# Does this coil topology work at high frequencies?

I have a cast iron wok and an induction hob. My understanding is that a ~24kHz AC in the hob's coil causes heating in the wok due to eddy currents from the varying magnetic field.

Sadly, the iron needs to be very close to the coil to work, and as such only the bottom part of the wok gets heated. I would like to heat a bit more than just a small part in the middle, and also be able to take advantage from the 'boost' ring which is otherwise out of reach of the wok. The basic idea is schematically depicted below, but I would like some advice on whether this topology would work, and if there are any additional considerations given the relatively high frequency.

The idea is to have a pancake coil wound of Litz wire placed just above the induction hob's 'boost' ring. This would be connected to a second, conical pancake coil (also made from Litz wire keeping in mind the skin effect) that is positioned very close to the pan to heat it inductively (while keeping the coil relatively cool).

Does the winding direction of the 'second' coil matter? My intuition was to place it reversed w.r.t. the bottom ring to 'push' the magnetic field more in the pan, but perhaps this actually reduces the efficiency by reducing the induction and thus the current? Should I worry about any side effects? (I presume the cooker works as an oscillating tank circuit, so perhaps a secondary coil would mess up the expected inductance of the circuit)

• This doesn't change anything about your question, but I think induction cooktops mostly work by hysteretic heating, not joule heating. Could be wrong on that though. Commented Mar 11, 2019 at 19:29
• @Hearth According to Wikipedia, less than 10% (also present in their source) is due to hysteretic heating. Commented Mar 11, 2019 at 19:33
• If you want the pan to become hot not only in the center region the easiest way to accomplish that is by using a standard frying pan instead of a wok. In other word: it's characteristic for a wok that the sides become much less hot than the center; if you don't want that, don't use a wok.
– Curd
Commented Mar 12, 2019 at 9:46
• @Curd But what's the fun in that? I asked this question because I like to learn about complex subjects in practical settings, not because I would be eating raw vegetables otherwise. Commented Mar 12, 2019 at 9:53

AFAIK Woks are intended to be heated unevenly, the bottom gets very hot while the sides are cooler the higher you go up the walls.

You might be better off by adding a thicker ferroelectric "heating ring" around the bottom or just placing it on a commercially available Induction Cooktop Converter Interface or diffusion disk, to more closely simulate the rising heat from flames.

High-temperature Litz Wire rarely goes above 300°C while a gas flame exceeds 1500°C and an electric stovetop 700°C, having direct contact of the Litz wire with the wok would not be a good idea. (Actual cooking temperature in the pan is much lower than this).

• I'm sorry if this was not clear from my question, but I do not want to heat the Litz wire, but use it to heat the wok itself (contactless, so the Litz wire stays relatively cool just like the coil inside the hob) through inductive heating. The heating ring is a good idea which I was also considering, but I would like to see if this idea is viable as well (less thermal mass = better response time during cooking, reducing the risk of charred vegetables) Commented Mar 11, 2019 at 20:28

I think Hearth's comment is correct. Induction cooking is done mostly by hysteretic heating. See the Wikipedia page Induction heating that mentions non ferrous materials are not suitable.

I suggest that you just need more iron on the cooktop. My wok is spun steel and about 1/16" thick so there is not much iron there to put through a hysteresis loop and thus heat.

• This does not answer the question at all. It's a thick cast iron wok, and gets very hot at the bottom. I'm interested only in the electromagnetic side of this 'transformer' idea (anything else is off-topic here, anyway) Commented Mar 12, 2019 at 9:25