Proximity effect (electromagnetism)

The concept of proximity effect ( eingedeutscht of english proximity effect: ger " neighborhood effect" ) called in electrical engineering, the impact of the current constriction or current displacement between two closely adjacent conductors under the influence of alternating currents due to the magnetic flux leakage between them, caused by opposing currents in the conductors.

Description

The picture shows the reason for this effect is sketched in the winding window of a transformer core for the simplest case. The displacement current in the conductors is also - indicated schematically, so it occurs already in the simplest case - the case of a single pair of conductors. It is clearly visible that the current flows are concentrated on the inner sides of the conductor and the conductor cross -section is not more fully exploited.

The proximity effect occurs in tightly packed coils of transformers of switching power supplies at higher frequencies in the example. In this case, he summarizes the effect of leakage fields between the conductor pairs, which then form the winding layers in the coils and transformers together.

The current displacements are caused by the so-called leakage flux of the alternating fields of the magnetic circuit.

The cause of the proximity effect is supported by:

  • Adjacent coil wires,
  • Finite conductivity of the magnetic material in the magnetic circuit ( stray field can be )
  • Air gaps in the magnetic circuit (extreme case of rod-core or air core coil ).

In addition, the stray fields also penetrate the winding wires and induce eddy currents there. It incurs additional line losses and therefore heat, the electrical quality deteriorates and power applications, the component can be damaged by temperature increase. These eddy currents occur in stray flux in addition to the proximity effect.

The proximity effect should not be confused with the skin effect, this also occurs with a single, free, straight conductor and is due to the displacement current in the conductor itself

This can be remedied by:

  • Symmetrical interleaving the windings and windings ( winding thrust ).
  • Possible single-layer windings for long cores.
  • No "dead" coils in the vicinity of AC current-carrying windings.
  • Reupholstering of the winding in the range of short air gaps (also effective for suppressing the eddy currents ).
  • Resonance coupling at transmitters from rod-core or air coils.
  • Dividing the solid wires to Hochfrequenzlitzen ( separately insulated stranded wires ), similar to the laminated cores for transformers. Here, it is not necessary that the strands are interlaced, as in the real RF - stranded, a twisting of the strands about the longitudinal axis is sufficient. A "wrong", that is just twisted, " Litz " already brings the desired improvement, since the wire strands on the same diameters change the situation in the stray field ( CTC ).

Through capacitive shunts at higher frequencies ( typically> 1 MHz. Too harmonics in switching power supplies! ) It comes to that, the advantage of the solution picks up again with the strand.

In practice, proximity and skin effect usually occur together.

For tape conductors is particularly important to pay attention to the direction of the stray fields; they should run along and not across them.

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