SINGLE-PHASE COMMUTATOR MOTORS 349
a component due to the rotation—a power e.m.f.—and a com-
ponent due to the alternation—a reactive e.m.f. The armature
coils of an alternating-current commutator motor, therefore, are
the seat of a system of polyphase e.in.fs., and at synchronism
the polyphase e.m.fs. generated in all armature coils are equal,
above synchronism the e.m.f. of rotation is greater, while
below synchronism the e.m.f. of alternation is greater, and in
the latter case the brushes thus stand at that point of the com-
mutator where the voltage between commutator segments is a
maximum. This e.m.f. of alternation, short-circuited by the
armature coil in the position of commutation, if not controlled,
causes a short-circuit current of .excessive value, and therewith
destructive sparking; hence, in the alternating-current commuta-
tor motor it is necessary to provide means to control the short-
circuit current under the commutator brushes, which results from
the alternating character of the magnetic flux, and which does
not exist in the direct-current motor; that is, in the alternating-
current motor the armature coil under the brush is in the posi-
tion of a short-circuited secondary, with the field coil as primary
of'a transformer; and as in a transformer primary and secondary
ampere-turns are approximately equal, if no = number of field
turns per pole and i = field current, the current in a single arma-
ture turn, when short-circuited by the commutator brush, tends
to become IQ = n0i, that is, many times full-load current; and
as this current is in opposition, approximately, to the field cur-
rent, it would demagnetize the field; that is, the motor field
vanishes, or drops far down, and the motor thus loses its torque.
Especially is this the case at the moment of starting; at speed,
the short-circuit current is somewhat reduced by the self-induc-
tance of the armature turn. That is, during the short time
during which the armature turn or coil is short-circuited by the
brush the short-circuit current can not rise to its full value, if
the speed is considerable, but it is still sufficient to cause destruc-
tive sparking.
197. The character of the commutation of the motor, and
therefore its operativeness, thus essentially depends upon the
value and the phase of the short-circuit currents under the com-
mutator brushes. An excessive short-circuit current gives de-
structive sparking by high-current density under the brushes
and arcing at the edge of the brushes due to the great and sud-
den change of-current in the armature coil when leaving the