Leakage inductance
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Leakage inductance is that property of an electrical transformer that causes a winding to appear to have some self-inductance in series with the mutually-coupled transformer windings. This is due to imperfect coupling of the windings and creation of leakage flux which does not link with all the turns.
The leakage flux alternately stores and discharges magnetic energy with each electrical cycle and thus effectively acts as an inductor in series in each of the primary and secondary circuits.
Leakage inductance is primarily caused by the design of the core and the windings. Voltage is dropped across the leakage reactance, resulting in poorer supply regulation when the transformer is placed under load.
[edit] Applications of Leakage Inductance
Leakage inductance can be an undesirable property, as it causes the voltage to change with loading. In many cases it is useful. Leakage inductance has the useful effect of limiting the current flows in a transformer (and load) without itself dissipating power (excepting the usual non-ideal transformer losses). Transformers are generally designed to have a specific value of leakage inductance such that the leakage reactance created by this inductance is a specific value at the desired frequency of operation.
Power distribution transformers are usually designed with a leakage reactance of between 1% and 10% of the full load impedance. If the load is resistive and the leakage reactance is small (<10%) the output voltage will not drop by more than 0.5% at full load, ignoring other resistances and losses.
Leakage reactance is also used for some negative resistance devices, such as neon signs, where a transformer action is required as well as current limiting. In this case the leakage reactance is usually 100% of full load impedance, so even if the transformer is shorted out it will not be damaged. Without the leakage inductance, the negative resistance characteristic of these gas discharge lamps would cause them to conduct excessive current and be destroyed. Devices (e.g. most fluorescent lamps) that do not require a transformer will just use a ballast. Transformers with variable leakage inductance are used to control the current in arc welding sets. In these cases, the leakage inductance limits the current flow to the desired magnitude.
[edit] Further reading
Texas Instruments Magnetics Design Handbook covers leakage inductance, its causes and effects as well as how to design it out of a transformer.
