Why do induction cooktops require ferromagnetic cookware?

[Paulusgnome]

I am playing around with induction heating at present.
I have a couple of Chinese-built royer oscillator drivers, one good for ~100W and its big brother (~1000W). Both of these will easily heat steel, stainless steel, copper, aluminium, lead, the 1000W driver even comes with a carbon crucible which also heats nicely when put inside the coil.
No real surprises here, it works after all just a transformer, the heating target just needs to be conductive in the same way that a transformer winding is good in copper or aluminium.
So why, then do cookpots for induction cooktops need to be ferromagnetic?
My working explanation is that the cooktops are optimised for ferromagnetic pots, and while they would heat a copper or aluminium, they cannot do so efficiently, and may overheat as a result.
Can anyone confirm or improve on my working explanation?

 

[jraef]

You are misinterpreting the operating principal to begin with. It's not acting like a transformer as an induction heater does, it's acting like a bad transformer CORE. It is using the magnetic flux of the cooktop element to cause eddy current heating of the iron in the cookware to create the heat. There is no current flow in the cookware, that would be potentially deadly! No iron, no eddy currents...

It's called an "induction cooktop" not because it's an induction heating system, but because calling it anything else with the term "flux" in it would cause too many snickers in the retail consumer world!


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[GroovyGuy]

It's the permeability of the metal. Take a look at the table in Wiki;

https://en.wikipedia.org/wiki/Permeability_(electromagnetism)

See also;

https://en.wikipedia.org/wiki/Induction_cooking

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[Paulusgnome]

As I understand it, if I heat a piece of brass or aluminium by placing it inside the coil of one of my royer oscillators, the field is inducing eddy currents in the brass which is then generating heat through resistive heating. The brass piece is acting exactly like a shorted single-turn secondary winding, forming the equivalent of an air-cored transformer with the oscillator coil as the primary. No ferrromagnetic material for the workpiece, brass, copper aluminium all heat up nicely.
Now, in an induction cooktop, the same principle is being applied, but we have to have a workpiece (the pot) made from iron, steel or stainless steel.
Could it be that this is because the ferromagnetic material couples to the induction coil more effectively, leading to a more efficient power transfer?

 

[IRstuff]

GroovyGuy's link to Wikipedia answers the question, but here's a more complete list. Iron has extremely high permeability and high resistance, both of which are conducive induction heating.

Inductive heating of brass requires a much larger RF generator than iron or steel, and you can get a decent induction cooktop that about the size of a old school laptop.

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[Paulusgnome]

Thanks for the help. I should have done some basic research before posting.
The wikipedia articles referenced be GroovyGuy above, and the article on induction heating are most helpful.
Induction cooktops require ferromagnetic cookpots because they rely on a combination of eddy-current and magnetic hysteresis to generate the heat. They operate at around 24KHz.
My toy induction heaters, when heating non-ferromagnetic metals, generate the heat purely through eddy current heating. The oscillators runs at an as-yet-unmeasured much higher frequency than the cooktop. I'd expect, if a small steel item is heated through the curie point, to see a change in frequency and power as the mode changes.

 

[IRstuff]

The original induction stoves required an RF supply that was MUCH larger than a breadbox, but it was more of a classical induction system that could probably handle anything metallic.

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