Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|improve this question
    
You forgot about induction cookers, they also come into this regulation and new phone chargers and toothbrushes. – Marko Buršič Jan 12 at 14:48
    
induction cookers fall into the household appliance exception and are much lower power than furnaces apps.fcc.gov/oetcf/kdb/forms/… and chargers and toothbrushes, etc can be shielded so im not sure its the same thing – S E Jan 12 at 14:50
up vote 8 down vote accepted

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.

share|improve this answer
    
You can't really produce a magnetic field instead of electromagnetic one. Variable magnetic and electric fields effectively generate each other. – Dmitry Grigoryev Jan 12 at 15:25
2  
@SE That's not true. The strength of the electrical field induced by magnetic field does not depend on the initial current, only on the strength of the magnetic field and the frequency. That's what Faraday's law says. – Dmitry Grigoryev Jan 12 at 15:38
    
see @pjc50 answer, the link on "why is an inductor not a good antenna". – Marla Jan 12 at 15:44
1  
What should I see exactly? The first answer states that "Indeed it can be a very good antenna". – Dmitry Grigoryev Jan 12 at 15:51

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.

share|improve this answer

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".

share|improve this answer
    
well there are FCC allocations made to those frequencies according to table 2.106 here ecfr.gov/cgi-bin/…. – S E Jan 12 at 14:57
    
I was not saying that there are no allocations. Of course there are. That's why the FCC will not approve the cooker you build if it radiates a lot of power into the "ether". So it is up to you to make sure that it does not. – FakeMoustache Jan 12 at 14:59

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)

share|improve this answer
    
I disgree that it can't transmit anything. Any conductor with an HF signal will transmit. It will not be a very effective antenna but it will transmit although at a very low level. Low but not zero. – FakeMoustache Jan 12 at 15:04
    
Induction heaters are regulated for emissions by FCC. See my answer. – Marla Jan 12 at 15:05
    
...and with good reason as these devices do emit RF signals. Even though they do not have an actual antenna inside. The FCC just wants to check that the "low" is actually as low as it should be. So regulations are needed. – FakeMoustache Jan 12 at 15:11

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.