Synchronous Generator Armature Reaction at Lagging Power Factor

Armature reaction in a synchronous generator refers to the effect produced by the magnetic field generated by the armature currents on the main magnetic field (produced by the field winding). The nature of this reaction depends heavily on the power factor of the load connected to the generator.

Understanding Armature Reaction Effects

The armature reaction can be classified based on the load's power factor:

• Demagnetizing Reaction: This occurs when the armature flux acts directly opposite to the main field flux, weakening the overall magnetic field. This happens primarily at leading power factors.
• Magnetizing Reaction: This occurs when the armature flux acts in the same direction as the main field flux, strengthening the overall magnetic field. This is less commonly discussed as a distinct category but is the opposite of demagnetization.
• Cross-magnetizing Reaction: This occurs when the armature flux is perpendicular to the main field flux. It distorts the main field flux pattern but does not significantly increase or decrease its strength. This effect is prominent at unity power factor.

Armature Reaction for Lagging Power Factor Loads

When a synchronous generator supplies a load with a lagging power factor, the current in the armature windings lags behind the phase voltage (and the main magnetic field flux wave). Specifically:

• The armature flux wave is approximately 90 degrees lagging relative to the main field flux wave.
• This lagging relationship means that a component of the armature flux opposes the main field flux (causing a demagnetizing effect).
• Simultaneously, another component of the armature flux is perpendicular to the main field, causing a distortion or cross-magnetizing effect.

Therefore, for a load with a lagging power factor, the armature reaction is a combination of these two effects: it is partly demagnetizing and partly cross-magnetizing.

The interaction leads to a reduction in the effective air gap flux and also distorts the flux distribution. This combination is characteristic of lagging power factor operation in synchronous generators.