332 ELECTRICAL APPARATUS
transformer, and for this purpose either a separate transformer
may be.used or the transformer feature embodied in the motor,
as in the so-called repulsion type of motors. This gives to the
alternating-current commutator motor a far greater variety of
connections than possessed by the direct-current motor.
While in its general principle of operation the alternating-
current commutator motor is identical with the direct-current
motor, in the relative proportioning of the parts a great differ-
ence exists. In the direct-current motor, voltage is consumed
by the counter e.m.f. of rotation, which represents the power
output of the motor, and by the resistance, which represents
the power loss. In addition thereto, in the alternating-current
motor voltage is consumed by the inductance, which is wattless
or reactive and therefore causes a lag of current behind the vol-
tage, that is, a lowering of the power-factor. While in the direct-
current motor good design requires the combination of a strong
.., * field and a relatively weak armature, so as to reduce the armature
™ - reaction on the field to a minimum, in the design of the alter-
nating-current motor considerations of power-factor predominate;
that is, to secure low self-inductance and therewith a high power-
1 factor, the combination of a strong armature and a weak field is
required, and necessitates the use of methods to eliminate the
harmful effects of high armature reaction.
As the varying-speed single-phase commutator motor has
found an extensive use as railway motor, this type of motor
will as an instance be treated in the following, and the other
types discussed in the concluding paragraphs.
II. Power-factor
190. In the commutating machine the magnetic field flux gen-
erates the e.m.f. in the revolving armature conductors, which
gives the motor output; the armature reaction, that is, the mag-
netic flux produced by the armature current, distorts and weakens
the field, and requires a shifting of the brushes to avoid sparking
due to the short-circuit current under the commutator brushes,
and where the brushes can not be shifted, as in a reversible motor,
i this necessitates the use of a strong field and weak armature to
keep down the magnetic flux at the brushes. In the alternating-
current motor the magnetic field flux generates in the armature
conductors by their rotation the e.m.f. which does the work of
the motor, but, as the field flux is alternating, it also generates