Understanding Armature Reaction in Alternators

Armature reaction is a phenomenon that occurs in an alternator (synchronous generator) when the generator is loaded. It is caused by the magnetic field set up by the armature current, which interacts with the main magnetic field produced by the field winding.

When the alternator supplies current to a load, the armature conductors carry this current. According to Ampere's law, this current creates its own magnetic field. This armature magnetic field superimposes onto the main magnetic field from the rotor (field winding), modifying the overall magnetic flux distribution in the air gap.

Effects of Armature Reaction

The effect of armature reaction depends significantly on the power factor of the load connected to the alternator:

• Unity Power Factor Load: The armature flux is largely cross-magnetizing. It distorts the main field, shifting the neutral magnetic axis. This distortion can lead to sparking at the brushes in DC machines, and while alternators don't have commutators like DC machines, flux distortion still impacts performance and waveform quality.
• Lagging Power Factor Load: The armature flux is predominantly demagnetizing. It opposes the main field, reducing the net flux in the air gap. A reduced flux leads to a decrease in the generated voltage. This is a significant effect in alternators operating at lagging power factors.
• Leading Power Factor Load: The armature flux is predominantly magnetizing. It aids the main field, increasing the net flux in the air gap. An increased flux leads to an increase in the generated voltage.

Therefore, armature reaction affects the generated voltage depending on the power factor and also distorts the flux, which can indirectly relate to brush sparking issues in some alternator types (though less directly than in DC machines) and waveform distortion.

Analyzing the Statements

Let's evaluate each given statement about the armature reaction of an alternator:

1. "It increases the generated voltage and sparking at brushes reduces."

   This statement is generally incorrect. Armature reaction typically reduces the generated voltage (at lagging PF) or keeps it relatively stable (at unity PF). It only increases voltage at leading PF. Furthermore, flux distortion caused by armature reaction can potentially increase issues like brush sparking (related to commutation in machines with brushes, though the primary impact in modern alternators is on voltage regulation and flux distribution).

2. "It reduces the generated voltage by distorting the main flux and it causes sparking at the brushes."

   This statement is generally correct, especially for lagging power factors where voltage reduction occurs due to the demagnetizing effect. Flux distortion also occurs, which can relate to sparking or other performance issues.

3. "It depends on both load current and power factor."

   This statement is correct. The magnitude of armature reaction flux is proportional to the load current. The direction and hence the nature of the armature reaction (demagnetizing, cross-magnetizing, or magnetizing) depend on the phase difference between the generated voltage and the armature current, which is determined by the load's power factor.

4. "When the generator is loaded, the armature carries the current; the armature current sets up its own magnetic field."

   This statement is the fundamental definition of what causes armature reaction. It is correct.

Conclusion

Based on the analysis, the statement that is NOT correct regarding the armature reaction of an alternator is the one claiming it increases generated voltage and reduces sparking. The effect is typically voltage reduction (at lagging PF) and potential increase in issues related to flux distortion.

The statement that is NOT correct is: "It increases the generated voltage and sparking at brushes reduces."