Leakage reactance

Leakage inductance is that property of an electrical transformer that causes a winding to appear to have some pure inductance in series with the mutually-coupled transformer windings.

Usually, this is an undesirable property, but it is sometimes deliberately introduced into a transformer that is used as a ballast for a gas discharge lamp such as a fluorescent lamp, or in a transformer used for arc welding. In this case, the leakage inductance limits the current flow to the desired magnitude.

Leakage inductance is primarily controlled by the design of the windings and the geometry of the magnetic core used to form the transformer.

Voltage drop across the leakage reactance results in often undesirable supply regulation with varying transformer load. But it can also be useful for harmonic isolation (attenuating higher frequencies) of some loads.

Leakage inductance applies to any imperfectly-coupled magnetic circuit device including motors; it reduces efficiency (ratio of power out to power in) of transformers and other inductive devices.

Leakage inductance can be estimated during design of a transformer from the dimensions of the windings on the core. More precise estimates can be obtained from solution of the magnetic field around the core, using computerized methods.

Several measurement methodologies are employed to assess transformer inductance. The primary one is measurement of various quantities at the transformer terminals, with one coil driven and the other coil open-circuited. Another methodology employs both open-circuit measurements as well as short-circuit measurements, where one coil is short circuited and the other driven. A different methodology is based on assessing magnetic flux of each winding in a "window" of the core where the coil windings pass through it. The resulting assessments of leakage inductance differ because different kinds of losses are taken into account by the methodologies.

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Wikipedia