Skin effect, frequency and heating

[elcraft]

Hi.

I'm trying to understand why smaller skin depths are better for induction heating.

Skin effect means that the highest the frequency is, the thinest the skin depth is (or the depth is simply smaller because of the material). That means the section of the workpiece with electric current in it is smaller, and according to R=ρ*L/S (ρ - electrical resistivity, L - length, S - section area) the resistance is higher, thus heating more.

But if the resistance is higher, the current decreases so that the power remains the same, right? So what makes the difference when the skin depth is changed?

 

[jonlg_uk]

Hi.



But if the resistance is higher, the current decreases so that the power remains the same, right? So what makes the difference when the skin depth is changed?

Think of it like this:

The resistance only gets higher because the temperature rises, due to increased current. Yes the current will decrease with increased temperature and resistance, but as soon as this current decreases the temperature decreases and the resistance will follow and the current will once again increase. The temperature will now increase again.

This isn't exactly what is going on but it gives you an idea.

 

[elcraft]

Ok but what difference makes a change in skin depth so that lower skin depths are better for heating?

 

[jonlg_uk]

Ok but what difference makes a change in skin depth so that lower skin depths are better for heating?

For a start, higher frequencies with shorter wave lengths will propagate less, in terms of distance than a lower frequency wave. Think of it like this if because higher frequency waves are oscillating more rapidly their energy dissipates sooner, thus they travel less than lower frequency with longer wave lengths.

Knowing this if we consider a high frequency EM wave impinging on the surface of a block of metal, you will see eddy currents form, a number of skin depths deep into the metal. The number of skin depths will depend on the resistivity of the metal block and the frequency of the impinging wave. High resistivity i.e. poorer conductivity means that the current will get attenuated more and as a result the temperature will increase. Yes the current will reduce, but voltages will "build up" because of the "electron pile up" caused by the high resistivitiy due to the temperature increase. As a result the power will remain constant (I*V), and it is the power dissapation that causes the heating effect.

In addition these swirling eddy currents create their own magnetic field that consequently opposes the magnetic field that created it. Thus it effectively acts as an insulator ensuring that the impinging magnetic field does not penetrate deeper and further into the metal and cause eddy currents at greater thicknesses.

With all this current confined to the surface of the metal the current density is much greater at the surface and as a result heating will take place as you have so much current passing through a relatively thin metal conductor.

Hope this helps.


J

 

[alphysicist]

Hello,

You are correct in understanding that skin effect is related to the decrease in current as frequency increases, resulting in a smaller skin depth and higher resistance. However, in induction heating, the goal is to transfer heat to the workpiece through the use of induced eddy currents. These eddy currents are generated by the alternating magnetic field produced by the induction coil.

When the skin depth is smaller, the surface area of the workpiece in which the eddy currents are induced is also smaller. This means that the heat is concentrated in a smaller area, resulting in a higher temperature. Additionally, because the current is concentrated in a smaller area, it is able to penetrate deeper into the workpiece, allowing for more efficient heating.

In contrast, a larger skin depth would result in a larger surface area of the workpiece being heated, but at a lower temperature. This is because the induced eddy currents are spread out over a larger area, resulting in a lower concentration of heat.

In summary, smaller skin depths are better for induction heating because they allow for a more concentrated and efficient transfer of heat to the workpiece. I hope this helps clarify the relationship between skin effect, frequency, and heating in induction heating.