Magnetic shielding

During the last decades, the topic “magnetic shielding” has gained interest. The increased research on shielding is partly due to the increased number of electromagnetic interference problems: electrical devices all generate electromagnetic fields that may disturb other electrical devices if  they are too close to each other. To guarantee the proper functioning of electrical apparatus and safe working conditions for humans, electromagnetic fields should be reduced field so that they don’t exceed the reference levels of the European Community [2004/40/EC] anywhere in the target area – i.e. the region to be shielded. The magnetic field can be shielded by adding passive and active shields:

  • Passive shields are sheets in material with high conductivity (copper), high permeability (ferrite) or both (steel). Multilayered shields consisting of alternating layers in steel and copper seem to have an even higher shielding efficiency.
  • Active shields consist of coils with suitable currents that generate an opposite field: in every point of the shielded area, the field of the active shield is (more or less) opposite to the original stray field so that they cancel each other. Both the coil positions and the currents in the coils must be calculated accurately in order to “destroy” the field in the whole shielded area.

Applications

Shielding of an induction heater

An induction heater is used for the heating of conductive objects. The device uses an excitation coil (Fig. 1 and Fig. 2) that generates a strong alternating magnetic field. This field causes eddy currents that heat the workpiece. However, the field also causes “electromagnetic polution” of the area surrounding the induction heater.

 

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Figure 1: Left: Principle of passive and active shielding of an induction heater. Right: Experimental setup of the induction heater at real scale: the diameter of the outer compensation coils is 3 m and the total height is 2.3 m.

The required field reduction can be achieved by building a metal cage around the heater, but then, the device is not accesible any more for the process operator. Consequently, only a small passive shield is chosen: a steel or a copper ring (see Fig. 2). To obtain enough field reduction, an active shield is added: this active shield consists of 9 compensation coils below the feet of the operator and 9 coils above his head. The current in the coils is controlled as shown in Fig. 1 in order to always keep the active shield efficient. The result is a reliable shielding that reduces the magnetic field in a given target area by more than a factor 10. Fig. 3 and Fig. 4 illustrate the field distributions obtained by finite element calculations. In Fig. 3, the induction heater is unshielded, resulting in a high magnetic field in the target area (the white rectangle). In Fig. 4, a passive and an active shield reduce the field in the target area more than 10 times.

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Figure 2: Magnetic field distribution of the unshielded induction heater. The arrows represent the field direction and the colour shows the amplitude of the magnetic induction in nT. The workpiece to be heated and the excitation coil are in the lower left corner. The white rectangle is the target area where the field should be reduced.
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Figure 3: Magnetic field distribution of the induction heater with passive and active shields. The arrows represent the field direction and the colour shows the amplitude of the magnetic induction in nT. The white line at r = 0.3 m is the passive shield in copper and the white dots represent the compensation coils of the active shield.

Relevant publications

P. Sergeant, L. Dupré, M. De Wulf and J. Melkebeek, "Optimizing active and passive magnetic shields in induction heating by a Genetic Algorithm", IEEE Trans. Magn., Vol. 39, No. 6, pp. 3486-3496, Nov. 2003.   

P. Sergeant, U. Adriano, L. Dupré, O. Bottauscio, M. De Wulf, M. Zucca and J. Melkebeek, "Passive and active electromagnetic shielding of induction heaters", IEEE Trans. Magn., Vol. 40, No. 2, pp. 675-678, Mar. 2004.

P. Sergeant and L. Dupré, "Software Control of an Active Magnetic Shield", IEE Proceedings Science, Measurement and Technology, Vol. 153, No. 1, pp. 13-21, Jan. 2006.

P. Sergeant, M. Zucca, L. Dupré and P.E. Roccato, "Magnetic shielding of a cylindrical shield in nonlinear hysteretic material", IEEE Transactions on Magnetics, Vol. 42, No. 10, pp. 3189-3191, Oct. 2006.
 

External links

Reference levels for magnetic fields

European Community, occupational exposure: 2004/40/EC

European Community, general public exposure: 1999/519/EC

ICNIRP (International Commission on Non-Ionizing Radiation Protection)

Health aspects of magnetic and electric fields

EMFS (Electric and Magnetic Fields): factual, comprehensive and fair information on power-frequency electric and magnetic fields

BBEMG (Belgian BioElectroMagnetic Group): effecten van 50 Hz elektrische en magnetische velden die ontstaan bij het transport en het gebruik van elektrische energie in het privé- en bedrijfsleven.

Contact

For more information: peter.sergeant@ugent.be luc.dupre@ugent.be