What happens to parts of induction cooker magnetic field not covered with the pan?

Question

An induction cooker works like this. There's a flat coil of wire under the cooker surface which is connected to a source of high frequency AC and it produces an oscillating magnetic field with field lines oriented vertically. A suitable pan (with thick steel bottom) is put on top of cooker surface and that magnetic field induces eddy currents in the pan bottom and those heat the pan bottom.

So it's a kind of an air core transformer. The coil under the cooker surface is the primary winding and the pan bottom is a shorted secondary winding. And there's no steel core so the energy magnetic flux "extends" from the cooker surface and "sticks into" the pan bottom and the pan bottom presumably absorbs all of that flux. It sound good so far. However modern induction cookers allow use of pans which are smaller then the coil diameter so that for example a cooker will have a coil 18 centimeters in diameter and pans with 10 centimeters diameter or larger are allowed and those are heated just fine.

A 18 centimeters coil forms a circle with square of 254 square centimeters and a pan bottom with 10 centimeters diameter forms a circle with square of 78 square centimeters. This way only about 30 percent of the coil surface is "covered". It looks like two thirds of magnetic flux "leaves" the coil and doesn't quite "stick into" the pan bottom.

What happens to that "uncovered" magnetic flux? Does it affect surrounding electronic devices or humans?

Answer 1

The manual of my induction cooker says:

The induction zones adapt themselves, until a certain limit, to the size of the cooking utensil. But the cooking-utensil must have a minimum size.

Then follows a table like:

• Right back : 145 - 180 mm
• Right front: 125 - 145 mm
• Left back : 145 - 180 mm
• Left front: 180 - 210 mm

I leave to the expert how they adapt themselves.

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As to Andy's fail safes: yes there are plenty. They even get annoying if they are hair triggered like in my induction cooker.

• Nothing on surface (Switches off before you have time to get a pan from the cupboard)
• No pan (Don't wait too long turning you pancakes)
• Water/moisture on the surface (spill a bit and it switches off )
• Too hot
• On for too long
• Object on the control surface(even a loaf of bread triggers the latter)
• Start beeping if I clean the surface with a wet cloth (whilst it is off!!)

Even the tiniest spillage causes mine to shut-down. I now dry my hands before touching the buttons.

Answer 2

A perfect coil producing a strong magnetic field does not consume energy when there is nothing taking that energy away. So, the 2 kW you mention in comments is not consumed when there is no pan on the cooker surface.

Given that the cooker is producing magnetic fields and those fields decay roughly as distance cubed, there won't be much you can measure beyond a metre away I would estimate. Compare this with radio waves - their intensity (H or E) decays with distance (not squared or cubed) and you can therefore see that the magnetic field should be very local to the cooker.

I don't know if there are fail safes built in but, if no energy is being taken by the "load" this is easily detected even at very low field intensities hence, I suspect, that the magnetic field is only ramped-up when there is a bona fide "load".

Answer 3

If you consider the inductor as two inductors in series, one under the pan, and one under free space, you can start to see why this works reasonably efficiently.

The inductor under the pan has an iron core (the pan itself) with relatively high permeability; that under free space has relative permeability 1. Now the first inductor is poorly coupled to its core, as it is underneath it instead of actually around it, but even so, if its relative permeability is only 10, (*) and each has the same number of turns, the inductor doing useful work has 10x the inductance and 10x the impedance of the other. So, in this case over 90% of the power is taken by the productive inductor.

(*) I don't know enough to compute the coupling between a planar wound inductor and an adjacent core so this is a speculative guess.

Answer 4

The answer is - it "leaks". This video shows it quite convincingly. Skip the beginning and fast forward to 02:18. The author uses an off-shelf induction cooker partially covered with some steel plate so that the cooker "feels metal" and turns on. He then conducts various experiments with "leaking" magnetic field. He also claims that the "leaking" field interferes with his lights electronic circuits.