Why doesn´t the coil heat up under the ceramic in induction cooking ?

Thread Starter

Cadena

Joined Nov 4, 2018
27
Hi everyone,

I would be interested to know exactly why the coil under the ceramic doesn´t warm up. If there is AC current passing through the coil, it must be heating up somehow since the cil is acting a resistor to the current flow, so how come that you don´t feel any heat coming from the induction cooker ? Also since the pan is being EM induced, there is a current flowing so would the user not feel the current as it passes through their hand ? Thank you in advance.
 

KeithWalker

Joined Jul 10, 2017
1,131
In an induction stove a coil of copper wire is under the cooking pot and an alternating electric current is passed through it. The resulting oscillating magnetic field induces an electrical current in the pot. This large eddy current flowing through the resistance of the pot results in resistive heating. The copper coil has very low resistance so it dissipates very little heat.
The coupling between the coil and the pot is magnetic. That is why you can't feel it.
Regards
Keith
 

andrewmm

Joined Feb 25, 2011
525
My thought,

Heat comes from resistance to the current flowing.
The coil is made out of copper, a good conductor, the pan is made out of "steel" , relatively a bad conductor.

Regarding the user,
they are a very high resistance, badly coupled to the coil, so low voltage induced.
 

MrAl

Joined Jun 17, 2014
7,748
Hi everyone,

I would be interested to know exactly why the coil under the ceramic doesn´t warm up. If there is AC current passing through the coil, it must be heating up somehow since the cil is acting a resistor to the current flow, so how come that you don´t feel any heat coming from the induction cooker ? Also since the pan is being EM induced, there is a current flowing so would the user not feel the current as it passes through their hand ? Thank you in advance.
Hello,

The kind you are thinking of at first is the resistive heating type of coil. That gets very very hot due to the large current in the coil wire itself and the somewhat larger resistance of the coil wire and the type of material used to create the wire is what causes the larger resistance. It is sometimes called I^2 R heating (that is "I squared R" heating). The main point though is the coil itself gets very hot and the hot coil transfers HEAT to the cooking pan from the bottom.

The induction kind is different. The resistance of the coil is low and so the coil itself does not get very hot. That is because it does not depend on the thermal transfer of heat to the pot or pan like the resistance heating type coil does. Instead, it depends on a magnetic field. The AC current in the coil generates a changing magnetic field that is coupled to the bottom of the cooking pan so there is little or no heat transfer taking place, only magnetic coupling.
As the field cuts the bottom of the cooking pan, the bottom starts to create small circulating currents called "Eddy Currents". The currents in the bottom then create resistance heating and that makes the bottom of the pan get very hot. The heat from the bottom then can cook the food. Note that the current in the coil has to be AC in order to transfer energy to the bottom of the pan. In the resistance type coil the current does not have to be AC, but it usually is anyway because most households have AC available and it works with AC too.

So the difference between the two is one cooks by direct resistance heating, and the other cooks by indirect resistance heating through magnetic coupling and the magnetic field itself does not get hot either, only the bottom of the pan.

To understand this better you can look up "Eddy Current" and see what you think.
 

MaxHeadRoom

Joined Jul 18, 2013
21,182
For a number of years I worked on Industrial versions of Induction heating and in every case, the work coil was water cooled, but this is mainly due to the very high radiated heat of the ferrous metal part being processed.
Without the part in place, the coil itself would not get that that hot. As no real 'Induced' heat or eddy current was present was present.
Max.

1595257780048.png
 

MrAl

Joined Jun 17, 2014
7,748
For a number of years I worked on Industrial versions of Induction heating and in every case, the work coil was water cooled, but this is mainly due to the very high radiated heat of the ferrous metal part being processed.
Without the part in place, the coil itself would not get that that hot. As no real 'Induced' heat or eddy current was present was present.
Max.

View attachment 212598
Yeah some heat will get coupled back to the coil from the bottom (or other surface) of the work piece.

Nice drawing BTW :)
 
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