Leakage inductance derives from the electrical property of an imperfectly coupled transformer whereby each winding behaves as a self-inductance in series with the winding's respective ohmic resistance constant. These four winding constants also interact with the transformer's mutual inductance. The winding leakage inductance is due to leakage flux not linking with all turns of each imperfectly coupled winding.

Leakage reactance is usually the most important element of a power system transformer due to power factor, voltage drop, reactive power consumption and fault current considerations.^[1][2]

Leakage inductance depends on the geometry of the core and the windings. 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.^[3]

Leakage inductance applies to any imperfectly coupled magnetic circuit device including motors.^[4]

Leakage inductance and inductive coupling factor

The magnetic circuit's flux that does not interlink both windings is the leakage flux corresponding to primary leakage inductance L_P^σ and secondary leakage inductance L_S^σ. Referring to Fig. 1, these leakage inductances are defined in terms of transformer winding open-circuit inductances and associated coupling coefficient or coupling factor ${\displaystyle k}$.^[5][6][7]

The primary open-circuit self-inductance is given by

${\displaystyle L_{oc}^{pri}=L_{P}=L_{M}+L_{P}^{\sigma ))$ ------ (Eq. 1.1a)

where

${\displaystyle L_{P}^{\sigma }=L_{P}\cdot {(1-k)))$ ------ (Eq. 1.1b)

${\displaystyle L_{M}=L_{P}\cdot {k))$ ------ (Eq. 1.1c)

and

• ${\displaystyle L_{oc}^{pri}=L_{P))$ is primary self-inductance
• ${\displaystyle L_{P}^{\sigma ))$ is primary leakage inductance
• ${\displaystyle L_{M))$ is magnetizing inductance
• ${\displaystyle k}$ is inductive coupling coefficient

It therefore follows that the open-circuit self-inductance and inductive coupling factor ${\displaystyle k}$ are given by

${\displaystyle L_{oc}^{sec}=L_{S}=L_{M2}+L_{S}^{\sigma ))$ ------ (Eq. 1.2), and,

${\displaystyle k={\frac {\left|M\right|}{\sqrt {L_{P}L_{S))))}$, with 0 < ${\displaystyle k}$ < 1 ------ (Eq. 1.3)

where

${\displaystyle L_{S}^{\sigma }=L_{S}\cdot {(1-k)))$

${\displaystyle L_{M2}=L_{S}\cdot {k))$

and

• ${\displaystyle M}$ is mutual inductance
• ${\displaystyle L_{oc}^{sec}=L_{S))$ is secondary self-inductance
• ${\displaystyle L_{S}^{\sigma ))$ is secondary leakage inductance
• ${\displaystyle L_{M2}=L_{M}/a^{2))$ is magnetizing inductance referred to the secondary
• ${\displaystyle k}$ is inductive coupling coefficient
• ${\displaystyle a\equiv {\sqrt {\frac {L_{p)){L_{s))))\approx N_{P}/N_{S))$^[a] is the approximate turns ratio

The electric validity of the transformer diagram in Fig. 1 depends strictly on open-circuit conditions for the respective winding inductances considered. More generalized circuit conditions are as developed in the next two sections.

Inductive leakage factor and inductance

A nonideal linear two-winding transformer can be represented by two mutual inductance-coupled circuit loops linking the transformer's five impedance constants as shown in Fig. 2.^{[6][16][17][18]}

where

• M is mutual inductance
• ${\displaystyle R_{P))$ & ${\displaystyle R_{S))$ are primary and secondary winding resistances
• Constants ${\displaystyle M}$, ${\displaystyle L_{P))$, ${\displaystyle L_{S))$, ${\displaystyle R_{P))$ & ${\displaystyle R_{S))$ are measurable at the transformer's terminals
• Coupling factor ${\displaystyle k}$ is defined as

${\displaystyle k=\left|M\right|/{\sqrt {L_{P}L_{S))))$, where 0 < ${\displaystyle k}$ < 1 ------ (Eq. 2.1)

The winding turns ratio ${\displaystyle a}$ is in practice given as

${\displaystyle a={\sqrt {L_{P}/L_{S))}=N_{P}/N_{S}\approx v_{P}/v_{S}\approx i_{S}/i_{P}=}$ ------ (Eq. 2.2).^[19]

where

• N_P & N_S are primary and secondary winding turns
• v_P & v_S and i_P & i_S are primary & secondary winding voltages & currents.

The nonideal transformer's mesh equations can be expressed by the following voltage and flux linkage equations,^[20]

${\displaystyle v_{P}=R_{P}\cdot i_{P}+{\frac {d\Psi {_{P))}{dt))}$ ------ (Eq. 2.3)

${\displaystyle v_{S}=-R_{S}\cdot i_{S}-{\frac {d\Psi {_{S))}{dt))}$ ------ (Eq. 2.4)

${\displaystyle \Psi _{P}=L_{P}\cdot i_{P}-M\cdot i_{S))$ ------ (Eq. 2.5)

${\displaystyle \Psi _{S}=L_{S}\cdot i_{S}-M\cdot i_{P))$ ------ (Eq. 2.6),

where

• ${\displaystyle \Psi }$ is flux linkage
• ${\displaystyle {\frac {d\Psi }{dt))}$ is derivative of flux linkage with respect to time.

These equations can be developed to show that, neglecting associated winding resistances, the ratio of a winding circuit's inductances and currents with the other winding short-circuited and at open-circuit test is as follows,^[21]

${\displaystyle \sigma =1-{\frac {M^{2)){L_{P}L_{S))}=1-k^{2}\approx {\frac {L_{sc)){L_{oc))}\approx {\frac {L_{sc}^{sec)){L_{P))}\approx {\frac {L_{sc}^{pri)){L_{S))}\approx {\frac {i_{oc)){i_{sc))))$ ------ (Eq. 2.7),

where,

• i_oc & i_sc are open-circuit and short-circuit currents
• L_oc & L_sc are open-circuit and short-circuit inductances.
• ${\displaystyle \sigma }$ is the inductive leakage factor or Heyland factor^[22][23][24]
• ${\displaystyle L_{sc}^{pri))$ & ${\displaystyle L_{sc}^{sec))$ are primary and secondary short-circuited leakage inductances.

The transformer inductance can be characterized in terms of the three inductance constants as follows,^[25][26]

${\displaystyle L_{M}=a{M))$ ------ (Eq. 2.8)

${\displaystyle L_{P}^{\sigma }=L_{P}-a{M))$ ------ (Eq. 2.9)

${\displaystyle L_{S}^{\sigma }=L_{S}-{M}/a}$ ------ (Eq. 2.10) ,

where,

• L_M is magnetizing inductance, corresponding to magnetizing reactance X_M
• L_P^σ & L_S^σ are primary & secondary leakage inductances, corresponding to primary & secondary leakage reactances X_P^σ & X_S^σ.

The transformer can be expressed more conveniently as the equivalent circuit in Fig. 3 with secondary constants referred (i.e., with prime superscript notation) to the primary,^[25][26]

${\displaystyle L_{S}^{\sigma \prime }=a^{2}L_{S}-aM}$

${\displaystyle R_{S}^{\prime }=a^{2}R_{S))$

${\displaystyle V_{S}^{\prime }=aV_{S))$

${\displaystyle I_{S}^{\prime }=I_{S}/a}$.

Since

${\displaystyle k=M/{\sqrt {L_{P}L_{S))))$ ------ (Eq. 2.11)

and

${\displaystyle a={\sqrt {L_{P}/L_{S))))$ ------ (Eq. 2.12),

we have

${\displaystyle aM={\sqrt {L_{P}/L_{S))}\cdot k\cdot {\sqrt {L_{P}L_{S))}=kL_{P))$ ------ (Eq. 2.13),

which allows expression of the equivalent circuit in Fig. 4 in terms of winding leakage and magnetizing inductance constants as follows,^[26]

${\displaystyle L_{P}^{\sigma }=L_{S}^{\sigma \prime }=L_{P}\cdot (1-k)}$ ------ (Eq. 2.14 ${\displaystyle \equiv }$ Eq. 1.1b)

${\displaystyle L_{M}=kL_{P))$ ------ (Eq. 2.15 ${\displaystyle \equiv }$ Eq. 1.1c).

The nonideal transformer in Fig. 4 can be shown as the simplified equivalent circuit in Fig. 5, with secondary constants referred to the primary and without ideal transformer isolation, where,

${\displaystyle i_{M}=i_{P}-i_{S}^{'))$ ------ (Eq. 2.16)

• ${\displaystyle i_{M))$ is magnetizing current excited by flux Φ_M that links both primary and secondary windings
• ${\displaystyle i_{P))$ is the primary current
• ${\displaystyle i_{S}'}$ is the secondary current referred to the primary side of the transformer.

Refined inductive leakage factor

Referring to the flux diagram in Fig. 6, the following equations hold:^[28][29]

σ_P = Φ_P^σ/Φ_M = L_P^σ/L_M^[32] ------ (Eq. 3.1 ${\displaystyle \approx }$ Eq. 2.7)

In the same way,

σ_S = Φ_S^σ'/Φ_M = L_S^σ'/L_M^[33] ------ (Eq. 3.2 ${\displaystyle \approx }$ Eq. 2.7)

And therefore,

Φ_P = Φ_M + Φ_P^σ = Φ_M + σ_PΦ_M = (1 + σ_P)Φ_M^[34][35] ------ (Eq. 3.3)

Φ_S^' = Φ_M + Φ_S^σ' = Φ_M + σ_SΦ_M = (1 + σ_S)Φ_M^[36][37] ------ (Eq. 3.4)

L_P = L_M + L_P^σ = L_M + σ_PL_M = (1 + σ_P)L_M^[38] ------ (Eq. 3.5 ${\displaystyle \approx }$ Eq. 1.1b & Eq. 2.14)

L_S^' = L_M + L_S^σ' = L_M + σ_SL_M = (1 + σ_S)L_M^[39] ------ (Eq. 3.6 ${\displaystyle \approx }$ Eq. 1.1b & Eq. 2.14),

where

• σ_P & σ_S are, respectively, primary leakage factor & secondary leakage factor

• Φ_M & L_M are, respectively, mutual flux & magnetizing inductance

• Φ_P^σ & L_P^σ are, respectively, primary leakage flux & primary leakage inductance

• Φ_S^σ' & L_S^σ' are, respectively, secondary leakage flux & secondary leakage inductance both referred to the primary.

The leakage ratio σ can thus be refined in terms of the interrelationship of above winding-specific inductance and Inductive leakage factor equations as follows:^[40]

${\displaystyle \sigma =1-{\frac {M^{2)){L_{P}L_{S))}=1-{\frac {a^{2}M^{2)){L_{P}a^{2}L_{S))}=1-{\frac {L_{M}^{2)){L_{P}L_{S}{^{'))))=1-{\frac {1}((\frac {L_{P)){L_{M))}.{\frac {L_{S}^{')){L_{M))))}=1-{\frac {1}{(1+\sigma _{P})(1+\sigma _{S})))}$ ------ (Eq. 3.7a to 3.7e).

Applications

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. In this case, actually working useful parameter is not the leakage inductance value but the short-circuit inductance value.

Commercial and distribution transformers rated up to say 2,500 kVA are usually designed with short-circuit impedances of between about 3% and 6% and with a corresponding ${\displaystyle X/R}$ ratio (winding reactance/winding resistance ratio) of between about 3 and 6, which defines the percent secondary voltage variation between no-load and full load. Thus for purely resistive loads, such transformers' full-to-no-load voltage regulation will be between about 1% and 2%.

High leakage reactance transformers are used for some negative resistance applications, such as neon signs, where a voltage amplification (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.

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. Transformer leakage reactance has a large role in limiting circuit fault current within the maximum allowable value in the power system.^[2]

In addition, the leakage inductance of a HF-transformer can replace a series inductor in a resonant converter.^[41] In contrast, connecting a conventional transformer and an inductor in series results in the same electric behavior as of a leakage transformer, but this can be advantageous to reduce the eddy current losses in the transformer windings caused by the stray field.