SINGLE-PHASE COMMUTATOR MOTORS 347
sating winding, and this quadrature flux, <l>i, lags nearly 90° be-
hind the main flux, <3>, since the secondary circuit is nearly non-
inductive, especially at speed.
In the repulsion motor with secondary excitation, 5, the quad-
rature flux, <i>i, is also very large, and practically constant, corre-
sponding to the impressed e.m.f., but lags considerably less than
90° behind the main flux, <i>, the secondary circuit being induct-
ive, since it contains the field coil, F. The lag of the flux, <i>i,
increases with increasing speed, since with increasing speed the
e.m.f. of rotation of the armature increases, the e.m.f. of self-
inductance of the field decreases, due to the decrease of current,
and the circuit thus becomes less inductive.
The series repulsion motors 6 and 7, give the same phase rela-
tion of the quadrature flux, $1, as the repulsion motors, 5 and 6,
but the intensity of the quadrature flux, $1, is the less the smaller
the part of the supply voltage which is impressed upon the com-
pensating winding.
V. Commutation
196. In the commutator motor, the current in each armature
coil or turn reverses during its passage under the brush. In the
armature coil, while short-circuited by the commutator brush,
the current must die out to zero and then increase again to its
original value in opposite direction. The resistance of the arma-
ture coil and brush contact accelerates, the self-inductance re-
tards the dying out of the current, and the former thus assists,
the latter impairs commutation. If an e.m.f. is generated in
the armature coil by its rotation while short-circuited by the
commutator brush, this e.m.f. opposes commutation, that is,
retards the dying out of the current, if due to the magnetic flux
of armature reaction, and assists commutation by reversing the
armature current, if due to the magnetic flux of overcompensa-
tion, that is, a magnetic flux in opposition to the armature
reaction.
Therefore, in the direct-current commutator motor with high
field strength and low armature reaction, that is, of negligible
magnetic flux of armature reaction, fair commutation is produced
with the brushes set midway between the field poles—that is,
in the position where the armature coil which is being commu-
tated encloses the full field flux and therefore cuts no flux and
has no generated e.m.f.—by using high-resistance carbon brushes,