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.

Advantages and disadvantages

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.

Usually, leakage inductance is an undesirable property, but it is sometimes deliberately introduced into a transformer used as a ballast for a gas discharge lamp such as a neon sign, fluorescent lamp, or a high-intensity discharge lamp. 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. Without the leakage inductance, the negative resistance characteristic of these gas discharge lamps would cause them to conduct excessive current and be destroyed.

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.

Transformers (electrical)