REGULATING POLE CONVERTERS 429
ponent of the alternating-current armature reaction, in the direc-
tion of the brushes, SFo cos r, that is:
$'2 = g: - $0 cos r = 9F (1 - cos2 r) = 3 sin2 r = ^Q sin r tan r.
233. The shift of the resultant magnetic flux, by angle T, gives
a component of the m.rmf. of field excitation, tf"/ = ^ sin T)
(where Sy = m.m.f. of field excitation), in the direction of the
commutator brushes, and either in the direction of armature
reaction, thus interfering with commutation, or in opposition to
the armature reaction, thus improving commutation.
If the magnetic flux is shifted in the direction of armature
rotation, that is, that section of the field pole weakened toward
which the armature moves, as in Fig. 202, the component $F"/
of field excitation at the brushes is in the same direction as the
armature reaction, ^'o, thus adds itself thereto and impairs the
commutation, and such a converter is hardly operative. In this
case the component of armature reaction, SF', in the direction of
the field flux is magnetizing. •
If the magnetic flux is shifted in opposite direction to the
armature reaction, that is, that section of the field pole weakened
which the armature conductor leaves, as in Fig. 203, the com-
ponent, $"/, of field excitation at the brushes is in opposite direc-
tion to the armature reaction, $'2, therefore reverses it, if suffi-
ciently large, and gives a commutating or reversing flux, <3?r, that
is, improves commutation so that this arrangement is used in
such converters. In this case, however, the component of arma-
ture reaction, $', in the direction of the field flux is demagnet-
izing, and with increasing load the field excitation has to be in-
creased by & to maintain constant flux. Such a converter thus
requires compounding, as by a series field, to take care of the
demagnetizing armature reaction.
If the alternating current is not in phase with the field, but
lags or leads, the armature reaction of the lagging or leading
component of current superimposes upon the resultant armature
reaction, $', and increases it—with lagging current in Fig. 202,
leading current in Fig. 203—or decreases it—with lagging cur-
rent in Fig. 203, leading current in Fig. 202—and with lag of the
alternating current, by phase angle, 6 = r, under the conditions
of Fig. 203, the total resultant armature reaction vanishes, that is,
the lagging component of synchronous-motor armature reaction
compensates for the component of the direct-current reaction,