 volts. Once this voltage is reached, the gate charge from the Mosfet that is on will discharge though the fast diode into the drain of the opposite Mosfet, effectivly shutting it off. With this Mosfet off, the other Mosfet has the opportunity to turn on. After this, the cycle repeats thousands of times per second. The 10K resistor is meant to deplete any excess gate charge on the Mosfet, because it is like a capacitor, and the Zener diode is meant to keep the gates of the mosfets at 12 volts or under so they do not explode. This high frequency high power oscillator is what allows metal objects to be heated.
Now, its time to build this thing!
Induction heating coil design plays a role in the effectiveness of the induction heating process. As a result, it is crucial to make the best selection when choosing its components.
The tubing of the coil must have high-conductive copper. The copper tube with an outer diameter of at least 0.32 cm ensures easy water cooling.
Nevertheless, the diameter could be as large as 5.1 cm for applications that need high power.
Multi-Turn coils are ideal for single-shot, large-diameter heating. These coils have multiple turns that increase the overall proportion of the coil. However, when the diameter of the coil exceeds 4x–8x, it becomes difficult to produce uniform heating.
Single-turn coils are suitable for heating bands with narrow diameters. The height and diameter of single-turn coils vary slightly with size.
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The design and structure of the work-coil lead have the main influence on the performance of the coil.
The coil has a tank circuit. All the leads connecting the tank capacitor to the coil have inductance. When the coil heating area has a higher inductance, it counterbalances the effect of lead inductance.
Coil bracing is another crucial factor to consider when designing an induction coil. Coil bracing stops the magnetomotive force from making unwanted movements.
When electric current flows in the coil and workpiece, it generates magneto motive forces. The force can become uncontrollable during induction heating. Therefore, braces are crucial.
Coupling is the term used for the gap between the induction coil and the workpiece.
For progressive heating, the coupling distance should be 0.19 cm.
However, if the workpiece is a magnetic material, the coupling distance should range from 0.25 to 0.38 inches.
The small gap between the coil and the workpiece is known as tight coupling, while the large gap is called loose coupling.
Most manufacturers anneal the coil tubing every few bends. It prevents the deformation of the coil.
The tubing is heated until it becomes bright red. After that, it is cooled rapidly with water. The process prevents the coil from fracturing during fabrication.
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