In an a.c. machine, any flux setup by load current which does not contribute to the useful flux of the machine is a leakage flux. the effect of this leakage flux is to setup self induce emf in the armature winding.

the leakage flux may be classified as following.
1.     Slots leakage
2.     tooth head leakage 
3.     coil end or overhanging leakage

the voltage induce in the armature windings by air gap flux is called air gap voltages .

The leakage flux also induce voltage in the armature windings. these taken into account by introduction of leakage reactance drops. Most of reluctances of the magnetic circuits for armature leakage fluxes are due to air paths. The fluxes are , therefore , nearly proportional to the armature currents producing them and are in phase with these currents. For this reason , the voltage induce in the armature winding can be taken into account by use of constant leakage reactance for the phases , which multiplied by the phasor currents , give the component voltages induce in the phases by leakage flux . these voltage are the leakage reactance drops and lead the currents producing them by 90 degree.

This leakage flux makes the armature winding inductive in nature. So winding possesses a leakage reactance, in addition to the resistance.

The effect of armature flux in an alternator depends on Magnitude of current flowing through the armature winding and Nature of power factor of the load connected to the alternator.

the synchronous reactance of an alternator is equal to Sum of leakage reactance and armature reaction reactance.


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