SINGLE-PHASE COMMUTATOR MOTORS 355

for instance the arrangement shown in Fig. 170, which represents
the development of a commutator. The commutator consists
of alternate live segments, S, and dead segments, S', that is, seg-
ments not connected to armature coils, and shown shaded in
Fig. 170. Two sets of brushes on the commutator, the one, B\,

&N

S S

I

I

Bo '

FIG. 170.—Commutation with external e.m.f.

ahead in position from the other, 52, by one commutator seg-
ment, and connected to the first by a coil, JV, containing an e.m.f.
equal in phase, but half in intensity, and opposite, to the e.m.f.
of alternation of the armature coil; that is, if the armature coil
contains a single turn, coil N is a half turn located in the main

I 234

FIG. 171.—Commutation by external e.m.f.

m,

field space; if the armature coil, A, contains m turns, -^ turns in
a
the main field space are used in coil, N. The dead segments, S',
are cut between the brushes, B\ and Bz, so as not to short-circuit
between the brushes.
In this manner, during the motion of the brush over the com-