ALTERNATING MOTORS FOR RAILWAY WORK. 171
The great advantage in synchronous motors for such
work lies in their freedom from lagging current, and
their insensitiveness to changes of voltage. A power factor
approaching unity such as can readily be obtained from
large synchronous motors reduces the difficulties of trans-
mission very materially, and particularly it diminishes the
necessary capacity in the generating station and in the
line.
In general transmission plants for a mixed load of
lights, synchronous and induction motors, the power
factor can be kept fairly high, with careful operation prob-
ably up to .85 or .go. This power factor means that for
operation at a given voltage ten to fifteen per cent
more ~urrent must be generated and transmitted than cor-
responds to the energy delivered. In addition a similar
amount of reserve voltage must be available to compensate
for the inductive drop in the line and the reaction of the
lagging current in the generators.
The total net effect then, of even this power factor is
to call for not less than twenty-five per cent extra capacity
in the generating plant. Were it not for the fact that
polyphase generators have a high output compared with
continuous current generators, even this increase would
be serious—as it is it is annoying. In plants operating
induction motors only, the increased capacity necessary by
reason of lagging current may be very much more serious,
and makes the synchronous motor a thing not lightly to
be put aside as impracticable.
III. Although the asynchronous polyphase motor is
now not unfamiliar and its theory is fairly well known: to
most engineers, its practical characteristics are not widely
understood.
We may best regard it as an alternating motor in
which the current is led into the armature by induction as
in an ordinary transformer instead of by brush contacts.
Its field and armature windings are so organized that the
currents in them bear to each other the relation necessary
to secure effective torque, asin any other motor. Whether
