Understanding Armature Reaction in Alternators at Unity Power Factor

Armature reaction is the effect of the magnetic field produced by the armature currents on the main field flux in a synchronous machine like an alternator. This effect changes based on the power factor of the load connected to the alternator.

When an alternator operates at unity power factor (UPF), the armature current is approximately in phase with the induced voltage. The magnetic field or Magnetomotive Force (MMF) created by these armature currents is then in quadrature, meaning it is 90 electrical degrees out of phase with the main field MMF produced by the field winding. This specific type of armature reaction at unity power factor is called cross-magnetization.

Let's evaluate each of the given statements about armature reaction at unity power factor:

• Statement 1: The maximum of the fundamental wave of the field will be opposite to the pole centres. At unity power factor, the cross-magnetizing effect distorts the main field flux. The flux density becomes stronger at one pole tip and weaker at the other, shifting the position of maximum flux density away from the geometric center of the pole. This statement relates to this distortion effect.
• Statement 2: The armature reaction MMF is directed perpendicular to the main field MMF. This is a core characteristic of cross-magnetizing armature reaction, which occurs at unity power factor. The armature MMF axis is physically perpendicular to the main field MMF axis, corresponding to a 90 electrical degree phase difference. This statement is true.
• Statement 3: The distribution of stator currents and fluxes of an alternator with 30° phase displacement. At unity power factor, the phase displacement between the armature MMF (due to stator currents and fluxes) and the main field MMF is 90 electrical degrees, not 30 electrical degrees. This statement claims an incorrect phase displacement for unity power factor operation.
• Statement 4: The distribution of stator currents and fluxes of an alternator with zero phase displacement. A zero phase displacement between armature MMF and main field MMF would mean they are either in phase (magnetizing) or in phase opposition (demagnetizing). These effects occur at lagging and leading power factors, respectively, not at unity power factor. This statement also describes an incorrect phase displacement for unity power factor operation.

The question asks which statement about the armature reaction at unity power factor is NOT true. Both statements 3 and 4 claim phase displacements (30° and 0°) that are not correct for the unity power factor condition, where the displacement is 90°. However, we need to select only one as the "NOT true" statement from the given options. Statement 3 specifically claims a 30° displacement.

Therefore, the statement that is NOT true about the armature reaction of an alternator with unity power factor is the one claiming a 30° phase displacement.