SINGLE-PHASE COMMUTATOR MOTORS 351

fundamental importance in the study of the alternating-current
commutator motor, and the control of this short-circuit current
the main problem of alternating-current commutator motor
design.

Various means have been proposed and tried to mitigate or
eliminate the harmful effect of this short-circuit current, as high
resistance or high reactance introduced into the armature coil
during commutation, or an opposing e.m.f. either from the out-
side, or by a commutating field.

High-resistance brush contact, produced by the use of very
narrow carbon brushes of high resistivity, while greatly improv-
ing the commutation and limiting the short-circuit current so
that it does not seriously demagnetize the field and thus cause
the motor to lose its torque, is not sufficient, for the reason that
the resistance of the brush contact is not high enough and also is
not constant. The brush contact resistance is not of the nature
of an ohmic resistance, but more of the nature of a counter
e.rn.f.; that is, for large currents the potential drop at the brushes
becomes approximately constant, as seen from the volt-ampere
characteristics of different brushes given in Figs. 167 and 168.
Fig. 167 gives the voltage consumed by the brush contact of a
copper brush, with the current density as abscissae, while Fig.
168 gives the voltage consumed by a high-resistance carbon
brush, with the current density in the brush as abscissae. It is
seen that such a resistance, which decreases approximately in-
versely proportional to the increase of current, fails in limiting
the current just at the moment where it is most required, that
s, at high currents.

Commutator Leads

198. Good results have been reached by the use of metallic
resistances in the leads between the armature and the commuta-
tor. As shown diagrammatically in Fig. 169, each commutator
segment connects to the armature, A, by a high non-inductive
resistance, CB} and thus two such resistances are always in the
circuit of the armature coil short-circuited under the brush, but
also one or two in series with the armature main circuit, from
brush to brush. While considerable power may therefore be
consumed in these high-resistance leads, nevertheless the effi-
ciency of the motor is greatly increased by their use; that is, the
reduction in the loss of power at the commutator by the reduction

,1)