Electrical Engineering ⇒ Topic : Armature Reaction

ARMATURE REACTION

When a machine operates at no-load, there exists in it only the m.m.f. of the main poles which creates the main flux Φ_m Fig. (a).

Under load when, when a current flows through the armature winding, an m.m.f. appears (which creates Φ_a) that interacts with the main m.m.f. Fig. (b). Hence the magnetic flux Φ_r that exists in a machine when it operates under load should be considered as the resultant flux created by the resultant m.m.f. Fig. (c).

 "The action of the armature m.m.f on the main m.m.f is termed as armature reaction".

                                        G.N.A.Geometrical Neutral Axis or plane  

                                           M.N.A.Magnetic Neutral Axis or plane

                          Fig. (a). Flux produced by main field of a generator.

                         Fig. (b). Flux produced by current in armature conductor

The effects of armature flux may be analysed by considering the flux to consists of two components that are at right angles to each other as shown in Fig.(d). These components are :

(i) Cross-magnetising or distorting component. This component is at right angles main field, because this to the component crosses the main field flux, it is known as the cross-magnetising component of the armature flux.

                                             Fig. (c) Shift of generator flux due to armature flux.

                                  Fig. (d) Components of armature flux and their relative position with respect to main field flux

(ii) Demagnetising component. This component is in the same plane as the,main-field flux.The direction of this component is opposite to the direction of th,e main field flux, with the result it tends to reduce the effect of the maintained flux.

It may be noted that both cross-magnetising (or distorting) and demagnetising effects increase with increase in the armature current