induction or hysteresis?

[Robert A Cohen <bbq@ESU.EDU>]

In the latest issue of The Physics Teacher, Brahmia and Horton raise the
question of whether the "induction cooktop" really utilizes the induction
of current to warm up the pan.  They note that a ferromagnetic pan is
needed and this implies an hysteresis effect.

I decided to check if this had been discussed on PHYS-L and, lo and
behold, it had.  In April of 1996, an ex-Westinghouse R&D engineer wrote
the following:

> This type of cooking element was first introduced by Westinghouse in the
> 70's.  A pancake coil made of Litz wire is driven by a high frequency
> power supply.  The magnetic field couples to a suitable pan made of
> steel with a copper clading to distribute heat more uniformily.  The
> eddy currents in the steel pan cause losses which heat the pan and cook
> the food.  The only heat on the ceramic top is the result of heat
> transfer from the pan backwards into the caramic and the coil which is
> located just below the surface.  It is an expensive though entirely
> useful way to cook food.  The trouble was that the cost was 2-3 times a
> normal electric heating element type stove.  It caught on in England for
> awhile, because Princess Margaret bought one.  A buying surge resulted
> in trying to "keep up with the ..."  One problem was the field that
> existed without a pan.  This high frequency field was not too compatible
> with cooks that had pacemakers, so we added a "pan detector" circuit
> that throttled back on the coil power supply if a pan was not covering
> (an capturing) the high frequency magnetic field.  It was an interesting
> program.  The Litz wire was necessary to reduce losses in the coil
> itself.

Later, someone in the same thread pointed out that if it was due to the
induction of eddy currents, any metallic pan would do and thus the heating
must be due to magnetic domains "flip-flopping".

Were the "induction cooktops" described above any different from the
hysteresis cooktops of today?

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| Robert Cohen               Department of Physics       |
|                            East Stroudsburg University |
| bbq@esu.edu                East Stroudsburg, PA  18301 |
| http://www.esu.edu/~bbq/   (570) 422-3428              |
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