Proximity effect (electrical engineering)
Proximity effect (of English. Proximity effect , proximity effect ") referred to in the electrical engineering the increase of the ohmic resistance for alternating current of a winding relative to the single turn. It occurs between closely spaced conductors through which alternating current flows. The causes are inhomogeneous current density and eddy current losses, in turn caused by an inhomogeneous alternating field.
description
In the picture, the creation of the effect in the winding window of a transformer core is sketched for the simplest case. The current displacement in the conductors is also indicated schematically - for the case of a single pair of conductors - so it already occurs in this simplest case. It is clearly visible that the current flows are concentrated on the inside of the conductors and the conductor cross-section is no longer fully utilized.
In the example, the proximity effect occurs in tightly packed windings of transformers of switched-mode power supplies at higher frequencies. In this case it then summarizes the effect of the stray fields between the individual conductor pairs, which then form the winding layers, in the coils and transformers .
Another example in which the proximity effect has to be prevented are the coils of induction hotplates .
The current displacements are caused by the so-called stray fluxes of the alternating fields of the magnetic circuit .
The cause of the proximity effect is promoted by:
- Adjacent coil wires,
- Finite magnetic conductivity of the material in the magnetic circuit (allows for a stray field)
- Air gaps in the magnetic circuit (extreme case with rod core or air core coil).
In addition, the stray fields also penetrate the winding wires and induce eddy currents there. Additional conduction losses and thus heat arise, the electrical quality deteriorates and, in power applications, the component can be destroyed by the increase in temperature. These eddy currents occur in addition to the proximity effect in the case of leakage flux.
The proximity effect must not be confused with the skin effect . The skin effect also occurs with a single, free and straight conductor. Both effects cause an ohmic resistance that increases with frequency, but the causes are different. The proximity effect also occurs with conductor packages made of thin wires at low frequencies. In practice, proximity and skin effects occur together in high-frequency coils.
This can be remedied by:
- Symmetrical nesting of turns and windings (thrust winding).
- If possible, single-layer windings for long cores.
- No “dead” windings in the vicinity of alternating current-carrying windings.
- Padding of the winding in the area of short air gaps (also effective for suppressing eddy currents).
- Resonance coupling for transformers made from rod core or air coils.
- Subdivide the solid wires into high-frequency strands (separately insulated stranded wires), similar to the laminated cores for transformers. It is not necessary here for the strands to be intertwined, as is the case with real HF strands; twisting the strands along the longitudinal axis is sufficient. A "wrong", i.e. H. only twisted , "high-frequency stranded wire" already brings the desired improvement, since the stranded wires with the same diameter change their position in the stray field (twisted conductor).
Capacitive shunts at higher frequencies (typically> 1 MHz. Even harmonics in switched-mode power supplies!) Mean that the advantage of the solution with the stranded wire is canceled out again.
In the case of strip conductors, particular attention must be paid to the direction of the stray fields; they should run lengthways and not across them.