Armature Reaction in Alternator or Synchronous Generator

Every rotating electrical machine works based on Faraday’s law. Every electrical machine requires a magnetic field and a coil (Known as armature) with a relative motion between them. In case of an alternator, we supply electricity to pole to produce magnetic field and output power is taken from the armature. Due to relative motion between field and armature, the conductor of armatures cut the flux of magnetic field and hence there would be changing flux linkage with these armature conductors. According to Faraday’s law of electromagnetic induction there would be an emf induced in the armature. Thus, as soon as the load is connected with armature terminals, there is a current flowing in the armature coil.

When current starts flowing through the armature conductor, it affects the main field flux of the alternator. This reverse effect is called armature reaction. Simply put, armature reaction is the effect of the armature’s magnetic field on the flux produced by the rotor field poles.
A current -carrying conductor produces its own magnetic field, which affects the main magnetic field of the alternator.
Armature reaction has two undesirable effects: it can distort the main field or reduce the main field flux, or both. These effects harm the machine’s performance. Distortion of the field is called a cross magnetizing effect, while reduction of the field flux is known as the demagnetizing effect.

Electromechanical energy conversion occurs through the magnetic field. The relative motion between the armature conductors and the main field induces an emf in the armature windings, depending on the relative speed and magnetic flux. Armature reaction can reduce or distort the flux, affecting the net emf induced and degrading the machine’s performance.

Armature Reaction in Alternator

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

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

The armature reaction of alternator or synchronous generator, depends upon the phase angle between, stator armature current and induced voltage across the armature winding of alternator.
The phase difference between these two quantities, i.e. Armature current and voltage may vary from – 90^o to + 90^o
If this angle is θ, then,

To understand actual effect of this angle on armature reaction of alternator, we will consider three standard cases,

1. When θ = 0
2. When θ = 90^o
3. When θ = – 90^o

Armature Reaction of Alternator at Unity Power Factor

At unity power factor, the angle between armature current I and induced emf E, is zero. That means, armature current and induced emf are in same phase. But we know theoretically that emf induced in the armature is due to changing main field flux, linked with the armature conductor.
As the field is excited by DC, the main field flux is constant in respect to field magnets, but it would be alternating in respect of armature as there is a relative motion between field and armature in the alternator. If main field flux of the alternator in respect of armature can be represented as

Then induced emf E across the armature is proportional to, dφ_f/dt.

Hence, from these above equations (1) and (2) it is clear that the angle between, φ_f and induced emf E will be 90^o.
Now, armature flux φ_a is proportional to armature current I. Hence, armature flux φ_a is in phase with armature current I.
Again at unity electrical power factor I and E are in same phase. So, at unity power factor, φ_a is phase with E. So at this condition, armature flux is in phase with induced emf E and field flux is in quadrature with E. Hence, armature flux φ_a is in quadrature with main field flux φ_f.
As this two fluxes are perpendicular to each other, the armature reaction of the alternator at unity power factor is purely distorting or cross-magnetising type.
As the armature flux pushes the main field flux perpendicularly, distribution of main field flux under a pole face does not remain uniformly distributed. The flux density under the trailing pole tips increases somewhat while under the leading pole tips it decreases.

Armature Reaction of Alternator at Lagging Zero Power Factor

At lagging zero electrical power factor, the armature current lags by 90^o to induced emf in the armature.
As the emf induced in the armature coil due to main field flux thus the emf leads the main field flux by 90^o. From equation (1) we get, the field flux,

Hence, at ωt = 0, E is maximum and φ_f is zero.
At ωt = 90^o, E is zero and φ_f has maximum value.
At ωt = 180^o, E is maximum and φ_f zero.
At ωt = 270^o, E is zero and φ_f has negative maximum value.
Here, φ_f got maximum value 90^o before E. Hence φ_f leads E by 90^o.
Now, armature current I is proportional to armature flux φ_a, and I lags E by 90^o. Hence, φ_a lags E by 90^o.
So, it can be concluded that, field flux φ_f leads E by 90^o.
Therefore, armature flux and field flux act directly opposite to each other. Thus, armature reaction of the alternator at lagging zero power factor is a purely demagnetising type. That means, armature flux directly weakens main field flux.

Armature Reaction of Alternator at Leading Power Factor

At leading power factor condition, armature current “I” leads induced emf E by an angle 90^o. Again, we have shown just, field flux φ_f leads, induced emf E by 90^o.
Again, armature flux φ_a is proportional to armature current I. Hence, φ_a is in phase with I. Hence, armature flux φ_a also leads E, by 90^o as I leads E by 90^o.
As in this case both armature flux and field flux lead, induced emf E by 90^o, it can be said, field flux and armature flux are in the same direction. Hence, the resultant flux is simply arithmetic sum of field flux and armature flux. Hence, at last, it can be said that armature reaction of alternator due to a purely leading electrical power factor is the magnetizing type.

Nature of Armature Reaction

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