Armature Reaction in Alternator (Armature Reaction in Synchronous Generator)

Armature Leakage Reactance

Leakage flux is the Magnetic flux created by the load current in an AC electrical machine but does not add to the Machine’s useful Magnetic flux. A Self-induced EMF is created in the Machine’s armature winding as a result of this leakage flux. The leakage flux may be Classified into three Categories as follows −

• Slot leakage
• Tooth head leakage
• Overhang or coil end leakage

The Armature Windings of Electrical machines experience EMFs as a result of leakage fluxes. By adding leakage Reactance drops and deviating by 90 degrees from the current producing them, these leakage EMFs are taken into Consideration.

Armature Reaction in Synchronous Generator

A magnetic flux is created when current passes through the armature winding of a synchronous generator or alternator, creating a Magnetomotive force (MMF). This armature flux interacts with the main rotor pole flux, changing it to a value that is either less than or greater than the original main pole flux in the alternator. The armature reaction can therefore be described as follows:

Armature reaction is the term for the impact of armature flux on the main field flux generated by the rotor field poles.

The rotor field poles in the alternator are what generate the majority of the field flux. The armature currents that create the armature flux are set up when the alternator is loaded. The main flux in the machine’s air gap is modified by the armature flux. Armature reaction is the name given to this armature flux action. The magnitude of the armature current and the load’s power factor both affect how the armature reaction behaves. Whether the armature flux opposes, supports, or distorts the main field flux depends on the load’s power.

Effect of Load Power Factor on Armature Reaction

• 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.

Summary of Nature of Armature Reaction

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 Armature reaction is brought about by a magnetic flux that rotates at synchronous speed and has a constant magnitude.
• When the Alternator supplies a load at unity power factor, then the armature reaction has cross-magnetising effect.
• The armature reaction is Partially Cross-magnetizing and partially demagnetizing when the alternator supplies a load at a lagging power factor.
• The armature reaction is Partially Cross-magnetizing and Partially magnetising. When the alternator is supplying a load at a leading power factor.
• 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°.