Figs. 1,411 and 1,412.—Section of armature and field showing distorting effect of armature reaction on the field. When a coil is opposite a pole as in fig. 1,411, no current is flowing (assuming no self induction) and the field is undisturbed, but, as the inductors pass under a pole face as in fig. 1,412, current is induced in them, and lines of force are set up as indicated by the dotted lines. This distorts the main field so that the lines of force are crowded toward the forward part of the pole face as shown.
Ques. Explain how the field becomes distorted by armature reaction.
Ans. Considering a slotted armature and analyzing the electrical conditions as the inductors move past a pole piece, it will be observed: 1, when the coil is in the position shown in fig. 1,411, the current will be zero, assuming no armature self-induction, consequently for this position the armature coil has no disturbing effect upon the field set up by the field magnet; 2, when the inductors have moved under the pole face, as in fig. 1,412, currents will be induced in them, and they will tend to set up a magnetic field as indicated by the dotted lines, and in direction, by the arrow heads. The effect of this field will be to distort the main field, strengthening one side of the pole and weakening the other side.
Fig. 1,413.—Section of armature and field showing weakening effect of armature reaction in the field. Self-induction being present (as it almost always is), the current lags more or less behind the pressure, so that when the coil is in the position of zero induction, as shown, the current has not yet come to rest. Accordingly, lines of force (indicated by the dotted lines) are set up by the current flowing through the coils which are in opposition to the field, thus weakening the latter. The dots and crosses in inductor sections, have their usual significance in defining the direction of current, representing respectively the heads and tails of arrows.
Ques. Explain how the field becomes weakened by armature reaction.
Ans. In all armatures there is more or less inductance which causes the current to lag behind the pressure a corresponding amount. Accordingly, the current does not stop flowing at the same instant that the pressure becomes zero, therefore, when the coil is in the position of zero pressure, as in fig. 1,413, the current is still flowing and sets up a magnetic field which opposes the main field as indicated by the dotted arrows, thus weakening the main field.
Ques. In what kind of armature is this effect especially pronounced?
Ans. In slotted armatures provided with coils of a large number of turns.
