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I was looking at getting an induction furnace for home/experimental use, and I just don't understand how these devices can operate within FCC limits. Dumping several kilowatts into an unshielded coil at frequencies in the hundreds of kilohertz seems like it would interfere with primary services in the area using those frequencies. Am I misunderstanding the operation of induction furnaces here or the FCC guidelines or both?
Here is an example of the type of furnace I am looking at.
The FCC does regulate induction heaters. The regulations mainly are concerned with interference with other users in the frequency spectrum.
The main intent of an induction heater is to produce a magnetic field, not an electomagnetic field (radio wave). Propagation of magnetic field and electromagnetic fields are a little different, and I will leave that discussion for another time. So ideally, the electromagnetic part is contained within the enclosure of the induction heater's enclosure, while the magnetic field portion is, of course, outside of the enclosure (the heating coil which produces the magnetic field).
The means of creating the magnetic field (switching power supply) can of course create undesired electomagnetic emissions. This is where manufacturers have to work more diligently to not cause interference.
The FCC has allotted frequency bands exclusively for industrial heating use. See this page for lots of boring details on industrial heaters.
The main enforcement for induction heaters is through complaints by other people using the frequency spectrum. At one time, a 200 KW 10 kHz induction heater, because of a poor installation, was effecting listeners of an AM radio station (1400 kHz) in St. Louis. Listeners complained, induction was located by FCC, problems corrected, every body happy then.
Air cored coils aren't good antennas: Why is an inductor not a good antenna?
In the EU regulations, if I remember rightly, the low point of testing was 250kHz; you could emit much more below that band without regulatory problems.
As the frequency gets lower, wavelength becomes very long, and everything that's not a long open length of wire becomes a bad antenna. Small appliances are at risk in the MHz to GHz range because that corresponds to the length of board traces which might act as antennas.
You assume that there are other services that can be interfered with in the same frequency band. Now what if there are none ? That is where the FCC comes in. They demand from manufacturers that their devices operate in a certain frequency band which is reserved for these type of applications ! The FCC will also set limits on the allowed RF transmissions (spurious signals) outside that frequency band.
If everyone just used all frequencies without any regulations most of the frequency spectrum would be unusable. So that is where the FCC (and ITU) come in, they regulate and assign frequency bands for certain applications to keep (almost) everyone happy.
Besides, to transmit such low frequencies (up to a couple 100 Kilohertz) effectively you need very large antennas because the wavelength is also very large. Since the coil in these heating/charging devices is small, very little signal "escapes" into the "ether".
FCC is about broadcasting radio waves, but induction coil isn't an antenna, it's just a coil, so these kilowats of power go into the heat, not into the air. Since there is no antenna attached that converts HF into transversal electromagnetic wave, then it cant broadcast anything so no subject to FCC regulative (IMO)
