182 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
of the motors was as good as could be desired, and the
abolition of the commutator is a very material gain, since
collecting rings give decidedly less trouble. Change of
armature resistance gave opportunity to pass smoothly
from one field connection to another without jerking the
car. Asappears from curves C II and C III, apparatus of
this kind has the very considerable advantage of fairly
constant speed over a wide range of drawbar pull. Al-
though polyphase induction motors are termed asynchron
ous they have so strong a tendency to run near synchronous
speed that they have the power of driving ahead regard-
less of grades unless grossly overloaded.
None of the methods of regulation as yet devised is
quite the equivalent of the series parallel control so exten-
sively used in continuous current practice, so far as effi-
ciency is concerned. It is possible to get, however, as
complete control of the speed and nearly as good efficiency
at all except the lowest speeds. In the line of work for
which alternating motors are most needed, i.e., long inter-
urban and similar lines the need of highly efficient control
at very low speeds is not so great as in ordinary street
railway work, since by far the largest aggregate output is
at the higher speeds.
The weak points of polyphase induction motors for
railway work are as follows:
I. Necessity for at least two trolley wires.
II. Lagging current.
Inasmuch as all true polyphase systems require at least three
working conductors, the best that can be done in supplying
polyphase current is to utilize the rails for one conductor
and provide separate trolley wires for the other two. In
rare instances it might be possible to use a third rail and a
single trolley wire or even to utilize the two track rails as
separate conductors, but such cases are likely always to be
exceptional. In conduit work, of course, two working
conductors are available without much difficulty, but for
general purposes the burden of two trolleys is difficult to
avoid.