Armature reaction in Alternator or Synchronous Generator

Armature reaction is an important aspect in DC generator and AC synchronous Generator or Alternator. Armature reaction in alternator is defined as the effect of armature flux on the main flux produced by the field poles.

An electric machine normally consists of field winding and armature winding. DC supply is given to the field winding to produce magnetic flux. The armature conductor is rotated at a synchronous speed with the aid of a prime mover.

When there exists a relative motion between the magnetic flux and armature winding, the armature conductor cuts the field flux. Hence there will be a change in flux linkage in the conductor.

According to Faraday’s law of Electromagnetic induction, an emf is induced in the armature conductors. When the load is applied to the armature terminals, the current starts flowing through the armature winding. Since the current is alternating in nature, it induces a flux in the conductor, called armature flux.

The armature flux thus produced will react with the main field flux and distort the effect of the main flux, called armature reaction in alternator or synchronous generator. Due to this distortion, the resultant flux will either strengthen or weaken.

The distortion may depend on the type of load applied to the alternator. A DC Generator also has more or less similar armature reaction effects. In this section, let us discuss the different armature reaction effects, that can be seen at different loads in detail.

Learn the construction and working of alternator to know about its working.

Armature reaction at unity power factor load

When a resistive load with a unity power factor is connected to the alternator, the load current will start to flow through the armature winding. As it is a pure resistive load, the armature current will be in phase with the induced voltage.

The armature current will produce its own flux in the conductor, which will also be in phase with the induced voltage. Since the induced emf lags behind the main field flux by 90⁰, the armature flux produced will also be delayed by 90⁰ with respect to the main flux. The below shows the phasor diagram at unity power factor load.

As the armature flux act on the main field flux perpendicularly, the distribution of main field flux under a pole face does not remain uniformly distributed. As you can see from the waveform that, the armature flux will cross and distorts the main field flux at one point, thereby weakening the main flux. This is said to be a cross magnetizing effect.

You can also notice, the armature flux also assists the main flux at another point. In this case, the armature reaction strengthens the main field flux. Due to these effects, the main field flux will get distorted, without causing much change to the generated voltage.

In other words, flux density at the trailing tip of the pole is increased while flux at the leading tip of the pole decreases. Due to this, the armature reaction at resistive load is said to have a distorting effect maintaining the constant average field strength.

Armature reaction at zero power factor lagging load

When a pure inductive load with zero lagging power factor is connected to the alternator, the load current starts to flow through the armature conductors.

The armature current will be delayed by 90⁰ and so the armature flux produced will also be shifted by 90⁰ with respect to the poles.

There will be a phase difference of 90⁰ between the armature flux and main field flux. It can be seen that the armature flux will be in direct opposition to the main flux. The below shows the phasor diagram at lagging power factor load.

Thus the main flux gets decreased in this loading condition. This effect of armature reaction on this load is said to be a demagnetizing effect.

Due to this, the main field flux gets weaken and so the emf induced will be reduced. To maintain the same value of generated emf, filed excitation will have to be increased to overcome the demagnetizing effect.

Armature reaction at zero power factor leading load

When a pure capacitive load with zero leading power factor is connected, the load current starts to flow through the armature conductors.

In this load condition, the load current will be advanced by 90⁰ and so the armature flux produced will also be advanced by 90⁰ with respect to emf induced. So the armature flux will be in phase with the main field flux, resulting in strengthening of the field flux. Thus the main flux gets increased in this loading condition. The below shows the phasor diagram at leading power factor load.

The armature reaction in this load is said to be a magnetizing effect. Due to this effect, the main field flux gets weaken and so the emf induced will be reduced. To maintain the same value of generated emf, filed excitation will have to be reduced to overcome the magnetizing effect.

For any intermediate power factor, the effect of armature reaction in alternator will be partly distorting and partly demagnetizing.

From the explanations, we can summarize that

1. When an alternator supplies a load at the unity power factor, the effect of armature reaction is partly cross magnetizing and partly distorting.
2. The effect of armature reaction is demagnetizing when an alternator supplies a load at a lagging power factor.
3. When an alternator supplies a load at the leading power factor, the effect of armature reaction is magnetizing.
4. When an alternator supplies a load at the intermediate power factor, the effect of armature reaction is partly distorting and partly demagnetizing.
5. The effects of armature reaction may cause the generated emf to vary. In order to overcome that, the main flux is varied to generate the rated voltage.

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