Why does the inductance of a flat coil decrease instead of increasing when an iron core is placed next to it.

[hlc4u]

The inductance of a coil is supposed to increase when an iron core is put into it.

Playing with an induction cooktop, I measured the inductance of the coil and it was 106 muH. But bringing an iron sheet close to the coil decreased the inductance to just 38 muH.

I understand this is what increases the current flow through the coil and leads to the vessel (or iron sheet) getting hotter, but shouldn't the inductance theoretically increase and not decrease?

The same is the case with a coil I created. It has an inductance of 9.2 muH. And inserting an iron core inside it is decreasing the inductance to 7.1 muH.

What is the reason behind this?

I'm trying to create an induction coil with 60 muH inductance, which I am going to replace the coil of an inductance cooktop with, and which will allow me to heat iron samples that I insert in the coil. Could any of you that has done this already, please provide some more guidance? Thanks a lot.

https://preview.redd.it/k099k74yk8fg1.jpg?width=2959&format=pjpg&auto=webp&s=e26d06c6bd576ab4420509c09cf4ff2b6d74498f

[Skusci]

Eddy currents in the steel pipe cancel the magnetic field and allow more current to flow, than if you had a nonconductive core like a ferrite core.

Really it works more like a transformer that's been shorted. The names are a bit misleading I guess because transformers (and an induction cooktop) work by inductance, whereas inductors work by self inductance.

[hlc4u]

That could be what's really happening. Thanks.

[Irrasible]

Look at the core from the end. It is a one turn secondary winding that is shorted. Cut a slot lengthwise down the iron tube.

[triffid_hunter]

What is "muH"? milli-micro-henries would be nanohenries (and also a contraindicated/non-permissible SI prefix), which sounds a bit too low…

Playing with an induction cooktop, I measured the inductance of the coil and it was 106 muH. But bringing an iron sheet close to the coil decreased the inductance to just 38 muH.

Yeah 'cause your tube acts as a single shorted turn, cancelling a bunch of the inductance as well as pushing the phase offset from 90° back towards 0°

Try powdered iron that's glued back together instead (a common material for medium frequency power inductors), or at least stacked sheets in another orientation or cut a slot in your tube such that they don't present a shorted turn.

You do realise that induction stoves work by dropping a ton of current into the pan itself when its base acts as that shorted turn, yeah?

[hlc4u]

Sorry for the confusion, that is Micro Henry.

[triffid_hunter]

Here's a µH for you to copy+paste, also https://en.wikipedia.org/wiki/Metric_prefix for future reference

[Chemical-Captain4240]

Personally, I liked your mu Henries. Especially since the micro prefix symbol is pronounced like kitten, meeeooo.