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I was trying to build a simple zone refining prototype, so I thought of How to simplifying induction heater for zone refining?

I throughly read What does a beginner need to know to build an inductive heater? And decide to drop this idea.

But all resources on internet and two very nice instructables:

  1. http://instructables.com/id/Powerful-yet-simple-induction-heater/
  2. http://instructables.com/id/30-kVA-Induction-Heater/

have one thing in common, and that is my question:

Why do all of them first rectify AC to DC & then convert it back to AC?

Is there a simpler way of doing this? I do understand 110V-220V is way too much for induction coil but I've heard of regulator IC, can they simplify this task?

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RinkyPinku
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  • Specifically addressing the regulator ICs, linear regulators (like an LM7805) only really work on DC, not AC. AC voltages are actually easier to change, you just use a transformer. I'm not familiar with induction heating, but the setup sounds very much like a Variable Frequency Drive (VFD), which is used with induction motors to control speed and (sometimes) torque, because you can vary the frequency of the output. I assume it has something to do with being able to change the frequency of the AC output. – michaelyoyo Mar 29 '15 at 01:08
  • @michaelyoyo I figured that if I step down 220V to ~ 30V the 5A current of mains would jump to about 35A. Please correct me if I am wrong because 35A is scary. – RinkyPinku Mar 29 '15 at 01:11
  • Remember that current isn't like voltage, it doesn't just exist, so there's only the possibility of 35A in the circuit, if say, the transformer was shorted. When dealing with mains voltages it's probably best to put a small fuse in line (it really depends on the situation to determine the rating).

    Just remember if you're not sure of what you're doing it's probably best to find someone who can be there watching to make sure nothing goes horribly wrong.

     – michaelyoyo Mar 29 '15 at 01:16
  • @michaelyoyo can you please give some link that explains current doesn't just exist, so there's only the possibility of 35A in the circuit, if say, the transformer was shorted. Really got me interested. – RinkyPinku Mar 29 '15 at 01:21
  • I'm not sure how much circuit theory you've done, but this is the best way I've found to introduce different concepts, http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html basically, current is analogous to water flow, when there's no loop, no water flows. It's the same with electricity, no path back, no current flow. Now, if you short the transformer, that's a path for electricity to flow, since there's very little of anything to impede it's flow, it'd be 35A (before the breaker tripped, obviously. That's not to say a 35A circuit isn't scary, it certainly is pretty dangerous. – michaelyoyo Mar 29 '15 at 01:26

4 Answers4

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For induction heating you want to work at a higher frequency than mains 50/60 Hz. As it says in that first instructable:

Induction heating works basically like a transformer inducing so called Eddy currents into a conductive workpiece.

This works at all frequencies but in the ragne of 20-250kHz ferrous metals heat the best.

So by rectifying and then re-oscillating you get to pick the frequency you run at, so you can pick a frequency that works most efficiently with the target material. Better frequency match = more heat for your current = cheaper to run, gets hotter faster, etc.

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Majenko
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    Our Prof had a 50 Hz electromagnet that worked as an induction heater. The eddy currents also had the effect of levitating the aluminium pot. We called it the flying pan. – tomnexus Mar 29 '15 at 06:33
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The answer is simply that the design has to be able to control the currents being induced in the target.

Obviously, if you power the induction coil directly from the AC main, then you have no control over what it is doing. It would be wildly inefficient to put a resistor in the circuit to control the current flow, and DC alone cannot induce the necessary magnetic field to accomplish the task, because it wouldn't result in the oscillations necessary for induction.

There are several methods by which to build a controllable, variable and efficient AC power supply from another AC supply - the simplest of which in most cases is a rectifier followed by an inverter.

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The problem with using ac direct from the mains is that it could give unwanted HF interference on the supply that would upset other equipment. Even a thyristor controlled rectifier gives 5th harmonic distortion and can only bee used up to a certain power of induction heater. Typically 400Kw on a 3 phase supply. (Retired induction heating engineer)

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All materials that conduct electricity can be heated by Induction heating, however the heat is generated by the resistance of the material to the induced current, therefore steel heats better than aluminum or copper. The frequency to be used depends on what you want to do. If you are surface hardening you only want to heat the surface rapidly by a low current penetration depth, then quench it leaving a hard coating with a softer more malleable interior e.g. Shockabsorber shafts would be hardened by around 400Khz. The lower the frequency the deeper the hardened depth. An estimate of penetration depth in mm can be calculated by 400 divided by the square root of the frequency. For forging the penetration depth should be around a third of the thickness of the material. A 10khz generator will heat a depth of 4mm and therefore will heat a 12mm bar to forging temperature. The centre of the bar heats up by conduction during the time it is taken to the forge machine. Too low a frequency will give current cancellation in the centre of the bar after the steel becomes non magnetic at curie point, about 800C, and the heating effect stops at that point.

John
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  • Please spend some time reformatting your answer. It is currently lacking any from of structure (paragraphs, coherency). As it stands the answer is hard to follow. – Tom Carpenter Oct 22 '16 at 20:35