Understanding Armature Reaction in an Alternator at Unity Power Factor

Armature reaction in an alternator refers to the effect of the magnetic field produced by the armature current on the main magnetic flux produced by the field winding (rotor). This effect depends heavily on the power factor of the load connected to the alternator.

Let's look at how armature reaction behaves at different power factors:

• Lagging Power Factor: When the alternator supplies current to a lagging load, the armature MMF (Magnetomotive Force) opposes the main field MMF. This results in a demagnetizing effect, causing a reduction or weakening of the main field flux.
• Leading Power Factor: When the alternator supplies current to a leading load, the armature MMF aids the main field MMF. This results in a magnetizing effect, causing an increase or strengthening of the main field flux.
• Unity Power Factor: When the alternator supplies current to a unity power factor load, the armature MMF is displaced by approximately 90 degrees electrically from the main field MMF axis. This means the armature flux neither directly opposes nor directly aids the main flux in the same direction. Instead, it acts in a way that distorts the main magnetic field distribution across the air gap. This effect is known as cross-magnetization. The main flux magnitude is largely unchanged, but its shape is altered, leading to a skewed distribution.

Therefore, at unity power factor, the armature reaction primarily causes a distortion of the main magnetic flux.

Considering the options provided:

• weakening of main flux: This occurs at lagging power factor, not unity power factor.
• distortion of main flux: This is the primary effect at unity power factor due to cross-magnetization.
• strengthening of main flux: This occurs at leading power factor, not unity power factor.
• magnetising effect: This is another term for the strengthening effect, which happens at leading power factor.

Based on the analysis, the armature reaction of an alternator at unity power factor is the distortion of the main flux.