Armature Reaction in Synchronous Machines

Armature Reaction in an Alternator

When a three-phase alternator is operating at no-load, there will be no current flowing through its armature winding. Hence, the magnetic flux produced in the air-gap will be due to rotor field poles only. But, when the alternator is loaded, the three-phase currents flowing through the armature winding will produce a rotating magnetic field in the air-gap. As a result, the resultant magnetic flux in the air-gap is changed. This effect is known as armature reaction, and may be defined as under −

The current flowing through the armature winding of a three-phase alternator, the resulting magnetomotive force (MMF) produces a magnetic flux. This armature flux interacts with the main pole flux, and causing the resultant magnetic flux to become either less or more than the original main pole flux. This effect of armature flux on the main pole flux is called armature reaction.

In a three-phase alternator, the effect of armature reaction depends upon the magnitude of the armature current and power factor of the load. Which means the power factor of the load determines whether the armature reaction flux distorts, opposes or assists the main field flux.

The following discussion explains the nature of armature reaction in synchronous machines for different power factors −

• Unity Power Factor − When the alternator supplies a load at unity power factor, i.e. purely resistive load, the effect of armature reaction is to distort the main field flux. This is called cross-magnetizing effect of armature reaction.
• Lagging Power Factor − When the alternator supplies a load at lagging power factor, i.e. purely inductive load, the effect of armature reaction is partly demagnetizing and partly cross-magnetizing. This causes a reduction in generated voltage.
• Leading Power Factor − When the alternator supplies a load at leading power factor, i.e. purely capacitive load, the effect of the armature reaction is partly magnetizing and partly cross-magnetizing. This causes an increase in generated voltage.

Armature Reaction in a Synchronous Motor

When the synchronous machine is operated in motoring mode, the armature reaction flux is in phase opposition, which means the nature of armature reaction is reversed what is stated for the alternator.

The following points explain the effects of armature reaction when the synchronous machine is operating in motoring mode −

• Lagging Power Factor − When the synchronous motor draws a current at a lagging power factor, the effect of armature reaction is partly magnetizing and partly cross-magnetizing.
• Leading Power Factor − When the synchronous motor draws a current at a leading power factor, the effect of armature reaction is partly demagnetizing and partly cross-magnetizing.

Summary of Nature of Armature Reaction

The summary of nature of armature reaction can be drawn for an alternator or synchronous generator supplying a balanced 3-phase load as follows −

• The magnetic flux causes the armature reaction is constant in magnitude and rotates at synchronous speed.
• When the alternator supplies a load at unity power factor, then the armature reaction has cross-magnetising effect.
• When the alternator supplies a load at lagging power factor, the armature reaction is partly cross-magnetising and partly demagnetising.
• When the alternator supplies a load at leading power factor, then the armature reaction is partly cross-magnetising and partly magnetising.
• If the magnetic flux causing the armature reaction is assumed to act independently of the main field flux, then it induces EMF in each phase which lags the respective phase currents by 90°.