Armature Reaction in Alternator at Unity Power Factor

Armature reaction in an alternator refers to the effect that the magnetic field produced by the armature current has on the main magnetic field produced by the rotor poles. This interaction changes the flux distribution in the air gap and affects the generated voltage.

The nature of the armature reaction depends on the power factor of the load connected to the alternator.

• Lagging Power Factor: For a lagging power factor load, the armature current lags the induced voltage. The magnetic field produced by the armature tends to oppose the main field. This effect is known as de-magnetizing.
• Leading Power Factor: For a leading power factor load, the armature current leads the induced voltage. The magnetic field produced by the armature tends to aid the main field. This effect is known as magnetizing.
• Unity Power Factor: For a unity power factor load, the armature current is in phase with the induced voltage. In this case, the magnetic field produced by the armature is approximately at 90 degrees (perpendicular) to the main magnetic field.

Effect of Unity Power Factor on Armature Reaction

When the load is at unity power factor, the armature current is in phase with the induced voltage. The resulting magnetic field of the armature acts across the main magnetic field. This effect is called cross-magnetizing.

The cross-magnetizing effect distorts the main magnetic field but does not directly strengthen or weaken it significantly across the entire pole face. Instead, it shifts the flux density distribution, causing a decrease in flux density at the leading pole tip and an increase at the lagging pole tip.

In summary, for unity power factor loads, the armature reaction in an alternator is predominantly cross-magnetizing.