27 Nov
2014
27 Nov
'14
10:53 p.m.
On 11/27/2014 09:51 PM, drlegendre . wrote:
In the case of most copper, antimagnetic stainless or
aluminum vessels, the
eddy currents actually propagate a bit too well - that is, the effective
resistance of the imposed current path is too low for any useful ohmic
heating to take place. In other words, the currents flow, but the voltage
drops are inconsequential.
Well, my view is that the substance being heated has to have a
sufficiently high relative permeability to work--and in a way, it's tied
into your resistive explanation. There isn't a lot of electrical
difference with 18 percent chromium stainless steels, but the 10 percent
nickel is a game killer in 18/10 stainless.
I've got a sheet of pure nickel, which is ferromagnetic. I should give
it a try on my cooker and report back.
A high-permeability ferromagnetic substance will tend to support
formation of a magnetic field at fairly shallow depths in the material,
and thus induction of a current and subsequent heating.
Were this a purely resistive effect, a sheet of very thin copper foil or
a graphite disk would also be great candidates for induction heating,
which (I can verify this by experimentation) they aren't. isn't. Of
course, the frequency has an effect as well.
I think of it as a transformer with a shorted winding. At induction
cooker frequencies, the coupling will be stronger if a ferromagnetic
core is used.
Maybe we're arguing two sides of the same coin?
--Chuck