Induction heating - DIY Experiments #8 - Make an induction forge

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• Hi everyone, today we gonna make a really cool experiment: we will use induction to heat metals. The idea is to produce a high-power electromagnetic field, that will induce a huge electric current in metals.

• To start we will reach the limits of our induction forge, and, as usual, we will explain how it works. For curious ones there is a detailed electric diagram at the end of the video. We test our machine; we notice that the consumed current increases immediately when we introduce metal in the coil, all this extra power is transformed into heat. First test: heat a big screwdriver, there it is, in only few seconds it becomes red-hot. We try bigger: a 50 grams weight. We put it inside the coil and try to maintain the current at 15 amps which represents a power of 360 Watts. In less than a minute, the weight becomes red hot and heats up to height hundred degrees; the coil doesn’t heat by itself, it’s mostly the weight that warms it up. It is very impressive to see such a hot object. We will see what we can do with aluminum foil. The thinnest parts become incandescent and end up releasing a very intense white light. Even better, we put an aluminum pencil sharpener in the forge; this metal has a melting point of only 660 °C, we can expect an interesting reaction; it doesn’t seem to like. For even more bizarre tests check out the end of the video. What if we tried to heat a kilogram weight? Because of the heat loss by conduction and convection, we can’t heat it red-hot but still, it reaches more than 300 °C.

• We will now explain briefly how our induction forge works. First of all, we need a system that transforms the direct current from the power supply into a sinusoidal current with a frequency near 100 kHz. For that, we use a simple but efficient assembly that we will present you in detail in another video about wireless power transfer. What happens in the coil? It produces an electromagnetic field, which, as it’s the case in a transformer, can be received by a second coil, which is then going to transform the field back it into electricity. The transmitted power density is so significant, that a single spin coil generates enough voltage and current to light up a car bulb. The retrieved current is very high, we can even melt a small wire that heats up with the Joule effect. You can imagine that a metal block immersed in the field is the same as a one spin wire on short-circuit with itself. The voltage is low but the current is huge, hundred Amps, which explains the heating.

• We are now going to make the link between our system and induction cooktops that you all know. We will also see if we can light a bulb using the electromagnetic field. The cooktop refuses to work if it doesn’t detect enough metal, so we place a pan so that the hotplate starts, and the bulb lights up. Obviously it wouldn’t be practical to place the pan in a coil. The transmitter is a flat coil made to heat a surface. We reproduce the induction cooktop, so that it will be clearer. You only have to roll up a wire and to cover it with a plate that resists to the temperature gradient. We tried with glass and we had a funny surprise. We add the small pan, let it heat up and we can, indeed, boil water in it.

• For those who want to make an induction forge, they will also need the power supply that goes with it. We are working with a very low voltage, which is safer, but high current to have a decent power. We made our own power supply 24V-18A by using 2 supplies from a first-generation Xbox 360.

• A little physics stop on what we call the Curie temperature. From a certain temperature, a ferromagnetic material is no longer sensitive to magnetism. We made a little pendulum that works with this phenomenon: when the metal rod is hot enough, the magnet doesn’t attract it anymore. Then once it’s cooled, it’s attracted again, this goes on and on. Why are we talking about the curie point? Well we find the same phenomenon in our induction forge. Take a look at the ammeter, the current increases slightly when the temperature rises, and then it falls down suddenly to a lower value when the metal is red-hot, only because it is no longer sensitive to magnetism. We understand that there is not only the electric current flowing in the metal which explains the heating, but also the losses by magnetic hysteresis for ferromagnetic metals until their curie point.

• Now we gonna overheat this electric motor while it is spinning. At first, it spins faster and faster. That’s it, it just “passed away”, and it is completely stuck. We won’t just stop here, we keep heating it up.

• Thanks for watching, hope you've enjoyed and consider subscribing for the next one ;)

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