Question

what do you mean by demagnetizing and cross-magnetizing effects of armature reaction in a dc machine?

Answer 1

The demagnetisation of DC generator is the effect of armature reaction . The armature reaction is the effect of armature fluxes over the field fluxes . The armature fluxes always opposes the field fluxes . The direction of the armature fluxes always 180° phase apart from the field fluxes or magnetisation fluxes . For the DC generator residual magnetism of the field poles ( high permeability) takes a vital part in generating the EMF across the armature . This field fluxes increases as the no load EMF across the armature increases this is called self excited DC generator , Because E is directly proportional to field fluxes. As EMF increases slowly the field fluxes also increases. But there is a problem as the EMF increases across the armature the load current (Ia) also gets higher then according to the LENZS law it will try to oppose the field fluxes or the magnetisation fluxes. This effect is called demagnetisation effect. The terminal voltage across the conductor is Vt = E-(Ia×Ra).

As the armature reaction increases the terminal voltage decreases so we have to compensate the armature reaction to maintain the field fluxes with the induced EMF to do that we use compensating winding. It provides the compensating fluxes which are 180° apart from the armature reaction fluxes.

Cross magnetisation is the distortion of the one field flux due to effect of another field fluxes 

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