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.
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!
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I had commented, "Talk to an induction heating manufacturer to help you with your project. The combined electrical and thermal design is not trivial. It wouldn't surprise me if you need 50 kW and 50 kHz induction heater".
Duolin contains other products and information you need, so please check it out.
Here is further information just to show that the thermal and mechanical considerations are a big part of the design.
The brass is about, "0.5&#; thick and a 5&#; diameter." Also, the copper is about 0.079 wall thickness. Both parts need to be heated to about degrees. So, you will need to apply about 5 times the power in the brass as you will apply to the copper. This will require a special induction coil or coils.
Since copper and brass both have high thermal conductivity, as you apply heat to the pipes, the heat will be rapidly being removed from the heat zone. The heat conducting down the length of both pipes. Thus, you need to apply heat at a very high rate to over come the heat loss due to conduction away from the heat zone.
EDIT1 : Using your 48 volts, you will need amps for 50 kW.
EDIT2 : There is a minimum power level required which must be greater than the amount of heat flowing away from the work zone. This is a thermal calculation. Additional power then is required to heat the material in the work zone to degrees. Additional power is then required to offset the induction power supply losses.
The wattage required has nothing to do with the resistivity or resistance of the workpiece.
I came up with 50 kW ESTIMATE using my years of experience and observations.
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