Operation Manual

THE INDUCTION HOB
Technical training
PRINCIPLE
- 8 -
CU3-INDUCTION-003UK-10/05
2.2. - Skin effect
An induced current in a metallic mass will only cause significant heating if it flows through a significant
resistor (P=RI
2
). A ferrite saucepan has only low resistivity. This is where a second natural
phenomenon occurs, which is called ‘Skin effect’.
2.2.1. - Definition
The propagation of the high-frequency current is not performed in the same way as a direct current.
Contrary to direct current, where current flows with consistency in a conductor, in HF its density varies
and decreases exponentially as you move away from the conductor surface.
¾ Example on copper wire supplied with high frequency
The current flows predominantly in wire periphery ‘e’. The decrease in
the effective cross-sectional area of the conductor causes an increase
in its resistance.
2.2.2. - Application
At a 20KHz frequency, and for a steel saucepan (magnetic ferritic material), the thickness of the
saucepan in which the induced currents flow is approximately 35 µm. This allows generating a current
in only a part of the saucepan bottom. The resistance becomes significant and the heating consequent
therein.
For a non-ferritic material, such as aluminium, the thickness is approximately 590 µm, the saucepan
behaves then as a quasi-zero resistor (short-circuit), which is prejudicial to electronics. The board will
take this discrepancy into account and will display the phenomenon by making the control panel flash.
Therefore, this type of material is not adapted.