The Surface Impedance approach enables simulation of high frequency induction heating in few minutes, without building a huge mesh. Induced heat in a steel strip is accurately calculated in 10 minutes for 300 kHz, while traditional FEM simulation takes about 10 hours.

## About the Surface Impedance approach

For induction heating with high frequencies, when EM field has very shallow penetrates into a workpiece, the skin depth may be measured in microns. Traditional FEM calculations, which require resolution of the skin layer by a mesh, takes hours and even days.

However, for such small penetration, induced current can be precisely calculated including a surface impedance to the equations and neglecting magnetic field inside the workpiece. In this case, we can avoid building a super fine mesh for the workpiece but use rough 2D surface mesh only.

The computational gain depends on frequency. While computational time of traditional FEM dramatically increases with frequency, exceeding a week for MHz cases, **the Surface Impedance approach always delivers results in several minutes**.

Read more about the Surface Impedance approach in __the paper by Nerg & Partanen (2001)__

## Calculation accuracy

The Surface Impedance approach **is mostly used to estimate induced current and Joule heat distribution**, since it is predicted within 97% of accuracy already at relatively low frequencies. The skin-depth to workpiece-thickness (*d/D*) is suggested as a non-dimensional criteria here.

While the Surface Impedance approach predicts apparent power in a coil and Joule heat in a workpiece already at *d/D* < 10%, accurate temperature prediction is possible only at *d/D* < 0.5%. Heat is defined as a surface source, alike a cooling condition (convection or/and radiation). Therefore, temperature is underestimated with up to 30% inaccuracy for lower frequencies.

So, the Surface Impedance approach, implemented in __CENOS Induction Heating__ simulation software, is a powerful design tool for high frequency installations.

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