In an AC electrical machine, the magnetic flux set up by the load current which does not contribute to the useful magnetic flux of the machine is known as leakage flux. This leakage flux sets up a self-induced EMF in the armature winding of the machine. The leakage flux may be classified into three categories as follows −
The leakage fluxes in the electrical machines induce EMFs in the armature windings. These leakage EMFs are taken into account by introduction of leakage reactance drops and lead the current producing them by 90°.
When current flows through the armature winding of the synchronous generator or alternator, the resulting Magnetomotive force (MMF) produces a magnetic flux. This armature flux reacts with the main rotor pole flux and causing the resultant flux in the alternator to become either less than or more than the original main pole flux. Therefore, the armature reaction can be defined as follows −
The effect of armature flux on the main field flux produced by the rotor field poles is known as armature reaction.
The main field flux in the alternator is produced by the rotor field poles. When the alternator is loaded, the armature currents are set up which produce the armature flux. This armature flux modifies the main flux in the air gap of the machine. This action of armature flux is called armature reaction. The effect of the armature reaction depends on the magnitude of armature current and on the power factor of the load. The power of the load determines whether the armature flux distorts, opposes or assists the main field flux.
Case 1 – Unity Power Factor Load − When the load has unity power factor i.e. pure resistive load, then the armature reaction distorts the main field flux in the air gap of the machine and not to weaken it.
Case 2 – Zero Lagging Power Factor Load− When the load power factor is zero lagging i.e. pure inductive load, then the armature reaction weaken the main field flux and not to distort it. Due to the reduction in the main field flux, the generated EMF is decreased.
Case 3 – Zero Leading Power Factor Load− When the load power factor is zero leading i.e. pure capacitive load, then the armature reaction strengthens the main field flux and not to distort it. This causes an increase in the generated voltage.
The summary of nature of armature reaction can be drawn for an alternator or synchronous generator supplying a balanced 3-phase load as follows −
The magnetic flux causes the armature reaction is constant in magnitude and rotates at synchronous speed.
When the alternator supplies a load at unity power factor, then the armature reaction has cross-magnetising effect.
When the alternator supplies a load at lagging power factor, the armature reaction is partly cross-magnetising and partly demagnetising.
When the alternator supplies a load at leading power factor, then the armature reaction is partly cross-magnetising and partly magnetising.
If the magnetic flux causing the armature reaction is assumed to act independently of the main field flux, then it induces EMF in each phase which lags the respective phase currents by 90°.