Leakage flux
The leakage flux (engl. Magnetic leakage flux ), or magnetic flux leakage with the symbols Φ, is a term used in electrical engineering and describes a portion of the magnetic flux which may occur in an array of two or more conductor loops and the magnetic coupling.
General
Due to a spatially adjacent arrangement of two or more conductor loops (these are implemented in technology, for example, in the form of coils and also referred to as turns), mutual inductances occur in one conductor loop in relation to the other conductor loops . The magnetic flux that changes over time causes - described by the law of induction - an electrical voltage in the coils or, if the conductor loops are closed, an electrical current .
Ideally, the magnetic coupling between the individual coils is perfect; the complete magnetic flux from the generating conductor loop penetrates all other neighboring conductor loops.
Since the magnetic flux is a source-free vector field (this fact is expressed by always closed flux density lines), the magnetic flux is also always closed, and there are no sources or sinks. In practically realizable applications, this fact and the inevitable spatial distance between several conductor loops result in a magnetic flux which is caused by one conductor loop, but does not pass through the other, neighboring conductor loops. As a result, this component - known as leakage flux - does not contribute to the magnetic coupling of the conductor loops to one another and is therefore to be understood as a kind of "flux loss". The leakage flux is schematically sketched in the adjacent figure by the thinly drawn outer flux lines which are generated by loop 1, but do not pass through loop 2.
Leakage inductance
The concept of the leakage inductance (or leakage inductance) describes those inductance component which is formed in magnetically coupled systems by the leakage flux. The leakage inductances, usually referred to as L s or L σ , play an essential role in the model of the transformer, for example. The leakage inductance is determined using the same procedures and methods as any other inductance, except that only the leakage flux Φ s is taken into account.
Technical application
In transformers of flux leakage is a key criterion. In order to maximize the efficiency in the power transmission, attempts are made to keep the leakage flux small through design measures. This requires a spatially close arrangement of the individual windings to one another.
A further technical improvement is achieved through the use of materials with high magnetic conductivity in the so-called core of the transformer: this allows the flux to be guided through the individual windings in a targeted manner in the core material, which is magnetically highly conductive. The leakage flux is therefore strongly influenced by the presence of the iron core. The leakage flux is therefore minimized by this measure, since the mostly surrounding air, similar to vacuum , only has a relatively poor magnetic conductivity and thus a large part of the magnetic flux can be guided in the magnetic iron core. The low magnetic conductivity of vacuum is also known as vacuum permeability and is a natural constant .
Depending on the geometric design of the core and the arrangement of the windings to be coupled, the leakage flux can be kept very small. With toroidal transformers , the leakage flux is only small if all turns of the windings are evenly distributed on the toroid.
For a particularly low leakage flux, the windings are bifilar or interleaved with each layer (see also output transformer ). Such transformers also have good transmission behavior up to high frequencies.
Transformers that do not have a core material are also referred to as "air transformers"; they have a comparatively high leakage flux. This results in a low degree of efficiency in power transmission, which is why practically all transformers used for power transmission have core materials with the highest possible permeability.
With special transformers such as leakage field transformers , the leakage flux is intentionally increased by spacing the individual windings in order to achieve short-circuit resistance and current limitation at the output. This is used for bell transformers, welding transformers and ballast transformers for neon advertising tubes ( fluorescent tubes ). There is often an adjustable magnetic " bypass " to regulate the current.
The level of leakage flux says something about the level of the short-circuit voltage . The greater the leakage flux, the greater the short-circuit voltage and the lower the secondary-side voltage stiffness.
literature
- Horst Stöcker : Pocket book of physics. 4th edition, Verlag Harry Deutsch, Frankfurt am Main, 2000, ISBN 3-8171-1628-4
- Günter Springer: Expertise in electrical engineering. 18th edition, Verlag Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9
- Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems. 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 1990, ISBN 3-8085-5002-3
Individual evidence
- ↑ Wikibooks: Modelle des Transformer (continuation of the former encyclopedia article "Modell des Transformer" as a book project)