Optimizing a transformer driven active magnetic shield in induction heating

Peter Sergeant (Ghent University, Gent, Belgium)
Luc Dupré (Ghent University, Gent, Belgium)
Jan Melkebeek (Ghent University, Gent, Belgium)



To design an optimal active shield for the mitigation of the magnetic stray field around an induction heating device.


The active shield consists of several compensation coils in series and generates a counter field opposite to the main field. One extra compensation winding – the “generating compensation winding” (GCW) – is positioned close to the excitation coil and works as the secondary winding of a transformer. The power in this winding is used to drive the other compensation coils (the active shield), which are the load of the transformer. A circuit with passive components is inserted between the GCW and the other compensation coils. The shield is optimal if it achieves a high field reduction, while the energy dissipation is low. By using a genetic algorithm (GA) that minimizes an objective function, the optimization algorithm finds the optimal geometry and the optimal current for the GCW and the other compensation coils. The objective function uses time harmonic and axisymmetric finite element calculations.


The transformer driven active shield reduces the magnetic field effectively. It is cheap and easy to build, but it works well only for one frequency.

Research limitations/implications

The shield is sensitive to tuning of the passive circuit and to changes in the frequency of the induction heater.

Practical implications

This transformer driven shield is an alternative for the classical active shield with external power supply.


An active shield that does not need an external power supply is a cheap solution for the shielding of magnetic fields.



Sergeant, P., Dupré, L. and Melkebeek, J. (2005), "Optimizing a transformer driven active magnetic shield in induction heating", COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, Vol. 24 No. 4, pp. 1241-1257. https://doi.org/10.1108/03321640510615580

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