Effect of armature flux on main field flux in alternators

  1.  Current is 90^ Lagging  ( zero power factor lag)

1. 

   purely lagging and leading load cases

   The armature flux  is always  in phase with the rotor the armature current Ǿ_a=NI_a hence the flux vector (Ǿ)will also lag the induced voltage by 90^ The main field flux or rotor flux is leading the induced emf E by 90, now the main field flux and armature flux are in  direct opposition and hence the net resultant vector will the difference of the two vectors and will be less than the original value.
   Hence the zero lagging power factor is purely demagnetizing in nature whereas in case it is lagging by some other angle than 90 then it will be demagnetizing and cross magnetizing both.
                                                                                       2.  Current is 90^ leading          
                                                                                            In this case as before the main field flux is leading the induced voltage by 90^(  application of lenz’s law) and the armature current ( stator current) is also leading the induced voltage by 90^ , hence the both vectors will be added to get the resultant and the resultant will be more than the original value hence in this magnetizing effect will occur , if the load is leading by some other angle then both magnetizing and cross magnetizing effect will occur
    
   3.   Armature current is in phase with induced voltage                           
   In this we can see in the phasor diagram that the resultant will be the phasor sum of field flux and armature flux which are at 90^ to each other and hence cross magnetizing effect will occur. The resultant vector will be displaced and will also have a magnitude greater than the original vector.
   
   
   So friends using simple phasor diagrams we can find the effect of armature flux on the main field flux for any angle