ARMATURE REACTION :

•     The armature reaction is the effect of the magnetic field produced by armature currents on the main magnetic field.

•     In the case of ac machines, the nature of armature reaction depends upon the p.f of the load- To analyze the effects of armature reaction, three extreme load conditions are considered,

                       u.p.f load,
                       zero p.f. lagging load
                       Zero p.f leading load.
   

• Consider a 3 phase, two pole alternator (fig.) having single layer winding.

•  The winding of each phase is assumed to be concentrated.  

        



   













 

•      The coil aa' will be directly under the middle of the pole faces when the emf has the maximum value (fig. a) and the in-phase component of current will also have maximum value in this position, (fig. b) It can be seen that on one side of the coil the armature field is aiding the main field whereas on the other side it is opposing the main field.

•     Thus u.p.f currents produce cross magnetization. In other words the armature reaction for u.p.f is distortional.

                 

                

•     In fig. c the coil is shown for ZPF lagging current condition.

•     On both sides of the conductor the armature flux is in direct opposition to the main flux.

•     The armature reaction in this case is wholly demagnetizing.

•     Hence ZPF lagging currents produce demagnetization. In fig. d the coil is shown for ZPF leading condition.

•      It can be shown that on both sides of the conductor, the armature field aids the main field.

•     The armature reaction is wholly magnetizing which results in greater induced e.m.f.

•     Hence ZPF leading currents produce magnetization.

•     The field and armature flux wave - forms are shown for upf, ZPF lag and ZPF lead in fig. e, f and g respectively.

•     The armature reaction is of constant magnitude but rotates at synchronous speed in a 3 phase synchronous machine.