Armature Reaction:
The effect of armature flux on the main field flux in any machine is known as armature reaction. In an Alternator, rotor is energized from a dc supply and made to rotate through the prime mover. When rotor rotates, rotating magnetic field of the rotor cuts the armature conductors and induces voltage (emf) in the stator windings of alternator. If there is no load connected across the alternator, the entire voltage induced in the armature winding appear as the terminal voltage.
When alternator↗ is loaded, current start to flow through the armature winding and produces its own magnetic field (since current carrying conductor produces magnetic field). The magnetic flux produce in the armature winding is known as armature flux.
According to Lenz law, the direction of armature flux is such that it opposes the cause which produces it. Since emf induced in the armature winding is due to main field flux(
). Hence armature flux opposes the main rotor field flux due to which net flux inside the machine decreases. This simply means that the terminal voltage across the alternator also reduces. The effect of armature flux on the rotor flux (main field flux) is known as armature reaction in alternator. The below diagram shows the opposition of armature flux to the main field flux (Armature Reaction).
The effect of armature flux on the terminal voltage of alternator is not same for all the time as it depends on the power factor (phase relationship between terminal voltage and armature current) of load. Consider the following conditions in order to understand the effect of armature flux on the terminal voltage of alternator.
Effect of Armature Reaction:
Firstly, let derive the relation between main field flux and induced emf in the armature windings of alternator. Let assume the voltage induced in the armature winding is Eph. The main field flux () of alternator and induced emf (Eph) in the armature can be expressed in equation form as:
……. eq. A
The main field flux can be written as:
By taking derivative w.r.t t, we have
Or we can write as:
By putting the value of 
in equation (A), we have
Since
Hence, we can write the above equation as
From this equation, we can say that the angle between the main field flux and induced emf is 
. Hence main field flux leads the induce emf (Eph) by. We can draw the fig.as:
Since we calculate the mathematical relation of main field flux and induced emf. Now, let discuss the effect of armature flux on the main field flux at different types of load.
Armature Reaction in Alternator at Unity Power Factor Loads:
Let assume that power factor of the load is unity (i.e. pure resistive load). In that case, angle between the induced emf (Eph) and current (Ia) is such that they are in phase with each other. Whenever current (Ia) flows through armature winding, it produces flux (
) which is known as armature flux. The direction of armature flux will be the same as that of armature current. Moreover, we also know that the angle between the main field flux and induced emf is . Hence, we can draw the fig. as:
Now, the angle between the main field flux and armature flux is such that the armature flux lags the main field flux by 90 degree. The effect of armature flux in this case is such that it will partly assist the main flux and partly opposes the main field flux. Hence net flux inside the machine remains same which means that the generated terminal voltage (emf) also remains same. This effect is known as cross magnetizing effect. The waveform in the case of resistive load is as follows:
In the above waveform, you can see that the armature flux waveform lags the main field flux by 90 degree.
Armature Reaction in Alternator at Leading Power Factor Loads:
Let assume that power factor of the load is leading (i.e. pure capacitive load). In case of capacitive load, current (Ia) will lead the induced emf (Eph) by . Since the direction of armature flux () is same as that of armature current. We can draw the fig.as
From the above fig. we can say that if the load connected across the alternator is capacitive, angle between the main field flux and armature flux is 0 degree. In that case, armature flux always supports the main field flux. This is known as magnetizing effect. It is called magnetizing effect because armature flux assists the main field flux. Hence net flux inside the machine increases which means that the generated terminal voltage (emf) increases in the case of pure capacitive load. The waveform in the case of capacitive load is as follows:
Armature Reaction in Alternator at Lagging Power Factor Loads:
When the load power factor is zero lagging ( i.e. pure inductive load), armature current lags the induced emf by . As described earlier, armature flux is produced due to the armature current and direction of the armature flux () is same as that of armature current. In that case, the angle between the main field flux and armature flux is 180 degree (out of phase). It is shown in the below fig.as
From the above fig. we can see that the angle between the field flux and armature flux is 180 degree (out of phase). Hence armature flux always opposes the main flux in the case of inductive loads. This effect is known as demagnetizing effect. It is called demagnetizing effect because armature flux always opposes the main flux. Hence net flux inside the machine decreases which means that the generated terminal voltage also decreases in the case of pure inductive load. The waveform in the case of inductive load is as follows:
In the above waveform, you can see that the armature flux waveform lags the main field flux by 180 degree.
Conclusion:
From the above discussion, we can say that armature reaction in alternator depend on the power factor of load. If the load power factor is unity, the effect of armature reaction in that case is cross magnetizing type. In case of lagging power factor load, the effect of armature reaction is demagnetizing. In case the load is at leading power factor, the effect of armature reaction is magnetizing type.