Leakage inductance is the property of an electrical transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 that causes a winding to appear to have some inductance in series with the mutually-coupled transformer windings. This is due to imperfect coupling of the windings and creation of leakage flux

Magnetic flux

Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...

 which does not link with all the turns of the winding.

The leakage flux alternately stores and discharges magnetic energy with each electrical cycle and thus effectively acts as an inductor

Inductor

An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

 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.

Definition of leakage inductance

The magnetic flux linked to both the primary winding and the secondary winding is said to be the
main flux, (φ₁₂ or φ₂₁). The magnetic flux which interlinks only with the primary winding, and does not interlink with the secondary winding, is said to be the primary leakage flux, φ_σ1. The magnetic flux which interlinks with the secondary winding, and does not interlink with the primary winding is said to be the secondary leakage flux, φ_σ2. The primary side leakage flux becomes the primary side leakage inductance, and the secondary side leakage flux becomes the secondary side leakage inductance. Defining k to be the coupling coefficient, and denoting the leakage inductances of the primary side and the secondary side as L_e1 and L_e2 respectively, it follows that:



If one winding of a transformer which has two windings is short-circuited, the inductance measured from the other winding is the leakage inductance. It is also called short-circuited inductance . Denoting the leakage inductances of the primary and secondary sides as L_sc1 and L_sc2 respectively, it follows that:

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 (accepting 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 sign

Neon sign

Neon signs are made using electrified, luminous tube lights that contain rarefied neon or other gases. They are the most common use for neon lighting, which was first demonstrated in a modern form in December, 1910 by Georges Claude at the Paris Motor Show. While they are used worldwide, neon signs...

s, 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

Negative resistance

Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...

 characteristic of these gas discharge lamps would cause them to conduct excessive current and be destroyed.

Transformers with variable leakage inductance are used to control the current in arc welding

Arc welding

Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct or alternating current, and consumable or non-consumable electrodes...

 sets. In these cases, the leakage inductance limits the current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

flow to the desired magnitude.

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.