D. C. COMMUTATING MACHINES 189

netic flux at the armature circumference therefore always has
the same shape, and its intensity is proportional to the current,
except as far as saturation limits it. As the result thereof,
shifting the brushes to the edge of the field poles, as in Fig. 95,
brings them in a field which is proportional to the armature cur-
rent and thus has the proper intensity as a commutating field.
Therefore with series-wound machines commutating poles are
not necessary for good commutation, but the shifting of the
brushes gives the same result. However, in cases where the direc-
tion of rotation frequently reverses, as in railway motors, the
direction of the shift of brushes has to be reversed with the re-
versal of rotation. In railway motors this cannot be done with-
out objectionable complication, therefore the brushes have to be
set midway, and the use of the magnetic flux at the edge of the
next pole, as commutating flux, is not feasible. In this case a
commutating pole is used, to give, without mechanical shifting
of the brushes, the same effect which a brush shift would give.
Therefore in railway motors, especially when wound for high
voltage, as 1200 to 2400 volts, a commutating pole is sometimes
used. This commutating pole, having a series winding just
like the main pole, changes proportionally with the main pole.
When reversing the direction of rotation, however, the armature
and the commutating poles are reversed, while the main poles
remain unchanged, or the main poles are reversed, while the arma-
ture and the commutating poles remain unchanged; that is,
the separate commutating pole becomes necessary because
during the reversal of rotation it has to be treated differently
from the main pole.

52. The commutating pole counteracts the armature reaction
only at the place of commutation, but not elsewhere, and the
field distribution resulting from the armature reaction thus is not
eliminated by the commutating pole, except locally. Thus in
machines having very low field excitation, and relatively high
armature reaction, as alternating-current commutating machines,
adjustable speed motors of wide speed range at the high-speed
position, boosters near zero voltage, etc., the load losses resulting
from excessive field distortion, the tendency to instability of
speed, .and the liability of flashing at the commutator at sudden
changes of load are not eliminated by the commutating pole,
but a more complete neutralization of the armature reaction is
necessary.