Every rotating electrical machine works on the basis of Faraday’s law. Every electrical machine requires a magnetic field and a coil (known as the armature) between which there is relative motion. In the case of an alternator, we supply electricity to the poles to generate a magnetic field and the output power is taken from the armature.

Due to the relative motion between the field and the armature, the conductors of the armatures cut the flux of the magnetic field and hence the flux linkage along these armature conductors will change. According to Faraday’s law of electromagnetic induction.An emf will be induced in the armature. Thus, as the load is connected to the armature terminals, current flows in the armature coil.

As current courses through the armature guide this current conversely affects the fundamental field transition of the alternator (or coordinated generator). This converse impact is called armature response in an alternator or simultaneous generator. As such, the impact of the armature (stator) transition on the motion prompted by the rotor field shafts is called armature reactance.
We definitely realize that a current-conveying guide creates its own attractive field, and this attractive field influences the super attractive field of the alternator.
This makes two unwanted impacts, it is possible that it mutilates the fundamental field, or it lessens the principal field motion or both. They debase the exhibition of the machine. At the point when the field is contorted, it is known as the cross-polarizing impact. Also, when the field transition diminishes, it is called demagnetizing impact.

Electromechanical energy is converted through a magnetic field as a medium. Due to the relative motion between the armature conductors and the main field, an emf is induced in the armature wingdings whose magnitude depends on the relative speed and magnetic flux. Due to the armature reaction, the flux is reduced or distorted, the net emf is also affected and hence the performance of the machine deteriorates.

Armature Reaction in Alternator:

In an alternator like all other synchronous machines, the armature reaction affects the power factor i.e. the phase relationship between the terminal voltage and the armature current.
Reactive power (lagging) is magnetic field energy, so if a generator supplies a lagging load, it means it is supplying magnetic energy to the load. Since this power comes from the excitation of the synchronous machine, the net reactive power in the generator is reduced.

Therefore, the armature reaction is demagnetizing. Similarly, the armature reaction has a magnetizing effect when the generator supplies a leading load (as the leading load takes the leading VAR) and in turn gives the generator a lagging VAR (magnetic energy). In the case of a purely resistivity load, the armature reaction is only cross-magnetizing.

Nature of Armature Reaction:

1. The armature reactance flux is constant in magnitude and rotates at synchronous speed.
2. When the generator supplies a load at unity power factor, the armature reaction is cross-magnetizing.
3. When the generator supplies the load at leading power factor, the armature reaction is partially demagnetizing and partially cross-magnetizing.
4. When the generator supplies the load at leading power factor, the armature reaction is partly magnetizing and partly cross-magnetizing.
5. The armature flux acts independently of the main field flux.