Armature Reaction in Alternator or Synchronous Generator

Armature reaction is the effect of armature mmf on the main field of a synchronous machine. Every rotating electrical machine still follows Faraday’s law: it needs a magnetic field and an armature coil, with relative motion between them. In an alternator the field winding is energised and armature conductors cut that flux. By Faraday’s law of electromagnetic induction an emf appears in the armature. When a load is connected, current flows in the armature winding.

That armature current produces its own mmf, which acts on the main field flux of the rotor poles. The interaction is armature reaction.
A current-carrying conductor has its own magnetic field, so the armature field can distort the main field or weaken it.
Distortion is the cross-magnetising effect. A drop in main flux is the demagnetising effect. Both change terminal voltage and extra heating for a given excitation.

Electromechanical energy conversion goes through the magnetic field. Relative motion between the armature conductors and the main field induces emf in the armature windings. That emf depends on speed and magnetic flux. If armature reaction reduces or distorts the flux, the net emf changes and so does terminal voltage.

Armature Reaction in Alternator

In an alternator, as in other synchronous machines, armature reaction depends on power factor, that is the phase of armature current relative to terminal voltage (or, in the ideal cases below, relative to induced emf).
A lagging load draws lagging Reactive power. That lagging current is an inductive mmf. The armature then has a demagnetising component, so air-gap flux falls unless field current is raised. The magnetic field energy of the load is supplied from the machine’s excitation.

So armature reaction is demagnetizing on a lagging load. On a leading load the armature mmf has a magnetizing component (the load returns lagging VARs). On a purely resistive load, armature reaction is cross-magnetizing only.

Armature reaction of an alternator or synchronous generator depends on the phase angle between stator armature current and the emf induced in the armature winding.
That angle, between armature current and voltage, can lie from – 90^o to + 90^o
If this angle is θ, then,

Three reference cases show how θ sets the armature reaction:

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. Armature current and induced emf are then in phase. Emf in the armature comes from changing main-field flux linking the armature conductors.
The field is DC-excited, so main-field flux is constant on the field magnets, but it is alternating as seen from the armature because of relative motion. If main field flux of the alternator as seen from the armature is

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

From equations (1) and (2), the angle between φ_f and induced emf E is 90^o.
Armature flux φ_a is proportional to armature current I, so φ_a is in phase with I.
At unity electrical power factor I and E are in phase. So at unity power factor, φ_a is in phase with E. Armature flux is then 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.
Because those two fluxes are perpendicular, the armature reaction of the alternator at unity power factor is purely distorting or cross-magnetising.
The armature flux crosses the main field at right angles, so flux density under a pole face is no longer uniform. Density under the trailing pole tips rises somewhat and under the leading pole tips it falls.

Armature Reaction of Alternator at Lagging Zero Power Factor

At lagging zero electrical power factor, the armature current lags induced armature emf by 90^o.
Emf in the armature coil comes from main field flux, so the emf leads the main field flux by 90^o. From equation (1) the field flux is

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 reaches its peak 90^o before E, so φ_f leads E by 90^o.
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.
Field flux φ_f leads E by 90^o.
Armature flux and field flux then act in opposite directions. Armature reaction of the alternator at lagging zero power factor is purely demagnetising: armature flux weakens main field flux.

Armature Reaction of Alternator at Leading Power Factor

At leading power factor, armature current I leads induced emf E by 90^o. Field flux φ_f also leads induced emf E by 90^o.
Armature flux φ_a is proportional to armature current I, so φ_a is in phase with I. Hence φ_a also leads E by 90^o as I leads E by 90^o.
Both armature flux and field flux then lead induced emf E by 90^o, so they lie in the same direction. Resultant flux is the arithmetic sum of field flux and armature flux. Armature reaction of the alternator at a purely leading electrical power factor is therefore magnetizing.

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 lagging 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 mmf is set by stator current. Field mmf is set by field current. In the air gap the two mmfs add.