According to the Wikipedia, one of the limitations of the induction cooker is that the bottom surface of the pot should be flat.

Accordingly, I commented on a question on Seasoned Advise, but I'd like to know whether this is actually true (My username is BaffledCook there).

For a wok, it seems reasonable to believe the induction cooker will not be effective, but how about a wobbly pan? How wobbly should it be before it really affects efficiency?

  • $\begingroup$ Wobbly pan doesn't matter, the magnetic field acts at a distance. So long as all the field lines from the cooker go through the bottom of the pan, you are fine. In order to be ineffective, it would have to curve more than a wok. I am surprised that a wok has any difference compared to a flat pan, the field lines shouldn't escape more from the wok by any significant amount. Is this folklore, or did somebody do the test? I don't think it matters at all. This should be an answer, but I don't know the exact coil configuration of a cooker and I didn't see one specified anywhere. $\endgroup$ – Ron Maimon Jul 15 '12 at 6:19
  • $\begingroup$ I think I found the possible reason that wok doesn't work. In an induction cooker, there is a safety feature that turns off the power if it senses that there is too little magnetic material in the environment. The sensor might be triggered by the different shape of the wok, turning off the current. I can't see why there would be an effect otherwise. $\endgroup$ – Ron Maimon Jul 15 '12 at 6:23
  • $\begingroup$ @RonMaimon, that's the doubt I have. A wobbly pan shouldn't affect efficiency, but there's the thing in the Wikipedia... So, did anybody test this? $\endgroup$ – GUI Junkie Jul 15 '12 at 9:43
  • $\begingroup$ Another thing is that I've bought an induction coffee maker that's slow and sometimes doesn't connect to the stove. I know it's another thing all together, but... It's aluminium, but there's some magnetic element in the bottom. $\endgroup$ – GUI Junkie Jul 15 '12 at 9:46

An induction cooker works with a flat coil setup similar to the one shown below. The diameter of this coil depends on the exact model (in the range of 30 cm). This implies that the field is also extending quite a bit into the air above the cooker.

enter image description here

As the large field can be potentially hazardous (think metal ring on finger) there are relatively strict safety standards for induction cookers. The field of of the coil measured at a horizontal distance of 30 cm from the end of the coil and along the vertical axis (z=0 is the center of the coil) is shown below (the absolute values are not comparable with the field above the center but the profile is certainly comparable). The data is from this study: "B-Field Exposure From Induction Cooking Appliances" by C. Viellard and coworkers.

enter image description here

As you can see the field strength decreases by about an order of magnitude in a vertical distance of 200 mm. A very wobbly pan might be less effective but only if it is warped by more than a few cm.

  • $\begingroup$ Your last sentence, was that cm or mm? $\endgroup$ – GUI Junkie Jul 22 '12 at 16:35
  • $\begingroup$ cm, mm will not make a significant difference. $\endgroup$ – Alexander Jul 22 '12 at 16:39
  • $\begingroup$ I guess that answers the question. Should Shaktyai's answer be of any concern? $\endgroup$ – GUI Junkie Jul 22 '12 at 16:44
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    $\begingroup$ @GUIJunkie: Shaktyai is correct that uneven surfaces can cause large temperature gradients. If this is a concern depends on the cookware. As a comparison a gas cooker also creates large temperature gradients. An uncoated steel pan won't be damaged, lightweight travel pots might not appreciate it if they work at all on an induction cooker. $\endgroup$ – Alexander Jul 22 '12 at 18:31
  • $\begingroup$ you have to go 10cm up to see a 20% decrease in the field, so the answer is no difference in effectiveness. $\endgroup$ – Ron Maimon Jul 23 '12 at 6:21

Induction will work whether the pan is flat or not. However Foucault's currents (eddy currents) locally induce important temperature gradients within the metal. Unflat pan generates a non uniform heating that in turns induces large deformations or even cracks in the steel. Furthermore the power transfered by microwaves to the pan is optimal if the field lines are perpendicular to the metal surface, and it is easier to achieve with a flat pan.

  • $\begingroup$ How 'unflat' should the pan be before that really creates problems? $\endgroup$ – GUI Junkie Jul 22 '12 at 16:42
  • $\begingroup$ A few cm would start to induce more deformations or cracks. $\endgroup$ – Shaktyai Jul 22 '12 at 17:08

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