Q. What do you understand by demagnetizing and cross magnetizing effects of armature reaction in a d.c. machine. Derive the expression for calculation of demagnetizing and cross magnetizing ampere - turns when brushes are given a shift from geometrical neutral axis.

Sol.  Let the brushes be given a forward shift in a generator or backward shift in a motor, so that brushes axis is now along the magnetic neutral axis (MNA). In other words, the brushes are given a shift through an angle θ and occupy a position BQ. The armature flux Φ_a = OB must be along the brush axis. Flux Φ_a can now be decomposed into two components; OC and OD. The component OC = Φ_a sinθ is opposing the main field Φ_f. Therefore, Oc has a demagnetic effect on the main field flux. The other component Od, being perpendicular to the main flux Φ_f, is the cross magnetizing armature flux. Note that the resultant flux Φ_R is obtained from the phasor sum of Φ_f and Φ_a, i.e. Φ_R = Φ_f + Φ_a. It is observed that the brush shift, the resultant flux gets reduced from its no - load value even if there is no magnetic saturation.

Draw RS making an angle θ with respect to GNA and on the other side of brush shift. The total ampere turns can now be divided into two groups. The conductors shown in angle ROP and QOS or the conductors lying in angle 4θ for every 360^o electrical (or 2θ for every 180^o electrical) are producing a flux opposite to the main field - flux. This can be verified by the right - hand grip rule. Hence the ampere turns due to the conductors contained in 4θ degrees for every 360^o electrical are demagnetizing in nature.