Calculation of Armature Reaction

In a generator without interpoles, the armature-reaction ampere-turns may be considered as being divided into two parts, namely, those produced by the conductors in the interpolar space included in angle 2α in Fig. 10-21 (c) and those produced by all other armature conductors.

The axis of the reaction mmf produced by the conductors and current in the angle 2α coincides with the axis of the main-field mmf; but, by Lenz's Law, the reaction mmf has a sense opposite to that of the main-field mmf, and so tends to demagnetize the main field. This component of the armature reaction is called the demagnetizing component. The mmf produced by armature current in the conductors outside the angle 2α has an axis perpendicular to that of the main field. The armature-reaction component produced by that mmf is called the cross-magnetizing component.

In a generator having interpoles (see the next article), the brushes are set at points midway between the poles; and so there can be no demagnetizing armature reaction. There is only a cross-magnetizing reaction.

Example 10-2. - A 4-pole generator has 720 armature conductors. The conductor current at full load is 25 amp. If the brushes are shifted forward in the direction of rotation 10 mechanical degrees, what will be the number of demagnetizing ampere-turns per pole?

Solution. - The number of conductors per pole is

In a 4-pole generator,

10 mechanical degrees are equal to 20 electrical degrees. Hence, the number of demagnetizing conductors per pole is

Since 40 conductors are equivalent to 20 turns, the number of demagnetizing ampere-turns pei pole is