Induction Heating 101

Induction heating is 95% voodoo, 5% science.

I hope that you will permit me to tackle a few misconceptions, including some particularly amusing ones in the Art of Tea magazine which VL may remember (though I've not bought this after the 3rd issue). I offer you the briefest of introductions to the black art of boiling your tea-water via induction.

The main idea is that you have an electromagnetic coil tucked away in the base of your cooker (#1, in the figure). Most of the tea-inductors I've seen are made by Kamjove, and I've used one of those myself for about three years (though have recently changed to a hot-plate). It's primary benefit is that it's quick, because it's efficient.

Run some alternating current (a.c.) through the coil and, in accordance with the Biot-Savart law, you'll generate a magnetic field, as is well-loved by most physics classes in school. This magnetic field passes up from the coil, through the ceramic hot-plate sitting on top of your cooker, and into the metallic structure of your kettle (#2 in the figure). Remember that the current was a.c., so it's cycling positive and negative; and so, the magnetic field is cycling at the same frequency. Nice and easy so far.

The idea is, of course, to heat the water (#3 in the figure). This can happen in two ways, depending on what your kettle's made from.

i. Eddy currents

When you pull your oar through the water, you leave behind swirling vortices, persistent turbulence. When the magnetic field flickers on and off in the pot, you get an electric current induced in it (according to Faraday's law), circulating much like that persistent turblence caused by the oar. As every user of the light bulb will know, running current through metal causes heat to arise through resistance. This heat can be used to get your water boiling.

In induction cookers, however, this effect is negligible, and heating occurs from the second of the two methods:

ii. Hysteresis loss

Check out the graph below. I know that most normal people are allergic to graphs, but hang in there.

This is what's happening as that magnetic field goes on and off in your kettle. That field is shown on the horizontal axis in the figure: "field strength". We start at P1. As the field gets stronger, that magnetic field is set up in the kettle. We can see the "flux density" (on the vertical axis) increasing as we increase the field strength, moving along that dashed line until we reach a maximum at P2.

Here's the useful bit: when the magnetic field flickers off again (moving back from P2 to P3), there remains some magnetism in the kettle. (This is what allows the creation of magnets, by the way.)

In fact, we have to reverse the field strength in order to get the flux density in the kettle down to zero again (P4). We reverse the field strength to its minimum (P5), and the flux density reaches its minimum. As we continue the cycle around once more (from P5 to P2), the flux density follows it as before - but "lagged". This cycle goes around and around as the a.c. current cycles between positive and negative.

This "lag" in the kettle's flux density is the hysteresis. The loop in the figure is a hysteresis loop, so called by engineering legend J.A. Ewing (although he was from the Other Place).

So how is this hysteresis heating my water?

The purple (lilac?) area shown above is the work done (per unit volume of your kettle's material) in getting your kettle magnetised. When we relax the magnetic field, some energy is regained - as shown by the yellow area in the above. The difference between the purple and the yellow area is energy lost in the kettle - as heat.

The bigger the area between the curves (i.e., the greater the hysteresis), the more effective your kettle is at heating your water. So, induction kettles are typically made out of materials which satisfy this.

The heat generated is also proportional to the frequency with which the field cycles on and off. This can be up to 20 KHz, or 20,000 flickerings per second. Some people don't like the idea of sitting next to a 20 KHz magnetic field. A friend visited us for tea after spending many months in a meditative retreat, and he expressed discomfort around it. "Your mileage may vary" depending on your attitude to such things.

So, that's how your inductor works.

"This article is so embarrassingly dull. What was that feeble human thinking?"

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Inductors are very efficient: very little energy leaks out of the pot into the air - most of it is transmitted into the kettle, which heats the water. In contrast, conventional hobs just get hot: they heat the kettle, because it's nearby, but lots of energy also escapes into the air. So, an induction cooker is very fast at heating water, compared to conventional heating.

We can see that the ceramic hot-plate isn't there to heat the kettle, because that's being performed by the field - in fact, it's there's to insulate the fairly delicate induction circuitry from the hot kettle. To stop it overheating, it needs to be cooled from underneath, and so tea-versions usually have a noisy fan roaring away. I don't like this about induction cookers. It really ruins the tea atmosphere.

Induction also has lots of other fun applications: as the water heats, the characteristics of the field change, which can be sensed by the induction unit, and so the field can be changed to compensate. This allegedly leads to induction kettles with temperature control - but they're usually fairly horrible. I've not seen one that I could trust so far, and it's infinitely easier (and more reliable) just to learn it yourself - and more satisfying.

Induction also requires some resilient materials. The magnetic field is always set up in the same way, and the location of the most heated areas is concentrated spatially - it's not an "all over" heat, like a conventional hot-plate would provide, but appears in regular "hot spots". You're heating the base of your vessel in a fixed pattern, repeatedly, and the (really rather significant) temperature differential across the metallic lattice can lead to stress fractures. Don't put your expensive Japanese kettle on an induction hob!

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So, that's induction. If you knew it all already, do please correct me where I'm wrong. If you didn't know (or care to know), then I hope it's been useful in some small way.

Either way, I hope I don't read any more stories about electricity passing through the water to heat it up!

Art of Tea, I'm looking at you...

"Your graphs displease me."